Type Annelids: characteristics, organ systems, the significance of worms in nature. Type annelids classes taxonomy The structure of the head lobe

To annelids belong primary annulus, polychaete and oligochaete worms, leeches and echiurids. In the type 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. Mostly these are free-living forms. The body of the annulus is subdivided into three parts: the head, the trunk, consisting of rings, and the anal lobe. Such a clear division of the body into sections is not found in animals that are lower in their organization.

The head of the rings is equipped with various sense organs. Many ringlets have well developed eyes. Some have particularly sharp eyesight, and their lens is capable of accommodation. True, the eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. The rings also have developed taste sensations. On the head and tentacles, many of them have special olfactory cells and ciliary pits that perceive various smells and the action of many chemical stimuli. The organs of hearing, arranged according to the type of locators, are well developed in the rings. Recently, auditory organs, very similar to those of the lateral line in fish, have been discovered in Echiurid marine rings. 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 rings consists of rings, or segments. The number of rings can reach several hundred. Other rings 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 rings. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "feet-like". Parapodia are lobe-shaped outgrowths of the body, from which tufts of bristles stick out. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all annulus. They are present in primary annulus and polychaete worms. In oligochaetes, only bristles remain. Primitive Leech acanthobdella has bristles. The rest of the leeches do without parapodia and bristles in motion. At echiuride no parapodia, and setae present only at the posterior end of the body.

Parapodia, nodes of the nervous system, excretory organs, sex glands, and, in some polychaetes, paired pockets of the intestine, are systematically repeated in each segment. This internal segmentation coincides with the external annulus. 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 the annulus. The elongation of the body necessitated repeated repetition, first of the organs of movement with their muscles and nervous system, and then of the internal organs.

The segmented secondary cavity of the body, or the whole, is extremely characteristic of the rings. 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 that covers the intestines, muscles, and all other internal organs. The body cavity is divided into segments by transverse partitions - dissipations. A longitudinal septum passes along the midline of the body - the mesentery, which divides each compartment of the cavity into the right and left parts.

The body cavity is filled with a 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 cavity fluid perform important functions. The 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 nutritious products, secretions of the endocrine glands, as well as oxygen and carbon dioxide involved in the breathing process inside the body of the rings.

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 partitions prevent the cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. Marine ships and submarines also have internal hermetic partitions. If the board is pierced, then the water that rushes into the hole fills only one damaged compartment. The remaining compartments, not filled with water, retain the buoyancy of the damaged ship. Similarly, in annuli, a violation 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 of the body wall of an echiurida can lead to its death. In addition to the respiratory and protective role, the secondary cavity acts as a receptacle for the reproductive products that mature there before being brought out.

ringlets, 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 coincide so much 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 chlorcruorin. Often the rings have red blood, similar in composition to the blood of vertebrates. Red blood contains iron, which is part of the hemoglobin pigment. Some rings, 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 developed the pigment hemerythrin, which contains five times more iron than hemoglobin.

In annuli, compared with lower invertebrates, metabolism and respiration proceed much more intensively. Some polychaete rings develop special respiratory organs - gills. In the gills, a network of blood vessels branches, and through their wall oxygen penetrates into the blood, and then spreads throughout the body. Gills can be located on the head, on the parapodia and on the tail.

The end-to-end intestine of the annulus consists of several sections. Each section of the intestine has its own specific function. The mouth leads to the throat. Some ringlets have strong horny jaws and denticles in the throat, which help to grasp live prey more firmly. In many predatory rings, the throat serves as a powerful weapon of attack and defense. The esophagus follows the pharynx. This department is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push the food into the following sections. In the wall of the esophagus there are glands, the enzyme of which serves for the primary processing of food. The esophagus is followed by the midgut. In some cases, goiter and stomach are developed. The wall of the midgut is formed by an epithelium very rich in glandular cells that produce a digestive enzyme. Other cells in the midgut absorb the digested food. In some rings, the midgut is in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths from the sides of the intestine. The hindgut ends with an anus.

Special organs - metanephridia - serve to excrete liquid metabolic products. Often they serve to bring out the germ cells - sperm and eggs. Metanephridia begin as a funnel in the body cavity; a convoluted canal runs from the funnel, which opens outwards in the next segment. Each segment contains two metanephridia.

Ringworms reproduce asexually and sexually. Aquatic rings frequently reproduce asexually. At the same time, their long body breaks up into several parts. After a while, each part regenerates 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 apart. In this case, the detached parts already have a head with all the necessary sense organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These rings have lost their segmented body cavity. This is partly why, apparently, they do not have the ability to reproduce asexually and restore lost parts.

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

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

Freshwater and terrestrial rings are most often hermaphrodites and have direct development. There are no free larvae in freshwater and terrestrial rings. This is due to the fact that fresh water has a salt composition of a completely different property than sea water. Sea water is more favorable for the development of life. Fresh water even contains some poisonous waters (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-permeability shells. Even more dense shells - shells - are formed in 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 growing more and more in connection with the development of the intensity of biological research.

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

Earthworms not only serve as bait for fishing and food for birds. They bring great benefits to man, loosening the soil, making it more porous. This favors the free penetration of air and water to the roots of plants and increases crop yields. Rummaging in the ground, the 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 earth 5 cm thick.

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

ring type includes several classes. The most primitive are marine primary rings - archiannelides. Polychaete rings and echiurids- inhabitants of the sea. Small-bristle rings 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 .

Annelids, also called annelids or annelids, include a huge number of animal species. Their body consists of numerous repeating ones, which is why they got such a name. The general characteristics of annelids unite about 18 thousand of their different species. They live on land in the soil and on the surface in tropical rain forests, in the sea water of the oceans and fresh water of rivers.

Classification

Annelids are a phylum of invertebrates. Their group is called protostomes. Biologists distinguish 5 classes of annelids:

Belt, or leeches;

Small-bristle (the most famous representative of this class is the earthworm);

Polychaete (sandworm and nereid);

Mysostomides;

Dinophylides.

Considering the general characteristics of annelids, you understand their important biological role in the processing and aeration of soils. Earthworms loosen the soil, which is beneficial for all the surrounding vegetation of the planet. To understand how many of them there are on earth, imagine that in 1 sq. meter of soil, aeration is carried out from 50 to 500 annelids. This increases the productivity of agricultural land.

Annelids are one of the main links in the food chains of the ecosystem, both on land and in the oceans. They feed on fish, turtles, birds and other animals. Even people use them as top dressing when breeding commercial fish species in both fresh and marine waters. Fishermen put worms on their hooks as bait when fishing with a line.

Everyone knows about the significance of medicinal leeches, which suck blood from sore spots, relieving a person from hematomas. Their medicinal value people have understood for a long time. Leeches are used for hypertension, increased blood clotting. Leeches have the ability to produce hirudin. This is a substance that reduces blood clotting and dilates the vessels of the human circulatory system.

Origin

Studying the general characteristics of annelids, scientists have found that they have been known since the Cambrian period. Considering their structure, biologists came to the conclusion that they originated from an older type of lower flatworms. The similarity is evident in certain structural features of the body.

Scientists believe that the main group of polychaete worms appeared first. In the process of evolution, when this type of animal passed to life on the surface and in fresh water, there were also small-bristle, later called leeches.

Describing the general characteristics of annelids, we note that this is the most progressive type of worms. It was they who first developed the circulatory system and the ring-shaped body. Paired organs of movement appeared on each segment, which later became the prototype of the limbs.

Archaeologists have found extinct annelids that had several rows of calcareous plates on their backs. Scientists believe that there is a definite connection between them and molluscs and brachiopods.

general characteristics

In grade 7, the type of annelids is studied in more detail. All representatives have a fairly characteristic structure. Both from the front and from the back, the body looks the same and symmetrical. Conventionally, it is divided into three main sections: the head lobe, numerous segments of the central part of the body, and the posterior or anal lobe. The central segmented part, depending on the size of the worm, may include from ten to several hundred rings.

The general characteristics of annelids include information that their sizes vary from 0.25 mm to a length of 5 meters. The movement of worms is carried out in two ways, depending on its type. The first way is by contracting the muscles of the body, the second is with the help of parapodia. These are the bristles that polychaete worms have. They have lateral bilobed outgrowths on the walls of the segments. In oligochaete worms, organs such as parapodia are absent altogether or have separately growing small bundles.

The structure of the head lobe

Annelids have sensory organs located in front. These are eyes, olfactory cells, which are also found on the tentacles. The ciliary pits are organs that distinguish between the effects of various odors and chemical irritants. There are also hearing organs that have a structure resembling locators. And, of course, the main organ is the mouth.

segmented part

This part is the same general characteristic of the type of annelids. The central region of the body consists of rings, each of which is a completely independent part of the body. Such an area is called a whole. It is divided by partitions into segments. They are noticeable when looking at the appearance. The outer rings of the worm correspond to the inner partitions. On this basis, the worms got their main name - annelids, or rings.

Such a division of the body for the life of the worm is very important. If one or more rings are damaged, the rest remain intact, and the animal regenerates in a short period of time. The internal organs are also arranged in accordance with the segmentation of the rings.

Secondary body cavity, or whole

In the structure of annelids, the following general characteristic is present: the skin-muscular sac has a coelomic fluid inside. It consists of the cuticle, skin epithelium, and circular and longitudinal muscles. In the fluid contained in the body cavity, the constancy of the internal environment is maintained. All the main functions of the body are carried out there: transport, excretory, musculoskeletal and sexual. This fluid is involved in the accumulation of nutrients, brings out all the waste, harmful substances and sexual products.

The type of annelids has common characteristics in the field of body cell structure. The upper (outer) layer is called the ectoderm, followed by the mesoderm with a secondary cavity lined with its cells. This is the space from the walls of the body to the internal organs of the worm. The fluid contained in the secondary cavity of the body, due to pressure, maintains a constant shape of the worm and plays the role of a hydroskeleton. The last inner layer is called the endoderm. Since the body of annelids consists of three shells, they are also called three-layered animals.

Worm food system

The general characteristics of annelids in grade 7 briefly describe the structure of the digestive system of the body of these animals. In the anterior part is the mouth opening. It is located in the first segment from the side of the peritoneum. The entire digestive tract has a through system of structure. This is actually the mouth, then there is a peripharyngeal ring that separates the pharynx of the worm. The long esophagus ends in the goiter and stomach.

The intestine has a common characteristic for the class of annelids. It consists of three departments with different purposes. These are the anterior, middle and hindgut. The middle compartment is made up of endoderm, while the rest are ectodermal.

Circulatory system

The general characteristics of annelids are briefly described in the 7th grade textbook. And the structure of the circulatory system can be seen in the schematic image above. Vessels are marked in red. The figure clearly shows that the circulatory system of annelids is closed. It consists of two long longitudinal vessels. This is the dorsal and abdominal. They are connected to each other by the annular vessels present in each segment, which resemble veins and arteries. The circulatory system is closed, the blood does not leave the vessels and does not spill into the body cavity.

The color of blood in different types of worms can be different: red, transparent and even green. It depends on the properties of the chemical structure of the respiratory pigment. It is close to hemoglobin and has a different oxygen content. Depends on the habitat of the annelids.

The movement of blood through the vessels is carried out due to the contractions of some parts of the dorsal and, less often, the annular vessels. After all, they don't. Rings contain special contractile elements in these vessels.

excretory and respiratory systems

These systems in the type of annelids (the general characteristics are briefly described in the 7th grade textbook) are associated with the skin. Respiration is carried out through the skin or gills, which in marine polychaete worms are located on the parapodia. The gills are branched thin-walled outgrowths on the dorsal lobes. They can be of different shapes: leaf-shaped, pinnate or bushy. The inside of the gills is pierced by thin blood vessels. If the worms are low-bristle, then breathing occurs through the moist skin of the body.

The excretory system consists of metanephridia, protonephridia, and myxonefridia, arranged in pairs in each segment of the worm. Myxonephridia are the prototype of the kidneys. Metanephridia are funnel-shaped, located in the coelom, from which a thin and short canal brings excretion products out in each segment.

Nervous system

If we compare the general characteristics of round and annelids, then the latter have a more advanced nervous system and sensory organs. They have a cluster of nerve cells above the parapharyngeal ring of the anterior lobe of the body. The nervous system is made up of ganglia. These are supra-pharyngeal and sub-pharyngeal formations connected by nerve trunks into a peri-pharyngeal ring. In each segment, one can see a pair of such ganglia of the ventral chain of the nervous system.

You can see them in the picture above. They are marked in yellow. Large ganglia in the pharynx play the role of the brain, from which impulses diverge along the abdominal chain. The sense organs of the worm also belong to the nervous system. He has many of them. These are the eyes, and the organs of touch on the skin, and the chemical senses. Sensory cells are located all over the body.

reproduction

Describing the general characteristics of the type of annelids (class 7), one cannot fail to mention the reproduction of these animals. They are mostly heterosexual, but some have developed hermaphroditism. The latter include well-known leeches and earthworms. In this case, conception occurs in the body itself, without fertilization from outside.

In many polychaetes, development occurs from the larva, while in the remaining subspecies it is direct. The gonads are located under the epithelium of the coelom in each or almost in each segment. When a rupture occurs in these cells, the germ cells enter the coelom fluid and are excreted through the organs of the excretory system to the outside. In many, fertilization occurs on the outer surface, while in underground soil worms, it occurs inside.

But there is another type of reproduction. In conditions favorable for life, when there is a lot of food, individual parts of the body begin to grow in individuals. For example, multiple mouths may appear. Subsequently, the rest grows. The worm breaks up into several separate parts. This is an asexual type of reproduction, when a certain part of the body appears, and the rest regenerate later. As an example, we can cite the ability of aulophorus to this type of reproduction.

In the article, you learned in detail all the main characteristics of annelids, which are studied in the 7th grade of the school. We hope that such a detailed description of these animals will help to learn knowledge more easily.

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

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

Polychaete annelids Polychaetes

The main features of the type:

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

Annelids General characteristics

Latin name Annelida

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

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 rings is expressed not only in the external, but also in the internal organization, in the repetition of many internal organs.

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

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

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

Internal structure

musculature

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

body cavity

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

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

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

Digestive system

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

excretory system

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

The circulatory system of annelids

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

Respiratory system

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

Nervous system and sense organs

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

Classification

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

1. Multi-brush rings (Polychaeta)

2. Echiurida (Echiurida)

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

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

3. Small-bristle rings (Oligochaeta)

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

4. Leeches (Hirudinea) >> >>

Phylogeny

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

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

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

As for the phylogenetic relationships within the four classes of annelids considered, they now seem to be 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 nodal group in the evolution of higher protostomes. Mollusks and arthropods originate from them.

Meaning of annelids

Polychaete worms.

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

Small-bristle worms.

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

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 also lie blood vessels and the nerve chain. Each segment has its own nodes of the 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.

The digestive system consists of the 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 tail 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 in comparison with 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.

Annelids, or annelids (from Latin annulus - ring) - 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 type has about 18 thousand species.

They belong to primary animals, the body is divided into segments, the number of which in some species reaches several hundred. Let's start studying annelid worms with classification.

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

Aromorphoses of annelids

The main details of the structure of annelids will be studied by us using the example of a typical representative - an earthworm (in the oligochaete section).

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