Type annelids. General characteristics, structure, reproduction, diversity and significance of annelids

Type annelids. Structure earthworm: external structure (view from the ventral side), cross section, circulatory and digestive systems (anterior end of the worm)

Representatives of the type Annelids (Annelida) are the most highly organized worms. Mostly live freely in the seas, fresh water, soil. About 9 thousand species are known.

The body is elongated, divided into relatively uniform (homonomic) segments (metameres) , separated by internal partitions, has a head, sometimes back, blades. Almost every segment has paired organs (nerve ganglia, excretory organs, etc.). They are round in cross section. The segments have bristles.

The integument is represented by a skin-muscular sac. Covered with a cuticle that is secreted by the epidermis. There are many mucous glands in the skin. Two layers of muscles: the outer layer is circular and the inner one is longitudinal. Most polychaete worms have organs of movement - parapodia . These are mobile muscular outgrowths of the body (primitive limbs), which consist of the dorsal and abdominal branches. They can cling to nearby objects.

They are filled with liquid secondary abdominal cavity (in general) , while in each segment there are a pair of coelomic sacs (except for the head and posterior lobe), which are located between the intestine and the body wall. In general, it differs from the primary cavity in that it is lined with a special epithelium, which, on the one hand, is adjacent to the wall of the body, and on the other, to the walls of the digestive tube and separates the cavity fluid from tissues and organs. The lining sheets grow together above and below the intestine and form the mesentery, which divides the whole into the right and left parts. The fluid is in constant motion, due to which it carries nutrients, oxygen, carbon dioxide, and gland secretions. Forms a hydrostatic skeleton. The secondary cavity develops in the middle of the mesoderm and is lined with epithelium of mesodermal origin.

Appearance (a) and structural diagram (b) of an earthworm

Digestive system of annelids

Digestive system annelids

The digestive system consists of three sections: anterior (ectodermal), middle (endodermal) and posterior (ectodermal). It is represented by the mouth, pharynx, esophagus (maybe goiter), muscular stomach, intestines, which ends with the anus. As part of digestive tract muscle cells appear. Predators have grasping appendages, sharp spikes or jaws in the throat.

excretory organs of annelids

excretory system of annelids

Excretory organs are represented by excretory tubes (metanephridia) , which begin with a funnel with cilia in the body cavity, continue with ducts and open outwards with an opening in the next segment. Each segment contains a pair of metanephridia.

The circulatory system of annelids

The circulatory system of annelids

The circulatory system is closed, consists of interconnected abdominal and spinal vessels, which in each segment are connected by annular ones. The blood flow is provided by contractions of the dorsal and several anterior annular vessels. Through the abdominal vessel, blood moves to back body, along the dorsal - to the front. The blood of most annelids is red, sometimes green, depending on the type of respiratory pigments. May be colorless, transparent or blue. In leeches, the circulatory system is reduced. The function of the blood is performed by the coelomic fluid.

Respiration of annelids

Breathing is through the surface of the body. Marine animals have gills (outgrowths of the body wall with blood vessels on the dorsal branch of the parapodia).

Nervous system of annelids

Nervous system of annelids

The nervous system (of the ladder-nodular type) consists of the supraglottic and subpharyngeal nerve nodes, which are connected by the peripharyngeal nerve ring, and the abdominal nerve chain.

Sense organs of annelids

The sense organs are poorly developed. Sensory cells and nerve endings are located in the epidermis. Individual active species have developed eyes, organs of chemical sense (olfactory pits), organs of touch (antennae, bristles, etc.), organs of balance.

Reproductive system of annelids

Sexual, sometimes asexual reproduction (budding, fragmentation). Mostly the genitals are only in certain segments of the body, sometimes in all. The oligochaete worms are hermaphrodites, the polychaetes are predominantly dioecious. Sexual dimorphism is not expressed. Fertilization is both internal and external. In marine forms, indirect metamorphosis is developed (with a larva, incomplete metamorphosis), in freshwater and terrestrial forms, direct metamorphosis.

Some annelids are capable of regeneration.

Variety of annelids

Type annelids: class Polychaetes or Polychaetes, class Oligochetes or Oligochaetes and class Leeches

Class Polychaete worms or Polychaetes (Polychaeta)

Class Polychaete worms or Polychaeta (Polychaeta): Amphitrite, Sea mouse, Green Nereis, Peskozhil and Serpula

Some species have respiratory organs - gills (body outgrowths on parapodia). Others breathe through the entire surface of the body.

The circulatory system is closed. They have organs of vision, a pair of tentacles (organs of touch). Expressed regeneration.

Separate animals. Sexual dimorphism is not expressed. Development in polychaetes is indirect. A larva emerges from an egg (trochophore) covered with cilia. Some species can reproduce asexually.

Representatives: peskozhil, nereis, pacific palolo and etc.

Class Small-bristle worms or Oligochetes (Oligocheta)

Class Small-bristle worms or Oligochetes (Oligocheta): Ripistes, Stilaria, Elosoma and Hetogaster

About 5 thousand species are known. Distributed mainly in fresh water and soil. Sizes vary greatly. Parapodia are absent. The head lobe is weakly expressed. Hermaphrodites. Fertilization is internal. The development is direct. The eggs are laid in a cocoon. Asexual reproduction occurs by fragmentation. Representatives: pipe maker, earthworm and etc.

Earthworm (Lumbricus terrestris)

Earthworm: appearance, internal structure, cross section, scheme of the movement of the worm in the soil, reproduction, mating of two individuals, exchange of reproductive products. Earthworms in summer and worms wintering in deep burrows

Dimensions reach 15 - 30 cm. Lives in the soil. Forms the soil, increases its fertility. It feeds on the remains of plants. The body is elongated, consists of segments, the number of which is 140 - 180. Near the 31st segment there is a special formation - a girdle. This is an organ that secretes a substance from which a cocoon is built, two individuals stick together during mating. Each segment has four pairs of setae (remnants of parapodia). The skin is covered with mucus secreted by the skin glands. The integuments are represented by a musculoskeletal sac, which consists of four layers: the mucous membrane of the cuticular lining, epithelial cells, the muscle layer, and a thin film. Musculature is represented by two layers of muscle fibers: annular septa. The body cavity is lined with epithelium, divided by partitions. It is filled with a liquid that performs the function of a hydrostatic skeleton.

Digestive system of an earthworm

The mouth opening at the anterior end of the body (cephalic lobe) leads to a thick-walled pharynx, which is located in the first 6 segments. The pharynx passes into a narrow esophagus, into which special limestone glands open, which neutralize humic acids (from the soil). Behind the esophagus is a goiter, which passes into the muscular stomach (the ectodermal section ends). Then the food passes into the middle and hind intestines, which end in the anus - in the posterior segment.

Excretory system of the earthworm

The excretory system is represented by metanephridia. The openings of the metanephridia are on the dorsal side. They open and then close. Not visible to the naked eye.

The circulatory system of the earthworm

The circulatory system is closed, consists of dorsal, abdominal and annular vessels. Six pairs of annular vessels that surround the esophagus and connect the dorsal and abdominal vessels are called hearts. Their walls, like the walls of the spinal vessel, pulsate. The blood is red, has hemoglobin in the plasma. It flows in the dorsal vessel to the head end, in the abdominal - in the opposite direction.

Respiratory system of an earthworm

There is no respiratory system. They breathe the entire surface of the body, so a large number of capillaries branch in it.

The nervous system of the earthworm

The nervous system is represented by the peripharyngeal nerve ring and the ventral nerve cord.

Sense organs of the earthworm

Sense organs: light-sensitive, tactile, chemical sensation cells. Light-sensitive cells are located on the head lobe.

The reproductive system of the earthworm

Reproduction is only sexual. On the ventral side, segment 14 has a pair of small, rounded female genital openings. On the 15th segment - in the form of transverse slits, a pair of male genital openings. Hermaphrodites. Gonads are located at the level of 9-15 segments. Fertilization is predominantly cross. During reproduction, two individuals exchange seminal fluid, which is stored in the seed receptacles. The secretions of the girdle form a kind of mucous sleeve. The worm crawls out of the clutch with its rear end forward. The edges of the muff stick together and form a cocoon. Eggs are laid in a cocoon (the egg and sperm are released from the seed receptacle). The cocoon lies in the ground. Young individuals develop from eggs. Development is direct. Such complex reproduction is due to the habitat, a small opportunity to meet a sexual partner.

Earthworms are capable of regeneration.

Leech class (Hirudinea)

Leech class (Hirudinea): False horse leech and Medicinal leech

Well developed digestive system. In bloodsuckers, the salivary glands produce a substance that prevents blood clotting. (hirudin) . The midgut forms lateral protrusions in which blood is stored. Metanephridia are located only in a few segments.

The function of the blood is performed by the coelomic fluid. The blood is colorless or red (has hemoglobin).

Hermaphrodites. Lose the ability to regenerate. Fertilization is internal. Direct development is characteristic.

Representatives: medicinal leech, horse leech and etc.

Meaning of annelids

Annelids are an important link in the food chain. Many species make up the food base of fish (for example, Nereis). From the Sea of ​​​​Azov, Nereis was relocated to the Caspian in order to maintain the food base of valuable industrial species fish.

Earthworms are soil formers, improve soil properties, enrich it with oxygen, organic matter, make it loose, facilitate the access of moisture, mix soil layers. Through the digestive system, the earthworm passes as much soil as its body weighs (approximately 4 - 5 g). Abroad (in Japan, America, etc.), earthworm cultures are bred to obtain soil, various plants (vegetables, herbs, etc.) are grown on them. The importance of earthworms as soil formers was noted by C. Darwin.

Some species of annelids are eaten by humans (palolo), used as live bait for fishing (sandworms, etc.). The ringed worm is used as food for aquarium fish.

Medical leeches are used for hypertension(increased arterial pressure), to reduce blood clotting, resorption of blood clots. For this, leeches are caught or specially bred. Get the substance hirudin, which is used in medicine, the perfume industry.

>>Earthworm. Variety of annelids and their common features

§ 16. Earthworm. The variety of annelids and their common features

The circulatory system of the earthworm serves to carry oxygen and nutrients primarily to the muscles. At earthworm two main blood vessels: the dorsal, through which blood flows from back to front, and the abdominal, through which blood flows from front to back. Both vessels in each segment are connected by annular vessels. Several thick annular vessels have muscular walls, due to the contraction of which blood moves. Thinner ones depart from the main vessels, then branching into the smallest capillaries. Oxygen from the skin and nutrients from the intestines enter these capillaries, and these substances are released from other similar capillaries branching in the muscles. Thus, the blood moves all the time through the vessels and does not mix with the cavity fluid. Such a circulatory system is called a closed system. 32 .

excretory system.

Liquid waste, processed substances enter the body cavity. Each segment contains a pair of tubules. Each tube has a funnel at the inner end, processed unnecessary substances enter it and are removed through the tube through the opposite end to the outside.

Nervous system.

A pair of nerve trunks runs along the entire body of the worm along the ventral side. In each segment, nerve knots are developed on them - a nerve chain is obtained. In the anterior part, two large knots are connected to each other by annular bridges - a peripharyngeal nerve ring is formed. From all nodes nerves depart to various organs 33 .

There are no special sense organs, but sensitive cells in the skin allow the earthworm to feel touch on its skin and distinguish light from darkness.

Reproductive system and reproduction.

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Significance in medicine of annelids

TO annelids include the well-known leeches. They are of great importance in medicine. They are used in the treatment of hypertension, reducing blood clotting, with blood resorption. For this, leeches are specially grown and their enzyme hydrurin is extracted, which has a medicinal property.

The biological significance of annelids in the ecosystem

polychaete worms in the ocean are known for their biological significance. Worms are an important link in the trophic chains of an ecosystem. Also, representatives of polychaete worms take part in cleaning sea ​​water, by processing organic substances in it.

Polychaetes used as food for fish.

• Freshwater representatives of oligochaetes are also fish food, more often used for aquarium fish. Tubifex from the class of oligochaetes act as a filter feeder for water bodies. They are also called ground beetles. Swallowing the soil, they directly digest organic matter, turning them into useful minerals that the soil needs so much.
• earthworms are also consumers in the ecosystem. Many animals feed on them: moles, shrews, frogs, many birds and some predatory beetles.

Remark 1

Some representatives of annelids are eaten by humans. For example, palolo.

The importance of worms in soil formation

As a result of digging the earth and laying passages, the worms increase the soil porosity, thus it becomes looser and increases in volume up to $30\%$. In loose soil, access to deep layers of soil, water and atmospheric air is facilitated. This is necessary for the roots of plants and the activity of beneficial microorganisms that are in the soil. The constant mixing of the surface layers of the soil by the worms saturates it with foliage and other organic residues.

Worms change with their vital activity Chemical properties soil. So mixed soil with intestinal mucus of the worm contains a greater amount of calcium, magnesium, ammonia, nitrates and phosphoric acid. The glands of the esophagus contribute to the neutralization of harmful soil acids. The waste products of the worms dry out and break up into microporous lumps that are not washed away by water.

Thus, earthworms change and improve the composition, structure and fertility of the soil.

Remark 2

Enchytreids feed on organic residues and participate in soil formation along with earthworms.

Negative meaning of annelids

medical leech their medicinal properties can lead to complications. But not the leech itself, but the presence of Aeromonas hydrophila bacteria in its throat. These bacteria cause skeletal muscle necrosis and sepsis. And the enzyme hydrurin itself can cause too much bleeding in the patient.

Leeches in natural conditions can attack animals and humans, thereby causing great harm. After all, for a person who did not suffer from thrombosis, a leech becomes dangerous. So after a bite, the wound bleeds for a long time, and can be infected. Water leeches can penetrate the oral cavity, nose and urinary tract, where they stick to the mucous membrane.

The phylum annelids are protostomes, the most highly organized of all worms. The type includes, according to various sources, 10-18 thousand species and is divided into three classes: polychaetes (the most numerous, more than 10 thousand species), oligochaetes and leeches. These worms live in water bodies, not excluding the ocean floor and the icy waters of the Atlantic, and in the soil. The exceptions are several species of leeches, which have adapted to terrestrial living in the humid forests of the tropics. Well, the most famous and familiar low-bristle worms in our latitudes are earthworms, so named for the custom, together, in dozens, crawl out of the soil during rain to breathe in oxygen. In addition, their presence is given out by small tubercles-excavations, especially clearly visible in spring - young worms loosen the soil, simultaneously saturating it with oxygen. We will talk about earthworms in more detail, and now we will consider important aromorphoses and structural features of annelids.

Aromorphoses of annelids

1. Overall - secondary body cavity, that is, a closed, fluid-filled space that separates the intestines from the walls of the body. It is important to emphasize that, unlike the primary cavity of roundworms, the whole annelids has an epithelial membrane, a lining. In multi- and low-bristle worms, in general, it occupies a fairly large volume. In the fluid that fills it, cells are free floating, participating in excretory, gas exchange and other processes.

2. Closed circulatory system- its occurrence is directly connected with the appearance of the coelom. As the worm larva develops, the secondary cavity displaces the primary cavity, the remnants of which turn into blood vessels.

3. Metamerism- segmentation, duplication internal organs, due to which the loss of a body part for the worm is not critical. Each ringlet has its own sex glands, excretory organs, nerve nodes, etc.

4. Parapodia- outgrowths conducive to movement on the sides of the body in polychaetes.

The structure of annelids

1. Body sizes from a quarter of a millimeter to three or even more meters.

2. In cross section, the body is close to a circle or oval. There are three sections: head (head lobe), trunk and anal lobe. Ringworms grow by forming new segments in the region of the anal lobe.

2. The body is segmented by constrictions into many homogeneous rings(segments). It is important that the whole also has partitions, in accordance with the outer division. Upper layer skin-muscular sac- cuticle, next - single-layer epithelium. Muscles of two types, circular on the outside, longitudinal lie deeper.

3. On the head, in addition to the mouth opening, different species may have eyes, tactile organs (various whiskers, palps, etc.).

4. Grow from the cuticle bristles, which can be very numerous along the entire length of the body.

Organ systems

1. Digestive system not closed, it is divided into three sections, represented by the anterior, middle (here nutrients are absorbed) and the posterior intestine. Some types of worms have acquired salivary glands.

2. Ringworms are heartless creatures, in their closed circulatory system there is only different types vessels through which blood moves. Interestingly, the red color for the blood of worms is not necessary - it all depends on the pigments.

3. Breath can be carried out in two ways - either by the surface of the body (as in oligochaete worms and leeches), or through primitive gills that are located on outgrowths-parapodia (in polychaete worms).

4. Nervous system begins in the head of the worm, where two nerve ganglia, supraglottic and subpharyngeal, connected by strands, form the peripharyngeal nerve ring. From the ganglion under the pharynx, a pair of nerve trunks with ganglia comes out and stretches along the body, connected by jumpers in each segment. This is the so-called ventral nerve cord.

5. sense organs quite well developed in active worms: tactile cells, eyes (not all species have them), chemoreceptors, an organ of balance.

6. excretory system presented in all rings in pairs metanephridia: tubules located in the coelom that open outward on the surface of the body.

Origin of annelids

1. The ancestors of annelids were free-living flatworms. How can you prove it? The larvae of polychaete worms are very similar to planarians. What is it expressed in? Trochophora, a polychaete larva, has cilia, ocelli, tube-shaped metanephridia with stellate cells and a "flickering flame" formed by the beating of cilia. In addition, the nervous system of the trochophore is very similar to nervous system planaria.

2. The oligochaetes evolved from the ancient polychaetes as a result of a simplification in their structure caused by life in the thickness of the soil.

3. Leeches are descended from ancient oligochaete worms.

Type annelids, or annelids, covers about 9,000 species of higher worms. This group of animals is of great importance for understanding the phylogenesis of higher invertebrates. Annelids are more organized than 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 the representative of the class of low-bristle (earthworm).

general characteristics

The body of the rings consists of segments. The segments of the body are outwardly identical. Each segment, except for the anterior one, which bears the oral opening, is provided with small bristles. These are the last remnants of the disappeared parapodia.

In annelids, worms have a well-developed skin-muscular sac, consisting of one layer of epithelium and two layers of muscles: the outer layer of the annular muscles and the inner layer formed by longitudinal muscle fibers.

Between the skin-muscular sac and the intestines 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 on one side to the body wall, and on the other, to the walls of the digestive tube. The lining sheets grow together above and below the intestines, and the mesentery formed from them is divided as a whole into the right and left sides. Transverse partitions divide the body cavities into chambers corresponding to the boundaries of the outer rings. The whole is 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 liquid, 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 the earthworm has a paired organ of the excretory system, consisting of a funnel and a convoluted tubule. Waste products from the body cavity enter the funnel. From the funnel comes a tubule, which enters the adjacent segment, forms several loops and opens outward with an excretory pore in the side wall of the body. Both the funnel and the tubule are provided with cilia that cause the movement of the secreted fluid. These excretory organs are called metanephridia.

Circulatory and respiratory systems

In most annelids, it is closed, consists 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 abdominal vessels. Blood moves through the vessels due to the rhythmic contractions of the spinal and anterior annular vessels.

In the earthworm, gas exchange occurs through the skin rich in blood vessels, and some sea rings have gills.

digestive

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

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

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

Some predatory annelid worms have cuticular "teeth" that serve to grasp prey. A layer of muscles appears in the intestinal wall, which ensures its independent peristalsis. The middle intestine passes into a short hindgut, ending in the anus.

Nervous system

Significantly more complicated than flatworms and roundworms. Around the pharynx there is a near-pharyngeal nerve ring, consisting of supra-esophageal and sub-esophageal nodes connected by bridges.

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 rings, the right and left nerve trunks converge, resulting in the formation of the abdominal nerve chain.

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

Regeneration

An earthworm, like hydra and ciliary worms, is capable of regeneration, that is, restoring lost parts of the body. If the 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), which lie inside the voluminous seminal sacs.

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

Earthworms are hermaphrodites, but dioecious forms are also found among the rings. On the body of the earthworm there is a girdle that produces mucus, from which a cocoon is formed. Eggs are laid in it and their development takes place there.

Development

In an earthworm, development is direct, but in some rings, a larva develops from a fertilized egg, that is, development occurs with transformation.

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

The value of annelids in nature

Many of the polychaete worms serve as the main food of fish, and therefore they 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 ​​\u200b\u200bAzov, 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 diet of sturgeons. The polychaete worm, called "palolo" by the natives of Polynesia, is eaten by them.

Earthworms feed on plant debris in the soil, which is passed through the intestines, leaving heaps of excrement consisting of earth on the surface. In this way, 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. Ch. Darwin also noted useful influence annelids on soil fertility.