Type annelids sense organs. Organ systems of annelids

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.

Table of biological taxonomy of annelids

Class*	Subclass	Infraclass	Squad
Polychaete worms, or polychaetes (lat. Polychaeta)
			Amphinomida Eunicida Phyllodocida
	Polychaeta incertae sedis (disputed species)
	Sedentaria	Canalipalpata	Sabellida Spionida Terebellida
	Sedentaria	Scolecida	Capitellida Cossurida Opheliida Orbiniida Questida Scolecidaformia
	Palpata		Polygordiida Protodrilida
	Errantia (sometimes called Aciculata)		Amphinomida Eunicida Phyllodocida
Belt class (Clitellata)	Leeches (Hirudinea)	Acanthobdellidea
	Leeches (Hirudinea)		Jawed or proboscis leeches (Arhynchobdellida) Proboscis leeches (Rhynchobdellida)
	Oligochaete worms		Capilloventrida Crassiclitellata Enchytraeida Haplotaxida (this includes the order Earthworms) Lumbriculida Oligochaeta incertae SEDIS (species uncertain)
Echiuridae			Echiura incertae sedis (disputed species) Unreviewed

There is also a superclass Annelida incertae sedis, which includes controversial species. There, according to the World Register marine species, such a controversial group as Myzostomida was included as an order, which other classifications classify as polychaete worms or even separate them into a separate class.

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 large number bristles for each 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(Oligochaetes). 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. Fixed number of body segments 33. Body cavity filled 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 numbering only about 170 known species, all of which are exclusively sea creatures. Echiurids were recently classified as annelids after DNA examinations, but previously it was 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 the division of the body into a number of cylindrical segments, or metameres, total quantity 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 movements of parapodia.

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

Basic general anatomical features of annelids in cross section

Digestive system annelids consists of an unsegmented gut that passes 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 and excretory and reproductive functions are sometimes served by one type of canals (and 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 corresponding color.

The circulatory system is usually closed, i.e. enclosed in well-developed blood vessels; in some species of polychaetes and leeches, an open-type circulatory system appears (blood and abdominal fluid mix directly in the sinuses of the body cavity). The main vessels - 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.

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

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

Reproduction Annelids occur 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. Eggs earthly 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 health

Charles Darwin book 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 They 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 in lately, from Asia. Northern deciduous forests have been particularly negatively impacted by invasive worms through loss of leaf litter, decreased soil fertility, changes in chemical composition soils 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 are studying aquatic annelids to monitor oxygen levels, salinity and pollution environment in fresh and sea water.

The jaws of polychaetes are very strong. These advantages 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

Cases of using leeches for medical purposes were known in China around 30 AD, India around 200 AD, ancient Rome around 50 AD and then throughout Europe. In 19th-century medical practice, the use of leeches was so widespread that supplies in some areas of the world were depleted, and some regions imposed restrictions or bans on their export (with medicinal leeches themselves considered an endangered species). More recently, leeches have been used in microsurgery for transplantation of organs and their parts, and skin areas. In addition, scientists claim that the saliva of medicinal leeches has an anti-inflammatory effect, and some anticoagulants contained in it prevent the growth of malignant tumors.

About 17 species of leeches are dangerous to humans.

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

Leeches can attach to human skin from the outside, or penetrate into internal organs(for example, respiratory 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

Key questions

Why are annelids considered “higher” animals?

What characteristics of annelids most contribute to their existence?

What benefits do earthworms bring to humans? What adaptive mechanisms have they developed against birds of prey?

In what specific way do some marine annelids reproduce?

Representatives of the Annelida phylum (annelida) have a soft, segmented body. There are three classes of worms of this type (Figure 13-1): polychaete worms(Polychaeta) - sea worms, oligochaete worms(Oligochaeta) - earthworms and similar worms - leeches(Hirudinea). In total, about 9,000 species of annelids are known.

Annelids are typical secondary cavity organisms in which the mesoderm lines the body cavity (Fig. 13-1). This layer, called peritoneum, fixes the internal organs in the fluid-filled coelom (body cavity), protects them and creates space for the development of organ systems. Annelids, like other secondary cavities, belong to higher animals.

The most characteristic distinctive feature This type of worm is characterized by the segmental structure of their body. This segmental structure has both internal and external manifestations. Most of the annelids' organs - muscles, nerve ganglia, excretory organs, blood vessels and, in some cases, gonads - are present in each segment. The exception is digestive system, which is not segmented and is the same for the entire organism. It is not entirely clear what advantages the segmented structure provided to the earliest forms of annelids, but it probably facilitated their movement.

Annelids move much faster than flatworms or nematodes.

13.1. Earthworms (Lumbricus terrestris) are typical representatives of annelids - oligochaetes

Earthworms have a round and relatively smooth body. With closer observation, the boundaries of each segment become noticeable. A slight magnification allows you to notice that on each segment there are four pairs of hard bristles. The body of the worm has a thick short belt - clitellum(Figure 13-2).

The internal structure of annelids is more complex. They can contain about 150 segments, which are separated from each other by connective tissue partitions. Within each segment there is a pair of excretory tubes called nephridia, nerve ganglia, muscles that fix the setae, and branches of the annular blood vessels. In addition, longitudinal and circular muscles are located in the walls of each segment.

The main blood vessels and intestines run along the entire body of the worm and look like long tubes. At the anterior end of the body, the intestine forms a number of special sections: the pharynx, crop and gizzard (Fig. 1.3-2). Food is stored in the crop for some time, and then ground and crushed in the gizzard. Worms ingest soil as it moves through their digestive tract and absorb almost everything it contains. organic compounds. In the front of the body, the dorsal and abdominal blood vessels connect and form five pairs of “hearts”.

It is difficult to determine the total number of earthworms living in the soil, but it can be assumed that when optimal conditions in 1 m 3 of land there can be over four hundred of them. In summer they usually live 50 cm from the surface of the earth, in winter they move to a depth of up to 1.5 m. The earthworm Lumbricus terrestris is distributed only in certain zones of the Earth, although many species of earthworms and freshwater oligochaete worms live everywhere.

When irritated, the earthworm secretes a certain amount of coelomic fluid through pores located on the dorsal surface of the body. Cells containing fat have bad smell, which makes the worms inedible to predators.

Like many previously described organisms, the exchange of gases during respiration of earthworms occurs through the walls of the body. Then, as in all organisms, these gases, having passed the outer membranes, dissolve in body fluids. If there is a lack of water in the soil, the worms begin to lose water, and if drought increases, they may die. To protect the epidermis from drying out, earthworms secrete mucus.

13.2. Precise coordination between the circular and longitudinal muscles ensures the movement of annelids on land and in water

All representatives of annelids have the ability to move. Under the thin cuticle and epidermal layer they have a rather thick layer of circular muscles, behind which there is an even thicker layer of longitudinal muscles (Fig. 13-2). The innervation of these muscles is carried out in such a way that only one layer is stimulated at a given time.

Coelomic fluid creates a hydrostatic or fluid skeleton against which muscle tension occurs. When the outer layer of circular muscles contracts, the diameter of the worm's body decreases, which entails lengthening of the segment. The increase in coelomic fluid pressure during contraction of the circular muscles causes relaxation of the layers of longitudinal muscles. Conversely, when an impulse causes contraction of the longitudinal muscle, the segment shortens and thickens and the circular muscles relax.

This system of opposing muscles is called antagonistic.

In annelids, there is no simultaneous contraction of all circular and longitudinal muscles. Muscle contractions pass in waves from the anterior to the posterior end of the worm's body. Therefore, in a photograph of a moving annelid, one can notice a number of shortened and thickened segments, alternating with more elongated and thinner ones. During the next cycle of movement, these areas will move to the back of the worm's body. To move forward, the worm also uses bristles, which cling to the ground.

13.3. In earthworms and other oligochaetes, internal fertilization occurs, during which not large number well protected eggs

A small number of deferrals Lumbricus eggs is compensated by the presence of complex reproductive structures and reliable protection of developing offspring. Adult organisms are hermaphrodites. Individuals are cross-fertilized by copulation. After the exchange of sperm, the worms separate, and later the clitellum of each animal forms a cocoon, which contains the laid eggs and the sperm of the other egg. The cocoon hardens in air and takes on a spindle-shaped shape. After the worm crawls out of it, the cocoon closes at the ends and takes the form of an oval capsule the size of a grain of rice. Each worm forms cocoons several weeks after mating. An individual cocoon contains from one to ten embryos. From the eggs developing inside the cocoon, young worms hatch, in which all the segments characteristic of adult individuals are formed.

Almost all polychaetes are inhabitants of the sea and at first glance seem to bear little resemblance to earthworms. True, their body is the same round, elongated and segmented, however, unlike earthworms, they have eyes, jaws, tentacles, gills, fins and others characteristic features, allowing them to exist in water and move actively.

The Nereid sea worm (Nereis) is a typical polychaete (Fig. 13-3). It reaches a length of 30 cm and leads a predatory lifestyle.

Using its powerful jaws, the nereid captures and crushes small marine organisms, living or dead. The jaws of these worms are connected to throat, which can turn out through the mouth, envelop food and transport it to the intestines.

Otherwise, the digestive system of Nereis is the same as that of earthworms.

Animals that actively move in their habitat need to be able to find food and escape from predators. For orientation in the surrounding space, polychaetes have four visual receptors (eyes), two palps and four pairs of touch receptors. In addition, the small indentations on the head apparently serve the function chemoreceptors.

Signals from these sensory organs go to two ganglia (groups nerve cells), located in the head of the body. The latter connect with two large ganglia at the beginning of the esophagus. Because these structures, located in the head of the worm, receive most of the nerves from the sensory organs, they are sometimes called the brain, although many biologists prefer to call them cerebral ganglion. The latter is connected to the ventral nerve trunk located along the lower wall of the coelomic cavity.

Each segment of the worm has parapodia in the form of fins, necessary for swimming and crawling. These paired, laterally located structures are extensions of the body walls that contain muscle tissue.

The parapodia fins allow the worm to move back and forth along the bottom and in the water, and the bristles located at the ends of the parapodia allow it to burrow into the mud. In many species, gills are associated with parapodia, although in all polychaetes respiration also occurs through membranes covering the walls of the body and parapodia.

The Nereid worm is not constantly on the move. It often lies in a U-shaped channel, which digs into the mud with its proboscis and lines it with secreted mucus. In this case, the nereid obtains oxygen by pumping water along the mucous canal with its parapodia.

13.5. Polychaetes often use two different ways reproduction

Most Nereis worms reproduce in the spring and summer. In them, like in most other polychaetes, eggs and sperm develop in the gonads of dioecious (male and female) organisms. Then the germ cells exit into the coelomic cavity, where they mature. Later, through a gap in the walls of the body, eggs and sperm enter the water. Fertilization occurs when eggs and sperm come together by chance.

Some worms, in addition to the reproduction discussed above, may have another method of releasing eggs and sperm. Worms are capable of budding independent specialized individuals that ensure reproduction. At the beginning of the spring breeding season, some worms begin to form new segments, from which long multi-segmented individuals with a head and enlarged gonads are formed at the posterior end of the body. The parapodia in this area of the body enlarge and become very effective swimming organs. At a certain time, at night, all these halves of worms specialized for reproduction break away from the parent organism, float to the surface and burst. The release of eggs or sperm occurs. The half of the worm left at the bottom of the sea begins to form a new hindquarters for the next season. This method of reproduction preserves the parent individual and allows the production of a large number of eggs and sperm simultaneously in a certain place.

Fertilized eggs of marine polychaetes develop into a swimming larva - trochophore, similar to a rotating top (Fig. 13-4). To move in water, each larva has a belt of flagella and two bundles of flagella at the upper and lower ends. The trochophore also has a vestigial mouth, gut and anus. After a short period of swimming in water, the lower end of the larva begins to elongate and segments. The palps appear, and after a series of complex transformations are completed, the adult worm is formed. Some mollusks have the same stages of trochophore development and the same complex metamorphosis.

13.6. The third group of annelids includes the class Hirudinae (leeches)

The leech can be recognized by the presence of two suckers located at both ends of the smooth body, which almost always consists of 32-34 segments. Alternately attaching themselves to the surface of objects with suction cups, leeches can move in a loop in a certain direction. Blood-feeding leeches have hard teeth inside the front sucker, which they use to gnaw through the host's skin and then suck out some blood. The anticoagulant effect of gerudin, which is chemical compound secreted by the thymus glands of leeches. This substance prevents blood clotting in the host's wound or in the leech's intestines.

The exciting stories associated with blood-sucking leeches are, of course, far-fetched. Currently, in a number of countries, people use leeches to remove blood from superficial bruises accompanied by minor hemorrhage. Previously, bloodletting with leeches was widespread and was used for almost any disorder from childhood diseases to cancer.

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 that great value, which has the type in question 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 opening 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.

Table 1. Features various types worms
Type	Skin-muscle bag	Digestive system	Circulatory system	Reproductive system	Nervous system	Body cavity
Flatworms	Includes layers of longitudinal and circular muscles, as well as bundles of dorso-abdominal and diagonal muscles	From the ectodermal foregut and endodermal midgut	Not developed	Hermaphrodite	Paired brain ganglion and several pairs of nerve trunks	Absent, filled with parenchyma
Roundworms	Only longitudinal muscles	From the ectodermal anterior and posterior gut and the endodermal midgut	Same	Dioecious	Peripharyngeal nerve ring and 6 longitudinal trunks	Primary
	From the external circular and internal longitudinal muscles	From the ectodermal foregut and hindgut and the endodermal midgut	Well developed, closed	Dioecious or hermaphrodite	Paired medullary ganglion, peripharyngeal nerve ring, ventral nerve cord	Secondary

Animals belonging to the type of annelids, or ringworms, are characterized by:

three-layered, i.e. the development of ecto-, ento- and mesoderm in embryos;
secondary (coelomic) body cavity;
skin-muscle bag;
bilateral symmetry;
external and internal homonomous (equivalent) metamerism or segmentation of the body;
the presence of major organ systems: digestive, respiratory, excretory, circulatory, nervous, reproductive;
closed circulatory system;
excretory system in the form of metanephridia;
nervous system, consisting of the suprapharyngeal ganglion, peripharyngeal commissures and paired or unpaired ventral nerve cord;
presence of primitive locomotion organs (parapodia)

Annelids live in fresh and sea waters, as well as in the soil. Several species live in the air. The main classes of the annelid phylum are:

polychaetes (Polychaeta)
oligochaetes (Oligochaeta)
leeches (Hirudinea)

Class polychaete ringlets

From the point of view of phylogeny of the animal world, polychaetes are the most important group of annelids, since their progressive development is associated with the emergence of higher groups of invertebrates. The body of polychaetes is segmented. There are parapodia consisting of dorsal and ventral branches, each of which carries an antennae. The muscular wall of the parapodia contains thick supporting setae, and tufts of thin setae protrude from the apex of both branches. The function of parapodia is different. Typically these are locomotor organs involved in the movement of the worm. Sometimes the dorsal barbel grows and turns into a gill. The circulatory system of polychaetes is well developed and always closed. There are species with cutaneous and gill respiration. Polychaetes are dioecious worms. They live in the seas, mainly in the coastal zone.

A typical representative of the class is the Nereid (Nereis pelagica). It is found in abundance in the seas of our country; leads a bottom lifestyle, being a predator, it captures prey with its jaws. Another representative, the sandbill (Arenicola marina), lives in the seas and digs holes. It feeds by passing sea mud through its digestive tract. Breathes through gills.

Class oligochaete ringlets

Oligochaetes originate from polychaetes. The external appendages of the body are setae, which sit directly in the body wall; no parapodia. The circulatory system is closed; skin breathing. Oligochaete ringlets are hermaphrodites. The vast majority of species are inhabitants of fresh water and soil.

A typical representative of the class is the earthworm (Lumbricus terrestris). Earthworms live in soil; During the day they sit in holes, and in the evening they often crawl out. Rummaging in the soil, they pass it through their intestines and feed on the plant debris contained in it. Earthworms play a large role in soil-forming processes; they loosen the soil and promote its aeration; they drag leaves into holes, enriching the soil organic substances; deep layers of soil are removed to the surface, and surface layers are carried deeper.

Structure and reproduction earthworm

The earthworm has an almost round body in cross-section up to 30 cm long; have 100-180 segments or segments. In the anterior third of the earthworm's body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side.

A characteristic feature of the internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells. Under the skin there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. When the circular muscles contract, the animal’s body elongates and becomes thinner; when the longitudinal muscles contract, it thickens and pushes the soil particles apart.

The digestive system begins at the front end of the body with the mouth opening, from which food enters sequentially into the pharynx and esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop, and a small muscular stomach (the muscles in its walls help grind the food). The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotten remains of plants, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil.

The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it - the abdominal vessel. In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one. The red color of blood is due to the presence of hemoglobin in the plasma. Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore earthworms are very sensitive to soil moisture and are not found in dry areas. sandy soils, where their skin soon dries out, and after rains, when there is a lot of water in the soil, they crawl to the surface.

The excretory system is represented by metanephridia. Metanephridia begins in the body cavity with a funnel (nephrostom) from which a duct emerges - a thin loop-shaped curved tube that opens outward with an excretory pore in the side wall of the body. In each segment of the worm there is a pair of metanephridia - right and left. The funnel and duct are equipped with cilia, causing the movement of excretory fluid.

The nervous system has a structure typical of annelids (see Table 1), two abdominal nerve trunks, their nodes are interconnected and form the abdominal nerve chain. The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in skin. The receptors for touch, taste, and smell are also located there. Like hydra, earthworms are capable of regeneration.

Reproduction occurs only sexually. Earthworms are hermaphrodites. At the front of their body are the testes and ovaries. Earthworms undergo cross fertilization. During copulation and oviposition, girdle cells on the 32-37th segment secrete mucus, which serves to form an egg cocoon, and protein fluid to nourish the developing embryo. The secretions of the girdle form a kind of mucous muff. The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the muff stick together and a cocoon is formed, which remains in the earthen burrow. Embryonic development of eggs occurs in a cocoon, and young worms emerge from it.

Earthworm tunnels are located mainly in the surface layer of soil to a depth of 1 m; in the winter they descend to a depth of 2 m. Earthworms penetrate into the soil through burrows and tunnels atmospheric air and water necessary for plant roots and the life of soil microorganisms. During the day, the worm passes through its intestines as much soil as its body weighs (on average 4-5 g). On each hectare of land, earthworms process an average of 0.25 tons of soil every day, and over the course of a year they throw out 10 to 30 tons of soil they processed to the surface in the form of excrement. In Japan, specially bred breeds of fast-reproducing earthworms are bred and their excrement is used for biological soil cultivation. The sugar content of vegetables and fruits grown in such soil increases. Charles Darwin was the first to point out the important role of earthworms in soil formation processes.

Annelids play a significant role in nutrition bottom fish, since in some places worms make up up to 50-60% of the biomass of the bottom layers of reservoirs. In 1939-1940 The Nereis worm was moved from the Azov Sea to the Caspian Sea, which now forms the basis of the diet sturgeon fish Caspian Sea.

Leech class

The body is segmented. In addition to true metamerism, there is false ringing - several rings in one segment. There are no parapodia or setae. The secondary body cavity was reduced; instead there are sinuses and gaps between organs. The circulatory system is not closed; the blood passes only part of its way through the vessels and pours out of them into the sinuses and lacunae. There are no respiratory organs. The reproductive system is hermaphroditic.

Medical leeches are specially bred and then sent to hospitals. They are used, for example, in the treatment of eye diseases associated with increased intraocular pressure (glaucoma), cerebral hemorrhage and hypertension. For thrombosis and thrombophlebitis, hirudin reduces blood clotting and promotes the dissolution of blood clots.

Annelids have the highest organization compared to other types of worms; For the first time, they have a secondary body cavity, a circulatory system, and a more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.

They live in soil, fresh and sea water.

External structure

The earthworm has an almost round body in cross-section up to 30 cm long; have 100-180 segments, or segments. In the anterior third of the body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side.

Internal structure

Muscles

Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.

The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.

Digestive system

The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotted plant remains, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil.

Circulatory system

The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is the abdominal vessel.

In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one.

The red color of blood is due to the presence of hemoglobin in the plasma. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.

Breath

Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, so the worms are very sensitive to wet soil and are not found in dry sandy soils, where their skin quickly dries out, but after rains, when there is a lot of water in the soil, they crawl to the surface.

Nervous system

In the anterior segment of the worm there is a peripharyngeal ring - the largest accumulation of nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.

This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.

Excretory organs

The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia - remove harmful substances into external environment as they accumulate.

Reproduction and development

Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. When mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.

During copulation (mating) and egg laying, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and a protein liquid for nutrition developing embryo. The secretions of the girdle form a kind of mucous coupling (1).

The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).

Sense organs

The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Earthworms are capable of regeneration (easily restore the back part).

Germ layers

The germ layers are the basis of all organs. Annelids have ectoderm (outer layer of cells), endoderm (inner layer of cells) and mesoderm ( intermediate layer cells) appear at the beginning of development as three germ layers. They give rise to all major organ systems, including the secondary cavity and the circulatory system.

These same organ systems are subsequently preserved in all higher animals, and they are formed from the same three germ layers. This is how higher animals repeat in their development evolutionary development ancestors

The phylum annelids, or ringworms, covers about 9,000 species of higher worms. This group of animals is of great importance for understanding the phylogeny of higher invertebrates. Annelids have a higher organization compared to flatworms and roundworms. They live in sea and fresh waters, as well as in the soil. The type is divided into several classes. Let's get acquainted with a representative of the class of oligochaetes (earthworm).

General characteristics

The body of the ringlets consists of segments. The body segments are identical in appearance. Each segment, except the anterior one, which bears the oral opening, is equipped with small bristles. These are the last remnants of the disappeared pair of podia.

Annelids have a well-developed skin-muscle sac, consisting of one layer of epithelium and two layers of muscles: an outer layer of circular muscles and an inner layer formed by longitudinal muscle fibers.

Between the skin-muscle sac and the intestines there is a secondary body cavity, or coelom, which is formed during embryogenesis inside the growing mesodermal sacs.

Morphologically, the secondary cavity differs from the primary cavity in the presence of an epithelial lining adjacent to the body wall on one side and to the walls of the digestive tube on the other. The lining leaves grow together above and below the intestines, and the mesentery formed from them divides the whole into right and left sides. Transverse partitions divide the body cavities into chambers corresponding to the boundaries of the outer rings. Entirely filled with liquid.

Organ systems

The appearance of a secondary body cavity provides annelids with a higher level of vital processes than other worms. Coelomic fluid, washing the organs of the body, along with circulatory system supplies them with oxygen, and also promotes the removal of waste products and the movement of phagocytes.

excretory

Each segment of an earthworm has a paired organ excretory system, consisting of a funnel and a convoluted tubule. Waste products from the body cavity enter the funnel. A canaliculus extends from the funnel, which enters the adjacent segment, forms several loops and opens outward with an excretory pore in the lateral wall of the body. Both the funnel and the tubule are equipped with cilia, causing the movement of the secreted fluid. Such excretory organs are called metanephridia.

Circulatory and respiratory systems

In most annelids it is closed, consisting of abdominal and dorsal vessels, which pass into each other at the anterior and posterior ends of the body. In each segment, an annular vessel connects the dorsal and ventral vessels. Blood moves through the vessels due to rhythmic contractions of the dorsal and anterior annular vessels.

In earthworms, gas exchange occurs through the skin, rich in blood vessels, and some ringworms have gills.

Digestive

It begins with the oral opening at the anterior end of the body and ends with the anal opening at the rear. The intestine consists of three sections:

Anterior (ectodermal);
average ( endodermal, unlike other departments);
posterior (ectodermal).

The foregut is often represented by several sections; oral cavity and muscular pharynx. The so-called salivary glands are located in the wall of the pharynx.

Some predatory annelids have cuticular “teeth” that are used to grasp prey. A layer of muscle appears in the intestinal wall, which ensures its independent peristalsis. The midgut passes into a short posterior intestine, ending in the anus.

Nervous system

Significantly more complicated compared to flat and roundworms. Around the pharynx there is a peripharyngeal nerve ring, consisting of the suprapharyngeal and subpharyngeal nodes, connected by jumpers.

On the ventral side there are two nerve trunks, which have thickenings in each segment - ganglia, which are connected to each other by jumpers. In many types of ringlets, the right and left nerve trunks come together, resulting in the formation of the ventral nerve cord.

Among the sense organs, annelids have antennae, eyes, and balance organs, which are often located on the head lobe.

Regeneration

An earthworm, like hydra and ciliated worms, is capable of regeneration, that is, restoration of lost body parts. If an earthworm is cut into two parts, then the missing organs will be restored in each of them.

The reproductive system consists of female gonads (ovaries), which are a complex of germ cells surrounded by epithelium, and male gonads (testes), lying inside voluminous seminal sacs.

Reproduction of annelids: 1 - copulation, 2 - egg laying, 3 - egg fertilization, 4 - cocoon laying

Earthworms are hermaphrodites, but among the ringworms there are also dioecious forms. The earthworm has a girdle on its body that produces mucus, from which the cocoon is formed. Eggs are laid in it and their development occurs there.

Development

In earthworms, development is direct, but in some ringworms a larva develops from a fertilized egg, i.e. development occurs with metamorphosis.

Thus, annelids have a number of progressive characters, which include the appearance of segmentation, coelom, circulatory and respiratory systems, as well as increasing the organization of the excretory and nervous systems.

The importance of annelids in nature

Many of polychaete worms They serve as the main food for fish, and therefore are of great importance in the cycle of substances in nature.

For example, one of the species of annelids, Nereis, living in the Sea of Azov, serves as food for commercial fish. It was acclimatized by Soviet zoologists in the Caspian Sea, where it multiplied intensively and is now an important integral part in the nutrition of sturgeon fish. Polychaete worm, called "palolo" by the natives of Polynesia, is used by them as food.

Earthworms feed on plant debris found in the soil, which is passed through their guts, leaving piles of excrement consisting of soil on the surface. By doing this, they contribute to mixing and, consequently, loosening the soil, as well as enriching it with organic substances, improving the water and gas balance of the soil. Even Charles Darwin noted beneficial influence annelids on soil fertility.