Annelids or annelids (lat. Annelida) are a type of highly organized invertebrate animals, characterized by the presence of a segmented coelom, which is responsible for external annulation. In the process of evolution, annelids evolved from ancient free-living flatworms.

The coelom is the cavity that separates the intestine from the body wall. It is a characteristic feature of ringed worms, since round and flat worms do not have it.

The most famous representatives of annelids for every person are leeches (subclass Hirudinea) and earthworms (suborder Lumbricina), which are also called earthworms. But in total there are more than 20 thousand species of these animals.

Taxonomy

Today, experts classify from 16 to 22 thousand modern animal species as annelids. There is no single approved classification of ringlets. The Soviet zoologist V.N. Beklemishev proposed a classification based on the division of all representatives of annelids into two superclasses: non-girdle worms, which includes polychaetes and echiurids, and girdle worms, which includes oligochaetes and leeches.

Below is the classification from the World Register of Marine Species website.

• Class Polychaetes (Polychaetes). Representatives of the class have connected lateral appendages (parapodia) bearing chitinous setae; The name of the group is determined by the presence of a large number of setae per segment. Head with or without appendages. In most cases - dioecious; gametes are discharged directly into the water, where fertilization and development occur; free floating and are called trochophores. Sometimes they reproduce by budding or fragmentation. The class includes more than 6,000 species, which are divided into free-living and sessile forms.
• Class Girdle (Clitellata). Representatives of the class have an insignificant amount or no bristles on their body. There are no parapodia. They are characterized by the presence of a unique reproductive organ - the girdle, which is formed from the remains of the cocoon and performs a protective function for fertilized eggs. The class has about 10,000 representatives.
  • Subclass Oligochaetes (Oligochetes). They live primarily in fresh water. They have setae that arise directly from the walls of the body, due to the small number of which (usually 4 on each segment) the subclass is called oligochaete. As a rule, they do not have appendages on the body. Hermaphrodites. Development is direct, there is no larval stage. There are about 3250 species.
  • Leech subclass. They inhabit mainly freshwater bodies, but there are also terrestrial and marine forms. There is a small sucker at the anterior end of the body and a large sucker at the posterior end. The fixed number of body segments is 33. The body cavity is filled with connective tissue. Hermaphrodites. Fertilized eggs are laid in a cocoon. Development is direct, there is no larval stage. There are about 300 species of representatives.

Class Echiura. This is a small group with only about 170 known species, all of which are exclusively marine inhabitants. Echiuridae were recently classified as annelids after DNA examinations, but previously it was a separate type. The reason is that their body is different - it does not have segmentation, like those of ringed animals. In some sources, the Echiurides are considered not as a separate class, but as a subclass of the Polychaetes.

Spreading

Annelids, depending on the species, live on land, in fresh and salt water.

Polychaete worms, as a rule, live in seawater (with the exception of some species that can also be found in freshwater bodies). They are food for fish, crayfish, as well as birds and mammals.

Oligochaete worms, to which the earthworm belongs, live in soil fertilized with humus or fresh water bodies.

Echiurids are distributed only in marine waters.

Morphology

The main characteristic of representatives of the phylum Annelida is considered to be the division of the body into a number of cylindrical segments, or metameres, the total number of which varies widely depending on the type of worm. Each metamer consists of a section of the body wall and a compartment of the body cavity with its internal organs. The number of outer rings of worms corresponds to the number of internal segments. The annelid body consists of a head region (prostomium); a body consisting of metameres; and a segmented posterior lobe called the pygidium. In some primitive representatives of this type the metamers are identical, or very similar to each other, each containing the same structures; in more advanced forms there is a tendency to consolidate certain segments and restrict certain organs to certain segments.

The outer shell of the annelid body (muscular sac) includes the epidermis surrounded by a cuticle, as well as well-developed, segmentally located muscles - circular and longitudinal. Most annelids have short external setae composed of chitin. In addition, on each metamere, some representatives of this type of animal may have primitive limbs called parapodia, on the surface of which bristles and sometimes gills are located. The spatial movement of worms is carried out either through muscle contraction or parapodia movements.

The body length of annelids varies from 0.2 mm to 5 m.

Basic general anatomical features of annelids in cross section

The digestive system of annelids consists of an unsegmented gut that runs through the middle of the body from the mouth, located on the underside of the head, to the anus, located on the anal lobe. The intestine is separated from the body wall by a cavity called the coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue called septa, which perforate the intestine and blood vessels. With the exception of leeches, the whole of annelids is filled with fluid and functions as a skeleton, providing muscle movement, as well as transport, sexual, and excretory functions of the body. If the integrity of the worm's body is damaged, it loses the ability to move properly, since the functioning of the body's muscles depends on maintaining the volume of coelomic fluid in the body cavity. In primitive annelids, each compartment of the coelom is connected to the outside via channels for the release of germ cells and paired excretory organs (nephridia). In more complex species, both excretory and reproductive functions are sometimes served by one type of canals (and the canals may be absent in certain segments).

Circulatory system. Annelids developed a circulatory system for the first time in the process of evolution. Blood typically contains hemoglobin, a red respiratory pigment; however, some annelids contain chlorocruorin, a green respiratory pigment that gives the blood its color.

The circulatory system is usually closed, i.e. enclosed in well-developed blood vessels; in some species of polychaetes and leeches, an open-type circulatory system appears (blood and cavity fluid mix directly in the sinuses of the body cavity). The main vessels - abdominal and dorsal - are connected to each other by a network of annular vessels. Blood is distributed in each segment of the body along the lateral vessels. Some of them contain contractile elements and serve as a heart, i.e. play the role of pumping organs that move the blood.

Respiratory system. Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most representatives of this type of invertebrates do not have any special organs for gas exchange, and respiration occurs directly through the surface of the body.

The nervous system typically consists of a primitive brain, or ganglion, located in the head region, connected by a ring of nerves to the ventral nerve cord. In all metameres of the body there is a separate nerve ganglion.

The sense organs of ringed fish usually include eyes, taste buds, tactile tentacles and statocysts - organs responsible for balance.

Annelids reproduce sexually or asexually. Asexual reproduction is possible through fragmentation, budding or fission. Among the worms that reproduce sexually, there are hermaphrodites, but most species are dioecious. Fertilized ringed eggs usually develop into free-swimming larvae. The eggs of terrestrial forms are enclosed in cocoons and larvae, like miniature versions of the adults.

The ability to restore lost body parts is highly developed in many multi- and oligochaete representatives of annelids.

Ecological significance

Earthworm is very important for maintaining soil condition

Charles Darwin, in The Formation of Vegetable Mold through the Action of Worms (1881), presented the first scientific analysis of the effect of earthworms on soil fertility. Some of the worms dig burrows in the soil, while others live exclusively on the surface, usually in damp leaf litter. In the first case, the animal is able to loosen the soil so that oxygen and water can penetrate into it. Both surface and burrowing worms help improve soil in several ways:

• by mixing organic and mineral substances;
• by accelerating the decomposition of organic substances, which in turn makes them more accessible to other organisms;
• by concentrating minerals and converting them into forms that are more easily absorbed by plants.

Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers in some cases.

Terrestrial annelids in some cases can be invasive (brought into a certain area by people). In glacial areas of North America, for example, scientists believe that almost all native earthworms were killed off by glaciers and the worms currently found in these areas (such as Amynthas agrestis) were introduced from other areas, primarily Europe , and more recently, from Asia. Northern deciduous forests have been particularly negatively impacted by invasive worms through loss of leaf litter, decreased soil fertility, changes in soil chemistry, and loss of ecological diversity.

Marine annelids may account for more than one-third of benthic animal species around coral reefs and in intertidal zones. Burrowing annelid species increase the penetration of water and oxygen into seafloor sediment, which promotes the growth of populations of aerobic bacteria and small animals.

Human interaction

Anglers find that worms are more effective baits for fish than artificial fly baits. In this case, the worms can be stored for several days in a tin jar filled with damp moss.

Scientists study aquatic annelids to monitor oxygen levels, salinity and environmental pollution in fresh and seawater.

The jaws of polychaetes are very strong. These advantages have attracted the attention of engineers. Research has shown that the jaws of this genus of worms are made of unusual proteins that bind strongly to zinc.

On the island of Samoa, catching and eating one of the representatives of annelids - the Palolo worm - is a national holiday, and the worm itself is considered a delicacy by local residents. In Korea and Japan, the worms Urechis unicinctus from the class Echiuridae are eaten.

Representatives of annelids that are eaten

About 17 species of leeches are dangerous to humans.

Medical leeches are used for hirudotherapy, and a valuable remedy, hirudin, is extracted from pharmaceuticals.

Leeches can attach to a person’s skin from the outside, or penetrate internal organs (for example, the respiratory system or gastrointestinal tract). In this regard, two types of this disease are distinguished - internal and external hirudinosis. With external hirudinosis, leeches most often attach to human skin in the armpits, neck, shoulders, and calves.

Misostomidae on sea lily

Skin helminthic infestation is an extremely unpleasant disease, especially when the location of the worms is the person’s face. How do they appear under the skin, what serves as a prerequisite, what varieties are capable of getting into this part of the human body, how to determine their presence, is there a chance to get rid of this scourge?

The main carriers of microscopic nematodes are the canine family, but cases of the disease in cats have also been recorded. India is the epicenter of the worm infection, but it is also found in other warm climates. Recently, the disease has been registered in places where it had not previously occurred.

Adult dirofilaria worms are round threads with a thickness ranging from a hundredth to one and a half mm. The body has pointed tips, the length of the female worm is 15 cm, the male is somewhat shorter. People called them the “Evil Thread” for the harm they cause to the body.

After blood-sucking insects bite sick animals, the larvae enter their body, where they mature to an invasive state. Then, the grown organisms are introduced into the human bloodstream with an insect bite and are located under the skin. They often choose the face as the site of invasion and get into the eyes. They develop there within 90 days and do not manifest themselves for a long time.

Symptoms of skin diseases

Treatment

Usually the worm is present alone and is removed from the skin surgically.

• To deprive him of the ability to move under the skin, Ditrazine is prescribed 2 days before the procedure.
• After surgery, antispasmodics, sedatives and anti-inflammatory drugs are used.
• The doctor may prescribe diethylcarbamazine or Ivermectin if necessary, but treatment is carried out under his supervision, since these drugs can cause a severe allergic reaction

Filariasis

These worm infestations are caused by a group of roundworm nematodes. They live in hot countries, feed on lymph, are located in different parts of the body, some types of worms prefer to settle under the skin, in the eyes. They often get stuck in the scalp and also get into the face.

The development of larvae occurs in the body of blood-sucking insects, and they are intermediate hosts. Dogs and cats carry filaria; the mechanism of entry into the human body is not much different from infection with dirofilariasis.

The larvae enter the bloodstream and spread throughout the body. More than one year will pass before a person realizes that filariae have settled in his face, under his skin:

• under the skin the worms gather in a ball, painless tumors the size of a pea or even a quail egg appear on it;
• the skin in this area turns slightly red;
• soon the bumps begin to itch;
• bacteria develop in them, then purulent inflammation cannot be avoided;
• severe pain occurs;
• sometimes the temperature rises;
• The larvae collected on the skin also change its color, in this place it becomes lighter or darker.

You can suspect filaria in the eye based on the following symptoms:

Treatment

Filaria from the skin and eyes are removed by surgery, antibiotics are prescribed to prevent re-inflammation from developing in the places from which the helminths were removed.

Cysticercosis

The causative agent is cysticercus (pork tapeworm larva), its habitat is raw meat and lard, sometimes water. It looks like a tiny oblong bubble, it contains the head of a pork tapeworm, it is very small and already has suckers and hooks.

The larva gets inside with food or is brought in with dirty hands, and is thrown from the intestines into the stomach along with vomit. In the human body it can settle in the liver and kidneys, brain, eyes, and can also be located under the skin, including on the face.

Once in the stomach, the larva loses its shell and begins to penetrate the intestinal mucosa, through it it enters the circulatory system, wandering throughout the body, and gets under the skin. There it gradually develops, changing shape, becoming round or taking on the appearance of a spindle, increasing to 15 cm .

Symptoms

The signs of cysticercosis under the skin are peculiar; the worms act not only locally, irritating tissues, but also affect the entire body, releasing toxins:

Doctors cannot make a favorable prognosis if the patient does not apply on time. As the cysticercus dies, it becomes discolored, which also does not go unnoticed by humans; this is expressed by allergies, skin rashes, and fever.

Treatment

Large subcutaneous formations are removed by a surgeon, but small ones do not require treatment. Therapeutic drugs are not used in this case, since the helminth, when dying, releases decay products, which causes severe allergies.

When the patient is near a body of water, the worms sense the water and begin to move, which causes severe pain to the person. Once in the water, the female begins to intensively lay a huge number of eggs; they are consumed by cyclops crustaceans, becoming infected with them and transmitting the infection.

The crustaceans in the stomach die, the larvae are released, passing through the walls of the stomach, and migrate with the blood into the soft tissues. After 90 days, the female settles into the skin and matures there for about a year.

Symptoms

Gradually, the head of the rishta approaches the surface of the skin, and the first signs of an allergy make themselves felt:

If the bladder becomes inflamed from infection, then the development of sepsis or gangrene is quite likely.

Treatment

• The manipulation is carried out extremely slowly, the long worm may break, then toxic liquid will enter the wound:
• during this period the patient takes Metronidazole, the drug weakens the helminth, it resists less;
• at the same time, antiseptic rules are carefully observed;
• the patient is given an injection for tetanus;
• antihistamines are prescribed;
• antibiotics.

There is another disease that has been repeatedly reported in the media. It was named after the American family in which it was first discovered in 2001. Now several thousand people claim that they suffer from this disease.

Patients are sure that worms are crawling on their face and under their skin; they are constantly changing their location, causing discomfort. Those who have encountered this phenomenon are constantly haunted by fear.

Some claim that thread-like worms emerge from wounds in the skin, while others claim that their worms then turn into beetles or butterflies. Patients even tried to get rid of them by applying hot iron to parts of the skin, but the relief did not come for long, and soon the disease manifested itself again in a different place.

Symptoms

Symptoms resemble those of dirofilariasis:

• first, unbearable itching appears on the skin of the face or other parts of the body;
• in these places the skin rises;
• then, there is a feeling that something is moving there, the seals are even moving under the skin;
• inflammation forms, and later bleeding purulent ulcers appear;
• There are thin threads in the skin that look like worms, but their etiology is still unclear.

What Science Says

Since nothing is still known about the origin of the pathology, the opinions of doctors are divided; there are several versions of the origin of the disease:

And the latter should not be excluded, since the media is often a provocateur of outbreaks of various phenomena in people with unstable psyches. There is an opinion that the reaction is caused by genetically modified organisms, or caused by experiments in the field of nanotechnology, but this is already at the level of speculation and fantasy.

Treatment

The disease is not recognized by official medicine, and methods for its cure have not been developed. When dealing with characteristic complaints of worms under the skin of the face, doctors, after a thorough examination, use antibacterial or antifungal drugs.

Indeed, Margellons disease responds to the use of drugs that destroy the fungal infection with a decrease in activity, the number of affected areas on the skin becomes smaller, and relapses have not yet manifested themselves. But the main treatment is to detoxify the body and increase immunity.

Animals; the most highly organized among all worms. For the first time, they have a set of organ systems characteristic of all higher groups of organisms, including mammals. Length from 2-3 mm to 3 m. The cylindrical or flattened body of annelids, as a rule, is clearly segmented. At the same time, partitions are formed in the body cavity, dividing it into separate segments. External and internal segmentation most often coincide, but sometimes only one internal segment corresponds to several external segments. More rarely, there is no segmentation. The first segment of the body is the head lobe, on which sensory organs can be located: antennae, palps, eyes.

The mouth opens on the lower surface of the second body segment. In polychaete worms, powerful chitinous jaws are formed in the oral cavity, capable of turning outward. They serve to capture and hold prey. In leeches, the mouth opening is surrounded by a sucker formed by the fusion of the first four segments. On the last segment of the body, the anus opens. On the sides of all segments, except 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 bristles, which less often may be completely absent.

Annelids are three-layered animals that develop ectoderm, endoderm and mesoderm. From the latter, a secondary body cavity (coelom) is formed, filled with cavity fluid. Due to the fact that the fluid is under pressure, 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-muscle sac, consisting of skin epithelium and underlying circular and longitudinal muscles. Thanks to him, worms are able to perform a variety of movements.

The digestive system of annelids is continuous and consists of three sections: the foregut, midgut and hindgut. The anterior and posterior sections of the intestine develop from the ectoderm, and the middle section 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 again, and in sipunculids it is absent. Paired ciliated funnels, which are repeated many times in each segment of the body, function as excretory organs in annelids. In this case, the funnel itself is located in one segment, and the excretory channel, passing through the partition between the segments, opens with an excretory opening on the side of the next segment of the body. The nervous system is represented by the peripharyngeal ring and the ventral nerve cord extending from it. On it, in each segment of the body, paired ganglia are located. Annelids are generally dioecious, but some species exhibit hermaphroditism. Development with metamorphosis, or direct.

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

Annelids are the most highly organized type of worms. Includes from 12 thousand (according to old sources) to 18 thousand (according to new) species. According to the traditional classification, annelids include three classes: polychaetes, oligochaetes, and leeches. However, according to another classification, polychaetes are considered in the rank of class, and oligochaetes and leeches are included in the rank of subclasses in the class Zyaskovye; 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.

During embryonic development, ectoderm, mesoderm and endoderm are formed. Therefore, they are classified as three-layered animals.

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

The coelom develops from the mesoderm. Unlike the primary cavity, the secondary cavity is lined with its own epithelium. In annelids, the whole is filled with fluid, which, among other things, performs the function of a hydroskeleton (supporting shape and support during movement). Coelomic fluid also transports nutrients, and 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 can 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 intestines. Between the walls there are also blood vessels and a nerve cord. Each segment has its own nodes of the nervous system (on the paired abdominal nerve trunk), excretory organs, gonads, and external outgrowths.

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

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

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

The digestive system consists of the foregut, midgut and hindgut. The walls of the intestine are formed by several layers of cells, they contain muscle cells, thanks to which food moves. The foregut is usually divided into the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The anus is located on the caudal blade. The process of absorption of nutrients into the blood occurs in the midgut, 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 blood 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 blade to the head), an abdominal vessel (blood moves from the head blade 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 (on the left and on the right). The 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 one, some annular vessels contract.

Blood carries nutrients from their intestines and oxygen supplied 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 in vertebrates, for example, the hemoglobin pigment is found in red blood cells. The pigments of annelids can be different (hemoglobin, chlorocruarine, etc.), so the color of blood is not always red.

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

Although annelids do not have a respiratory system and usually breathe over the entire surface of the body, gases are transported through the circulatory system rather than by diffusion through tissue 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 (coelom). On the other side, the tubes open outward through the surface of the body. Each annelid segment contains two metanephridia (right and left).

The nervous system is more developed compared to roundworms. In the head lobe, a pair of fused nodes (ganglia) form something like a brain. The ganglia are located on the peripharyngeal ring, from which the paired abdominal chain extends. It contains paired nerve ganglia in each body segment.

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

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

Annelids are thought to have evolved from worms with undifferentiated bodies, similar to ciliated worms (a type of flatworm). That is, in the process of evolution, two other groups of worms evolved from flatworms - round and annelid.