Zoology of invertebrates. Annelids Nervous system of polychaetes

Class Polychaeta

with all the colors of the rainbow bristles. Serpentine phyllodoces (Phyllodoce) swim and crawl quickly. Tomopteris (Tomopteris) hang in the water column on their long whiskers.

The class of polychaetes differs from other ringlets by a well-separated head section with sensory appendages and the presence of limbs - parapodia with numerous setae. Mostly dioecious. Development with metamorphosis.

General morphofunctional characteristics

External structure. The body of polychaete worms consists of a head section, a segmented body and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (peristomium), which is often complex as a result of fusion

with 2-3 body segments (Fig. 172). The mouth is located ventrally on the peristomium. Many polychaetes have eyes and sensory appendages on their heads. Thus, in a Nereid, on the prostomium of the head there are two pairs of ocelli, tentacles - tentacles and two-segmented palps, on the peristomium below there is a mouth, and on the sides there are several pairs of antennae. The trunk segments have paired lateral projections with setae - parapodia (Fig. 173). These are primitive limbs with which polychaetes swim, crawl or burrow into the ground. Each parapodia consists of a basal part and two lobes - dorsal (notopodium) and ventral (neuropodium). At the base of the parapodium there is a dorsal barbel on the dorsal side, and a ventral barbel on the ventral side. These are the sensory organs of polychaetes. Often the dorsal barbel in some species is transformed into feathery gills. Parapodia are armed with tufts of bristles consisting of an organic substance close to chitin. Among the setae there are several large setae-acicules, to which muscles are attached from the inside that move the parapodia and tuft of setae. The limbs of polychaetes make synchronous movements like oars. In some species leading a burrowing or attached lifestyle, parapodia are reduced.

Skin-muscle bag(Fig. 174). The body of polychaetes is covered with a single-layer dermal epithelium, which secretes a thin cuticle onto the surface. In some species, certain areas of the body may have ciliated epithelium (a longitudinal ventral stripe or ciliated bands around the segments). Glandular epithelial cells in sessile polychaetes can secrete a protective horny tube, often impregnated with lime.

Under the skin lies circular and longitudinal muscles. The longitudinal muscles form four longitudinal ribbons: two on the dorsal side of the body and two on the abdominal side. There may be more longitudinal strips. On the sides there are bundles of fan-shaped muscles that drive the parapodium blades. The structure of the skin-muscle sac varies greatly depending on lifestyle. The inhabitants of the ground surface have the most complex structure of the skin-muscle sac, close to that described above. This group of worms crawls along the surface of the substrate using serpentine body bending and parapodia movements. The inhabitants of calcareous or chitinous tubes have limited mobility, as they never leave their shelters. In these polychaetes, strong longitudinal muscle bands provide a sharp lightning-fast contraction of the body and retreat into the depths of the tube, which allows them to escape from attacks by predators, mainly fish. In pelagic polychaetes, the muscles are poorly developed, since they are passively transported by ocean currents.

Rice. 172. External structure of the Nereid Nereis pelagica (according to Ivanov): A - anterior end of the body B - posterior end of the body; 1 - antennae, 2 - palps 3 - peristomal antennae, 4 - eyes, 5 - prostomium, 6 - olfactory fossa, 7 - peristomium, 8 - parapodia, 9 - setae, 10 - dorsal antennae, 11 - pygidium, 12 - caudal appendages , 13 – segment

,

Rice. 173. Parapodia of Nereis pelagica (according to Ivanov): 1 - dorsal antenna, 2 - notopodium lobes, 3 - setae, 4 - neuropodium lobes, 5 - ventral antenna, 6 - neuropodium, 7 - acicula, 8 - notopodium

Rice. 174. Cross section of a polychaete worm (according to Natalie): 1 - epithelium, 2 - circular muscles, 3 - longitudinal muscles, 4 - dorsal antennae (gill), 5 - notopodium, 6 - supporting seta (acicula), 7 - neuropodium, 8 - funnel of nephridium, 9 - canal of nephridium, 10 - oblique muscle, 11 - abdominal vessel, 12 - ovary, 13 - abdominal antenna, 14 - setae, 15 - intestine, 16 - coelom, 17 - dorsal blood vessel

Secondary body cavity- in general - polychaetes have a very diverse structure. In the most primitive case separate groups mesenchymal cells cover the inside of muscle bands and the outer surface of the intestine. Some of these cells are capable of contraction, while others are capable of turning into germ cells that mature in the cavity, only conventionally called secondary. In more difficult case The coelomic epithelium can completely cover the intestines and muscles. The coelom is fully represented in the case of the development of paired metameric coelomic sacs (Fig. 175). When paired coelomic sacs close in each segment above and below the intestine, the dorsal and abdominal mesenteries, or mesenteries, are formed. Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissepiments. The wall of the coelomic sac, lining the inside muscles of the body wall, is called the parietal layer of mesoderm, and the coelomic epithelium , covering the intestine and forming the mesentery, is called the visceral layer of mesoderm. Blood vessels lie in the coelomic septa.

Rice. 175. Internal structure polychaetes: A - nervous system and nephridia, B - intestine and whole, C - intestine, nervous and circulatory systems, side view (according to Meyer); 1 - brain, 2 - peripharyngeal connective, 3 - ganglia of the abdominal nerve chain, 4 - nerves, 5 - nephridium, 6 - mouth, 7 - coelom, 8 - intestine, 9 - diosepiment, 10 - mesentery, 11 - esophagus, 12 - oral cavity, 13 - pharynx, 14 - muscles of the pharynx, 15 - muscles of the body wall, 16 - olfactory organ, 17 - eye, 18 - ovary, 19, 20 - blood vessels, 21 - network of vessels in the intestine, 22 - annular vessel , 23 - muscles of the pharynx, 24 - palps

The whole performs several functions: musculoskeletal, transport, excretory, sexual and homeostatic. Cavity fluid maintains body turgor. When the circular muscles contract, the pressure of the cavity fluid increases, which provides the elasticity of the worm's body, which is necessary when making passages in the ground. Some worms are characterized by a hydraulic method of movement, in which the cavity fluid, when muscles contract under pressure, is driven to the front end of the body, providing energetic forward movement. Overall, nutrients are transported from the intestines and dissimilation products from various organs and tissues. The organs for excreting metanephridia by funnels open as a whole and ensure the removal of metabolic products and excess water. In the whole, there are mechanisms to maintain the constancy of the biochemical composition of the fluid and water balance. In this favorable environment, gonads form on the walls of coelomic sacs, germ cells mature, and in some species even juveniles develop. Derivatives of the coelom - coelomoducts - serve to remove reproductive products from the body cavity.

Digestive system consists of three sections (Fig. 175). The entire anterior section consists of derivatives of the ectoderm. The anterior section begins with the oral opening located on the peristomium on the ventral side. The oral cavity passes into the muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like the finger of a glove. In predators, the pharynx consists of several layers of circular and longitudinal muscles, is armed with strong chitinous jaws and rows of small chitinous plates or spines that can firmly hold, wound and crush captured prey. In herbivorous and detritivorous forms, as well as in sestivorous polychaetes, the pharynx is soft, mobile, adapted for swallowing liquid food. Following the pharynx is the esophagus, into which ducts open salivary glands, also of ectodermal origin. Some species have a small stomach

The middle section of the intestine is a derivative of the endoderm and serves for final digestion and absorption of nutrients. In carnivores, the midgut is relatively shorter, sometimes equipped with paired blind side pouches, while in herbivores, the midgut is long, convoluted, and usually filled with undigested food debris.

The hind intestine is of ectodermal origin and can perform the function of regulating water balance in the body, since there water is partially absorbed back into the coelom cavity. Fecal matter forms in the hindgut. The anal opening usually opens on the dorsal side of the anal blade.

Respiratory organs. Polychaetes mainly have cutaneous respiration. But a number of species have dorsal cutaneous gills formed from parapodial antennae or head appendages. They breathe oxygen dissolved in water. Gas exchange occurs in a dense network of capillaries in the skin or gill appendages.

Circulatory system closed and consists of the dorsal and ventral trunks, connected by annular vessels, as well as peripheral vessels (Fig. 175). Blood movement is carried out as follows. Through the dorsal, largest and most pulsating vessel, blood flows to the head end of the body, and through the abdominal - in the opposite direction. Through the annular vessels in the front part of the body, blood is distilled from the dorsal vessel to the abdominal one, and in the back part of the body - vice versa. Arteries extend from the annular vessels to parapodia, gills and other organs, where a capillary network is formed, from which blood collects into venous vessels that flow into the abdominal bloodstream. In polychaetes, the blood is often red due to the presence of the respiratory pigment hemoglobin dissolved in the blood. Longitudinal vessels are suspended on the mesentery (mesentery), annular vessels pass inside the dissepiments. Some primitive polychaetes (Phyllodoce) lack a circulatory system, and hemoglobin is dissolved in nerve cells.

Excretory system polychaetes are most often represented by metanephridia. This type of nephridia appears for the first time in the phylum annelids. Each segment has a pair of metanephridia (Fig. 176). Each metanephridia consists of a funnel, lined inside with cilia and open as a whole. The movement of the cilia drives solid and liquid metabolic products into the nephridium. A canal extends from the funnel of the nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In the convoluted channels, ammonia is converted into high molecular weight compounds, and water is absorbed as a whole. U different types Polychaetes' excretory organs can be of different origins. Thus, some polychaetes have protonephridia of ectodermal origin, similar in

Rice. 176. The excretory system of polychaetes and its connection with coelomoducts (according to Briand): A - protonephridia and genital funnel (in a hypothetical ancestor), B - nephromyxium with protonephridium, C - metanephridia and genital funnel, D - nephromyxium; 1 - coelom, 2 - genital funnel (coelomoduct), 3 - protonephridia, 4 - metanephridia

structure with those of flatworms and roundworms. Most species are characterized by metanephridia of ectodermal origin. In some representatives, complex organs are formed - nephromyxia - the result of the fusion of protonephridia or metanephridia with the genital funnels - coelomoducts of mesodermal origin. Additionally, the excretory function can be performed by chloragogenic cells of the coelomic epithelium. These are peculiar storage buds in which grains of excreta are deposited: guanine, uric acid salts. Subsequently, chloragogenic cells die and are removed from the coelom through nephridia, and new ones are formed to replace them.

Nervous system . Paired suprapharyngeal ganglia form the brain, in which three sections are distinguished: proto-, meso- and deutocerebrum (Fig. 177). The brain innervates the sense organs on the head. Periopharyngeal nerve cords extend from the brain - connectives to the abdominal nerve cord, which consists of paired ganglia, repeating in segments. Each segment has one pair of ganglia. The longitudinal nerve cords connecting the paired ganglia of two adjacent segments are called connectives. The transverse cords connecting the ganglia of one segment are called commissures. When the paired ganglia merge, a nerve chain is formed (Fig. 177). In some species, the nervous system becomes more complex due to the fusion of ganglia from several segments.

Sense organs most developed in motile polychaetes. On the head they have eyes (2-4) of a non-inverted type, goblet-shaped or in the form of a complex eye bubble with a lens. Many sessile polychaetes living in tubes have numerous eyes on the feathery gills of the head. In addition, they have developed organs of smell and touch in the form of special sensory cells located on the appendages of the head and parapodia. Some species have balance organs - statocysts.

Reproductive system. Most polychaete worms are dioecious. Their gonads develop in all segments of the body or only in some of them. The gonads are of mesodermal origin and form on the wall of the coelom. The germ cells from the gonads enter the whole, where their final maturation occurs. Some polychaetes do not have reproductive ducts and the germ cells enter the water through breaks in the body wall, where fertilization occurs. In this case, the parent generation dies. A number of species have genital funnels with short channels - coelomoducts (of mesodermal origin), through which the reproductive products are excreted out into the water. In some cases, germ cells are removed from the coelom through nephromyxia, which simultaneously perform the function of the reproductive and excretory ducts (Fig. 176).

Rice. 177. Nervous system of polychaetes: 1 - nerves of the antennae, 2 - neopalps, 3 - mushroom body, 4 - eyes with a lens, 5 - nerves of the peristomal antennae, 6 - mouth, 7 - peripharyngeal ring, 8 - abdominal ganglion of the peristomium, 9- 11 - parapodia nerves, 12 - ganglia of the ventral nerve chain, 13 - nerve endings of the nuchal organs

Reproduction Polychaetes can be sexual or asexual. In some cases, alternation of these two types of reproduction (metagenesis) is observed. Asexual reproduction usually occurs by transverse division of the worm's body into parts (strobilation) or by budding (Fig. 178). This process is accompanied by the regeneration of missing body parts. Sexual reproduction often associated with the phenomenon of epitoky. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in body shape during the period of maturation of reproductive products: segments become wide, brightly colored, with swimming parapodia (Fig. 179). In worms that develop without epitocy, males and females do not change their shape and reproduce in benthic conditions. Species with epitocy may have several life cycle options. One of them is observed in Nereids, the other in Palolos. Thus, in Nereis virens, males and females become epitocous and float to the surface of the sea to reproduce, after which they die or become prey to birds and fish. From eggs fertilized in water, larvae develop, settling to the bottom, from which adults are formed. In the second case, as in the palolo worm (Eunice viridis) from Pacific Ocean, sexual reproduction is preceded by asexual reproduction, in which the anterior end of the body remains at the bottom, forming an atokny individual, and the posterior end of the body is transformed into an epitokny tail part filled with sexual products. The back parts of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization occurs. Epitocene individuals of the entire population emerge to reproduce simultaneously, as if on a signal. This is the result of the synchronous biorhythm of puberty and biochemical communication of sexually mature individuals of the population. The massive appearance of reproducing polychaetes in the surface layers of water is usually associated with the phases of the Moon. Thus, the Pacific palolo rises to the surface in October or November on the day of the new moon. The local population of the Pacific Islands knows these periods of reproduction of palolos, and fishermen en masse catch palolos stuffed with “caviar” and use them for food. At the same time, fish, seagulls, and sea ducks feast on worms.

Development. The fertilized egg undergoes uneven, spiral crushing (Fig. 180). This means that as a result of fragmentation, quartets of large and small blastomeres are formed: micromeres and macromeres. In this case, the axes of the cell cleavage spindles are arranged in a spiral. The inclination of the spindles changes to the opposite with each division. Thanks to this, the crushing figure has a strictly symmetrical shape. Egg crushing in polychaetes is determinate. Already at the stage of four blastomeres, determination is expressed. Quartets of micromeres give derivatives of ectoderm, and quartets of macromeres give derivatives

Rice. 178. Development of polychaetes (family Sylhdae) with metagenesis (according to Barnes): A - budding, B - multiple budding, C - alternation of sexual reproduction with asexual

Rice. 179. Reproduction of polychaetes: A - budding of the polychaete Autolytus (no Grasse), B, C - epitocous individuals - female and male Autolytus (according to Sveshnikov)

endoderm and mesoderm. The first mobile stage is the blastula - a single-layer larva with cilia. The blastula macromeres at the vegetative pole plunge into the embryo and the gastrula is formed. At the vegetative pole, the primary mouth of the animal is formed - the blastopore, and at the animal pole, a cluster of nerve cells and a ciliated crest - the parietal plume of cilia - is formed. Next, the larva develops - a trochophore with an equatorial ciliary belt - a troch. The trochophore has a spherical shape, a radially symmetrical nervous system, protonephridia and a primary body cavity (Fig. 180). The blastopore of the trochophore shifts from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anal opening breaks through later at the vegetative pole, and the intestines become through.

Previously, there was a point of view that in all polychaetes the mouth and anus are formed from a blastopore. But, as was shown by the research of polychaete specialist V.A. Sveshnikov, this situation represents only a special case of the development of polychaetes, and in most cases only a mouth is formed from the blastopore, and the anus forms independently at later phases of development. In the area of ​​the posterior end of the larva, in the immediate vicinity of the anus, on the right and left sides of the intestine, a pair of cells appears - teloblasts, located in the growth zone. This is the rudiment of mesoderm. The trochophore consists of three sections: the head lobe, the anal lobe and the growth zone. -In this area, the zone of future growth of the larva is formed. The structural plan of the trochophore at this stage resembles the organization of lower worms. The trochophore successively turns into a metatrochophore and a nectochaete. In the metatrochophore, larval segments are formed in the growth zone. Larval, or larval, segmentation involves only ectodermal derivatives: ciliary rings, protonephridia, rudiments of the setal sacs of future parapodia. Nektochaete is distinguished by the fact that it develops a brain and an abdominal nerve cord. The setae from the setal sacs are exposed, and the parapodial complex is formed. However, the number of segments remains the same as in the metatrochophore. Different types of polychaetes may have different numbers of them: 3, 7, 13. After a certain time pause, postlarval segments begin to form and the juvenile stage of the worm is formed. In contrast to larval segmentation, postlarval segments in juvenile forms capture derivatives of not only ectoderm, but also mesoderm. At the same time, in the growth zone, teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a metanephridia funnel is formed. The secondary body cavity gradually replaces the primary one. At the borders of contact of the coelomic sacs, dissepiments and mesenterium are formed.

Due to the remaining primary body cavity, longitudinal vessels of the circulatory system are formed in the lumen of the mesentery, and circular vessels are formed in the lumens of the septa. Due to the mesoderm, the muscles of the skin-muscular sac and intestines, the lining of the coelom, gonads and coelomoducts are formed. The nervous system, metanephridia channels, foregut and hindgut are formed from the ectoderm. The midgut develops from the endoderm. After metamorphosis is completed, an adult animal develops with a certain number of segments for each species. The body of an adult worm consists of a head lobe, or prostomium, developed from the head lobe of the trochophore, several larval segments with a primary cavity, and many postlarval segments with a coelom and an anal lobe without a coelom.

Thus, the most important features of the development of polychaetes are spiral, determinate fragmentation, teloblastic anlage of mesoderm, metamorphosis with the formation of trochophore larvae, metatrochophore, nectochaete and juvenile form. The phenomenon of the dual origin of metamerism in annelids with the formation of larval and postlarval segments was discovered by the prominent Soviet embryologist P. P. Ivanov. This discovery shed light on the origin of annelids from oligomeric ancestral forms.

Sequential phase change individual development polychaetes from oligomeric to polymeric reflect a phylogenetic pattern. Comparative morphological data indicate that the ancestors of polychaetes had a small number of segments, i.e. they were oligomeric. Among modern polychaetes, the closest to ancestral forms are some primary ringlets of the class Archiannelida, in which the number of segments usually does not exceed seven. Manifestations of primitive organizational features at the trochophore and metatrochophore stages (primary cavity, protonephridia, orthogon) indicate the relationship of coelomic animals with the group of lower worms.

The biological significance of the development of polychaete worms with metamorphosis lies in the fact that the floating larvae (trochophores, metatrochophores) ensure the dispersal of species that, as adults, lead a predominantly bottom lifestyle. Some polychaete worms show care for their offspring and their larvae are inactive and lose their distribution function. In some cases, live births are observed.

The meaning of polychaete worms. Biological and practical significance Polychaete worms are very numerous in the ocean. The biological significance of polychaetes lies in the fact that they represent an important link in trophic chains, and are also important as organisms that take part in the purification of sea water and the processing of organic matter.

substances. Polychaetes have food value. To strengthen the food supply of fish in our country, for the first time in the world, the acclimatization of nereids (Nereis diversicolor) in the Caspian Sea, which were brought from the Azov Sea, was carried out. This successful experiment was carried out under the leadership of Academician L.A. Zenkevich in 1939-1940. Some polychaetes are used as food by humans, such as the Pacific palolo worm (Eunice viridis).

Let us consider the general appearance, lifestyle, structure and organ systems of polychaete worms using the example of the sea worm - Nereis, which is a typical representative of this class.

General view. Nereis is big worm up to 10 centimeters long (Fig. 36). The body of the worm is elongated and slightly flattened; it is formed by more than 150 segments. At the head end of the body there are palps and tentacles, two pairs of eyes, antennae and an olfactory fossa. The body segments have paired lateral outgrowths and perform the function of legs. At their ends there are bristles, which cling to the surface of the bottom and allow the worm to move. At the posterior end of the body, the trunk segments merge into the anal lobe, which contains the anus.

The body of Nereis is covered with a thin cuticle. Two layers of subcutaneous muscles and skin form a musculocutaneous sac.

Lifestyle. Nereis lives in coastal zone seas at shallow depths in burrows that it digs in the sand. Feeds on algae and various small animals

Internal structure (Fig. 37). Directly behind the skin-muscle sac in the body of the worm there is a cavity. Unlike the cavity of roundworms, it is lined with a layer of integumentary cells and is therefore called the secondary body cavity. (Remember what the body cavity of roundworms is called and explain why.) Each segment of the body has its own isolated cavity filled with a special watery liquid.

The principle of creating isolated segments - compartments - is used by designers when developing projects for large ships and submarines, where each compartment is hermetically sealed. Thanks to this, the ship does not sink in the event of an accident in one of the compartments.

Digestive system. The intestine stretches along the entire body and consists of three sections: the foregut, middle and hind intestine. The mouth opening opens into the pharynx, in which teeth are located to help hold prey. The pharynx passes into a narrow esophagus. Next comes the midgut, which looks like a straight tube. Food is digested in it. The other intestine opens outwards through the anus.

Excretory system. Each segment of the body has a pair of excretory canals. One end of this channel opens into the body cavity, and the other goes out.

Respiratory system. The function of the respiratory organs is performed by the dorsal antennae and skin. Blood vessels run directly under the skin and in the dorsal antennae. This arrangement of blood vessels allows the body to remove carbon dioxide and enrich the blood with oxygen, d) “The Nereis circulatory system consists of two vessels - dorsal and abdominal, which are connected by annular vessels. Blood circulates throughout the body due to the rhythmic contraction of the dorsal and anterior annular vessels.

The nervous system of Nereis is well developed and consists of a cerebral ganglion, shaped like a peripharyngeal ring. Two nerve trunks extend from it along the ventral side of the body, which form thickenings in each segment.

Sense organs. The organs of vision (4 eyes) are located at the head end of the worm's body. The function of the organs of touch is performed by the antenna palps on the head and lateral outgrowths. In addition, Nereis has olfactory pits that help the animal sense chemicals dissolved in water. The eyes are the most important sensory organ of polychaete worms. If the real eyes disappear in stationary polychaete worms, ocelli of various structures appear. In worms that lead a motionless life in their armor, these interchangeable eyes appear not just anywhere, but on the gills. But this is still a small thing. Some species of worms have their mouths, so to speak, backwards, with their eyes at the anus. You won't see this in any other animal.

Reproduction. Polychaete worms are dioecious animals, but appearance male and female cannot be distinguished.

Gonads that produce germ cells are formed in each segment of the worm, and these cells finally mature in the body cavity. From it, the germ cells exit through the excretory canals into environment, where fertilization occurs. On a moonlit night, many worms leave their burrows, rise and accumulate near the surface of the sea, releasing reproductive cells into the water. This is when the local population of the Pacific Islands harvests worms, because for them it is a delicious food.

Nereis can also reproduce asexually, when individual segments begin to grow, gradually turning into a new organism. Sometimes a scattering or chain of fused worms is formed, consisting of many individuals (30).

Life cycle. The larva, emerging from the egg, lives in the water column. Its spherical body has no parts; it is surrounded by cilia, with the help of which the larva swims. Subsequently, its segmentation occurs. Gradually the larva switches to a bottom lifestyle. Diversity of agatochaete worms. The class Polychaete worms, which are divided into two subclasses, has over 7,500 species (Fig. 38).

The subclass Vagrant worms include worms that actively move and eat algae, small crustaceans, other worms and even mollusks. The length of these worms reaches three meters. Stray worms move along the bottom or swim. In smooth species of worms, the body is transparent, and the head end contains large black eyes. A representative of this subclass is Nereis.

The subclass Sitting worms include worms whose skin secretes special substances, which subsequently begin to harden, forming a transparent shell - the exoskeleton. In some worms, grains of sand or fragments of mollusk shells are attached to this shell, further compacting it. There are also worms whose body covers are penetrated by lime, forming an outer shell - a skeleton in the form of hard tubes. The entrance to the tube can be closed with a special cap. The body of immobile worms is not clearly divided into segments. These animals breathe with gills located at the head end of the body. Sessile worms feed, filtering out small organisms living in the water column. A well-known representative of this subclass is the sea sandstone, a large worm up to 30 centimeters long. Fish feed on sea sandstones.

Polychaetes are a phylum of annelids, thus being relatives of our common earthworms.

Habitat

Polychaete worms are long-bodied inhabitants of the sea. However, some species have adapted to living in fresh water bodies, as well as on land - in deep layers of soil.

Appearance and structure

The similarity with earthworms is found primarily in the structure of the body, which is divided into many segments. The length of the polychaetes (so called polychaete worms in Greek) varies from 2 millimeters to three meters.

tubular polychaete sea ​​worm photo

Large species may have several hundred segments. Each segment contains a repeating set of internal organs:

• Coelomic bags;
• Genital ducts;
• Excretory organs.

Parapodia extend from each segment - lobe-shaped outgrowths on which there are chitinous bristles. This feature gave the name to the entire group of worms. In some species, on the head segment there is a bunch of tentacles that act as gills.

Another feature of polychaete worms is their developed eyes, which have a complex structure. They also have some kind of vestibular apparatus - statocysts. These are bubbles containing solid spherical bodies - statoliths.

polychaete worms photo

When the body changes its position, statolites roll along the walls of the vesicle and irritate the cilia of the epithelium, the nerve impulse from which is transmitted to the central nervous system, after which the animal restores balance.

The entire variety of polychaete worms is divided into free-swimming types and sessile types - attached to the seabed.

Nutrition

Polychaete worms feed either on detritus, that is, decaying organic matter, or on animal food. Sessile species extract detritus from the water column using their tentacles, which simultaneously serve as gills.

polychaete annelid photo

Free-swimming worms extract detritus from the soil by eating it or digging it out using long tentacles. Predatory families of polychaete worms are, for example, Nereids and Glycerides.

Reproduction

Polychaetes in most cases are dioecious animals. However, they do not form true gonads (organs that produce germ cells). Germ cells emerge from the coelomic epithelium.

Fertilization is external. The eggs hatch into larvae called trochophores. These are planktonic organisms that are microscopic in size and swim with the help of many cilia. After some time, they settle to the bottom and transform into adult animals.

Compared to other types of worms, annelids exhibit features of a higher organization and constitute an important link in the evolution of the animal world.

Although they belong to protostomes, like and, but, unlike them, they have a secondary body cavity with its own epithelial lining (the so-called coelom).

These worms got their name from their clearly defined division of the body into segments, or rings. Hence their short name “rings”. The ring type is genetically related to others, more complex types- and arthropods.

Most ringworms have a well-developed circulatory system, which is absent in other types of worms. Often the development of respiratory organs (gills) is observed in ringlets. The excretory organs, built according to the type of metanephridia, also became more complex. Deeper differentiation is typical for ringlets digestive systems s (mouth, pharynx, esophagus, crop, stomach, intestines, anus), as well as a more complex nervous system, which includes, in addition to the supra-pharyngeal and sub-pharyngeal ganglion and the peripharyngeal ring, the abdominal nerve chain.

Sense organs of annelids

The sense organs (eyes or their rudiments, tentacles, bristles, etc.; the primary ringlets have statocysts) received further development. Some annelids in ontogenesis go through the stage of a kind of larva - a trochophore, which repeats in its development some features of the distant ancestors of annelids. The emergence of metamerism, the essence of which consists in the systematic repetition in each segment of all internal and external organs of the body, should be considered very significant. An important stage in the evolution of worms was the development of parapodia in the rings - the rudiments of legs.

The genetic connection between ringworms and lower worms is known to be established through nemerteans, the study of which is not provided for in the school zoology course. Therefore, the question of the origin of annelids in high school cannot be addressed accordingly. The teacher should confine himself to a general indication of a special type of worm-like animals existing in nature (nemerteans), a number of primitive features of which suggest their origin from ancient ciliated worms, and on the other hand, some structural and developmental features indicate their relationship with annelids. The ancestors of annelids, in all likelihood, led a freely mobile predatory lifestyle, which contributed to a significant improvement in their organization. Their initial habitat was the sea, and then, in the process of evolution, some of the ringlets adapted to life in fresh water, as well as in soil.

Nervous system of annelids

Due to the metameric structure of the nervous system, each segment of the body has ganglia from which nerves extend, containing both sensory fibers that perceive irritations coming from receptors and motor fibers that conduct irritations to the muscles and glands of the worm. Consequently, ringlets have an anatomical and morphological basis for reflex activity in a wide range. It should be borne in mind that the head ganglia of the worm (supra- and subpharyngeal) with the help of sensory organs receive from the outside such irritations that are not perceived by other parts of the body. However, despite the leading role of the head nerve centers, unconditioned reflex reactions in the rings can also be carried out locally, in each segment of the body, which has its own ganglia. Moreover, the closure of the reflex arc can occur according to the type receptor - sensory axon - motor axon - muscle cell. In this case, the central nervous system only regulates the level of muscle activity.

The meaning of annelids

Annelids play a significant role in the cycle of substances in nature and occupy a prominent place in many biocenoses of land and sea. No less great is the practical importance of ringlets as a source of food for commercial fish and as an active factor in the soil-forming process. Some species of sea ringlets (polychaetes) have the ability to selectively absorb and accumulate chemicals dispersed in water in their bodies. For example, they found a concentration of cobalt ranging up to 0.002%, and nickel - from 0.01 to 0.08%, i.e. many thousands of times higher than in water. This ability is also characteristic of other ocean inhabitants, which opens up for humans the prospect of extracting rare elements directly from sea water with the help of invertebrates.

The food relationships of ringworms are very diverse and affect many groups of invertebrates, excluding insects, with which they do not have direct food contacts.

Types of annelids

Currently, over 7,000 species of ringlets are known, grouped into several classes, of which only two are studied in high school: the class Polychaete annelids, or Polychaetes, and the class Oligochaetes, or Oligochaetes. Polychaetes are important for understanding the origin of annelids and at the same time are of interest as an ancestral group in relation to other classes of annelids, and polychaetes can serve as an example of the adaptation of annelids to existence in fresh water and soil. The study of live ringlets is carried out at school only on representatives of the class of oligochaetes (earthworms). Familiarization with polychaete ringlets is carried out on exhibits of zoological museums, using wet preparations.

The phylum Annelida includes three classes: Polychaetes, Polychaetes, and Leeches. Characteristics of the type are given using the example of the most numerous class - Polychaetes.

Class Polychaeta

The scientific name of the class “polychaetes” means “polychaetes” in Greek. These worms are the most numerous of the ringworms; there are over 5,000 species of them. Most live in marine bodies of water, inhabiting all areas and depths of the World Ocean. They are found both in the water column and at the bottom, penetrating into the soil layers or remaining on the surface. Among the polychaetes there are predatory and peaceful views, i.e. carnivores and herbivores. Both of them use sharp, strong jaws when eating food. Pelagic worms chase fish fry; bottom worms eat algae, hydroid polyps, other worms, small crustaceans and mollusks. Those living in the soil pass sand with particles of organic substances through their intestines.

Many polychaetes build themselves tube-houses in which they hide from enemies; others live in burrows and, in case of danger, burrow into the ground (sandworms). The lifespan of polychaetes does not exceed 2-4 years. In some species, care for the offspring is clearly expressed (bearing young - in the brood pouch and special cavities or under the cover of dorsal scales).

Polychaete larvae often settle on the bottom of ships and, together with other fouling organisms, cause harm, reducing the navigability of ships. Since polychaetes do not have a hard skeleton, they serve as complete and easily digestible food for fish, constituting an important element of their food supply.

Polychaetes, with a few exceptions, are marine animals that live in extremely diverse ecological conditions.

Body structure of polychaetes

The body of polychaetes is segmented and consists of a head lobe (prostomium), body segments and anal lobe (pygidium). On the head lobe there are sensory organs: touch (on the palps), vision (simple eyes), chemical sense. The body is elongated, worm-shaped, the number of segments varies greatly. Segments of the body can be identical in structure (homonomous metamerism) or different in both structure and functions (heteronomous metamerism). Metamerism is the division of the body of animals into similar sections - metameres located along the longitudinal axis of the body. Polychaetes are characterized by the process of cephalization - the inclusion of one (or more) segments of the body into the head section.

The body segments are equipped with paired lateral motor appendages - parapodia. In fact, parapodia are the first primitive limbs to evolve in invertebrates. Each segment bears a pair of parapodia. The parapodium consists of two branches: dorsal (notopodium) and ventral (neuropodium). Each branch contains a tuft of bristles. In addition to thin identical setae, the branches of the parapodia contain thick supporting setae. The sizes, shapes of parapodia and setae within the class are very diverse. Often in sessile forms the parapodia are reduced.

The body of polychaetes is covered with a thin cuticle formed by a single-layer integumentary epithelium. The epithelium contains single-celled glands that secrete mucus onto the surface of the worms' body. In sessile polychaetes, skin glands secrete substances for the construction of tubes in which worms live. The tubes can be inlaid with grains of sand or impregnated with calcium carbonate.

Under the epithelium there are two layers of muscle - circular and longitudinal. The cuticle, epithelium and layers of muscle form a skin-muscle sac. From the inside, it is lined with single-layer epithelium of mesodermal origin, which limits the secondary body cavity, or coelom. Thus, the coelom is located between the body wall and the intestine. In each segment, the coelom is represented by a pair of sacs filled with coelomic fluid. It is under pressure, and individual cells - coelomocytes - float in it. Contacting above and below the intestines, the walls of the pouches form a two-layer partition - the mesentery (mesentery), on which the intestines are suspended from the body. At the border between the segments, the walls of adjacent coelomic sacs form transverse partitions - dissepiments (septa). Thus, the septa are divided as a whole into a certain number of transverse sections.

Functions of the secondary body cavity: supporting (liquid internal skeleton), distribution (transport of nutrients and gas exchange), excretory (transport of metabolic products to the excretory organs), reproductive (in general, maturation of reproductive products occurs).

The mouth leads into a muscular pharynx, which in predatory species may contain chitinous jaws. The pharynx goes into the esophagus, and then the stomach follows. The above sections make up the foregut. The midgut has the shape of a tube and is equipped with. own muscular lining. The hindgut is short and opens with an anal opening on the anal lobe.

Polychaetes breathe through the entire surface of the body or with the help of gills, into which some parts of the parapodia turn.

The circulatory system is closed. This means that it circulates in the animal’s body only through the vascular system. There are two large longitudinal vessels - dorsal and abdominal, which are connected in segments by ring vessels. A very dense capillary network is formed under the epithelium and around the intestine. The capillaries also intertwine the convoluted tubules of the metanephridia, where the blood is freed from waste products. There is no heart; its functions are performed by a pulsating spinal vessel, and sometimes by annular vessels. Blood flows from front to back through the abdominal vessel, and from back to front through the spinal vessel. Blood may be red due to the presence of iron-containing respiratory pigment, or it may be colorless or have a greenish tint.

The excretory organs in primitive polychaetes are represented by protonephridia, and in higher ones - by metanephridia. The metanephridium is a long tubule that opens into a generally ciliated opening. The genital funnels (genital ducts) fuse with the metanephridium tubules, and a nephromyxium is formed, which serves to remove metabolic products and germ cells. Metanephridia are located metamerically: 2 in each body segment. The excretory function is also performed by chloragogenic tissue - modified coelomic epithelial cells. Chlorogenic tissue functions according to the principle of a storage bud.

Nervous system of polychaetes

The nervous system consists of paired cerebral ganglia, the peripharyngeal nerve ring and the ventral nerve cord. The abdominal nerve cord is formed by two longitudinal nerve trunks, on which two adjacent ganglia are located in each segment. Sense organs: organs of touch, chemical sense and vision. The organs of vision can be quite complex.

Reproduction of polychaetes

Polychaete worms are dioecious, sexual dimorphism is not pronounced. The gonads are formed in almost all segments, have no ducts, and the reproductive products exit as a whole, and out through nephromyxia. In some species, reproductive products are released into the water through breaks in the body wall. Fertilization is external, development proceeds with metamorphosis. The polychaete larva - trochophore - swims in plankton with the help of cilia. In the trochophore, two large mesodermal cells lie on the sides of the intestine - teloblasts, from which the sacs of the secondary body cavity subsequently develop. This method of laying down the coelom is called teloblastic and is characteristic of protostomes.

In addition to sexual reproduction, polychaetes have asexual reproduction, timed to coincide with the period of maturation of the reproductive products. At this time, some species rise from the bottom (atokic forms) and lead a planktonic lifestyle (epitokic forms). Epitoke forms are morphologically very different from atoce forms. In these animals, the back of the body can form a head and separate from the front. As a result of regeneration processes, chains of individuals are formed.

Polychaetes serve as food for many species of fish - benthophages, large crustaceans and marine mammals.

At school, students get acquainted with polychaetes using the example of representatives of two families - nereids and sandworms. In addition to the information provided about them in the school textbook, some additional data is given below.

Nereids

Students should be informed that there are over 100 species of Nereids in nature. They belong to the subclass of vagrant polychaetes. The body of Nereids is often painted in green tones, cast in all the colors of the rainbow. Nereids of the White Sea feed on kelp and other algae, as well as small animals; Some species of nereids from the seas penetrate through the mouths of rivers into rice fields, where they gnaw young rice shoots, causing damage to the seedlings. One of the tropical Nereids even moved to land and began to live far from seashore on banana and cocoa plantations, where it lives in humid environment, feeding on rotting leaves and fruits. These facts show that marine forms of polychaetes can adapt to life in fresh water and on land, which sheds light on the origin of ringlets living in fresh water bodies and in moist soils (oligochaetes, leeches).

Some types of nereids live only in clean water and cannot tolerate the presence of even small amounts of hydrogen sulfide in it, while others can live in polluted water bodies with rotting in the silt. organic substances. Consequently, nereids, like other aquatic organisms, can serve as indicators of water quality.

As a result of the artificial relocation of Nereids from the Azov Sea to the Caspian Sea, the nutrition of the valuable fish species inhabiting it has significantly improved. For example, silt, rich in detritus, used to lie on the bottom of the Caspian Sea as if it were “dead capital”; now it serves as food for Nereids, which, in turn, constitute the main food for fish (sturgeon, stellate sturgeon, bream, etc.). The success of the acclimatization of Nereids, carried out under the leadership of Academician L.A. Zenkevich, opened up broad prospects for the reconstruction of the food supply of not only the Caspian, but also the Aral Sea and entailed a number of other similar measures for the reconstruction of marine fauna.

Nereids are capable of forming temporary connections of a conditioned reflex type. For example, one of the White Sea Nereids was systematically illuminated simultaneously with feeding at the moment it emerged from the tube. After several sessions, the worm began to crawl out on lighting alone, without reinforcing this stimulus with food. Then this reflex was converted to darkening, and even later to changing the degree of illumination.

Nereid trochophores have remarkable maneuverability in swimming, which is facilitated not only by the streamlined shape of the larva, but also to a greater extent- peculiar movements of the cilia in the bands covering the body of the trochophore. This movement creates special currents of water that carry the larva forward, and changing the mode of operation of the cilia allows it to move in a variety of directions. Using the principles of trachophora propulsion, a model of a submarine with rotary engines was proposed in the United States. Thus, knowledge of the characteristics of the trochophore found application in technology after the ringlet larva became an object of bionics.

Sand veins

The silty and sandy soils of the littoral zone are inhabited by greenish-brown polychaetes (20-30 cm long), which lead a burrowing lifestyle. They belong to the subclass of sessile polychaetes and feed on plant dotrite, swallowing and passing soil with organic residues through their intestines.

In the littoral zone of the White Sea at low tide, you can see traces of the activity of sandworms in the form of many trapping funnels and cone-shaped emissions of sand. Sand deposits are arranged in upper layers coastal shallows have curved burrows with two exits to the surface. A funnel is formed at one end of the burrow, and a pyramid is formed at the other. The funnel is a sock that has settled near the worm’s mouth as a result of the sandworm’s absorption of soil along with rotting algae, and the hummock is another portion of sand thrown out that has passed through the intestines of the worm. Calculations have shown that sand extractors are capable of renewing and processing up to 16 tons of soil per 1 hectare of sea coast per day.

Class Oligochaeta

The scientific name for this class, “oligochaetes,” comes from a Greek word that means “oligochaetes.” Oligochaetes evolved from polychaetes by changing some structural features due to their transition to other habitats (fresh water, soil). For example, they completely lost parapodia, tentacles, and some species - even gills; The larval stage, the trochophore, disappeared and a cocoon appeared, protecting the eggs from the effects of soil particles.

The sizes of oligochaetes range from 0.5 mm to 3 mm. About 3,000 species of oligochaetes are known, the vast majority of which are soil inhabitants. Several hundred species live in fresh water and very few (several dozen species) belong to marine forms.

Oligochaetes are inhabitants of soil or fresh water; marine representatives are extremely few in number. The parapodia of oligochaetes are reduced; only a limited number of setae are preserved. Oligochaetes are hermaphrodites.

Body structure of oligochaetes

The body of oligochaetes is elongated and has homonomic segmentation. No cephalization processes are observed; there are no sensory organs on the head lobe. Each body segment bears 4 tufts of setae, the number and shape of which are different. The body ends with an anal lobe.

The body of oligochaetes is covered with a thin cuticle, which is secreted by single layer epithelium, rich in mucous glands. The secreted mucus is necessary for the worm to ensure respiration processes, and also facilitates the animal’s movement in the ground. There are especially many glands concentrated in the girdle area - a special thickening on the body that takes part in the process of copulation. The muscles are circular and longitudinal, the longitudinal ones are more developed.

In the digestive system of oligochaetes, complications associated with feeding habits are observed. The pharynx is muscular and leads into the esophagus, which expands into the goiter. In the crop, food accumulates, swells and is exposed to enzymes that break down carbohydrates. The ducts of three pairs of calcareous glands flow into the esophagus. Calcareous glands serve to remove carbonates from the blood. Carbonates then enter the esophagus and neutralize humic acids, which are contained in rotting leaves - food for worms. The esophagus flows into the muscular stomach, in which food is ground. On the dorsal side of the midgut, an invagination is formed - typhlosol, which increases the absorptive surface of the intestine.

In the circulatory system, the role of “hearts” is performed by the first five pairs of annular vessels. Breathing occurs through the entire surface of the body. Oxygen dissolved in mucus diffuses into a dense capillary network located under the integumentary epithelium.

Excretory organs are metanephridia and chloragogenous tissue covering the outer surface of the midgut. Dead chloragogenous cells stick together in groups and form special brown bodies, which are brought out through unpaired pores located on the dorsal surface of the worms' body.

The nervous system has a typical structure, the sense organs are poorly developed.

Reproduction of oligochaetes

The reproductive system is hermaphrodite. The gonads are located in several genital segments. Fertilization is external, cross. During copulation, the worms stick together with girdle mucus and exchange sperm, which is collected in the seminal receptacles. After this, the worms disperse. A mucous muff forms on the girdle, which slides towards the anterior end of the body. Eggs are deposited in the muff, and then the partner's sperm is squeezed out. Fertilization occurs, the muff slides off the body of the worm, its ends close, and a cocoon is formed, inside which direct development of the worms occurs (without metamorphosis).

Oligochaetes can reproduce asexually - by architomy. The body of the worm is divided into two parts, the front part restores the rear end, and the back part restores the head.

Earthworms play an important role in soil-forming processes, loosening the soil and enriching it with humus. Earthworms serve as food for birds and animals. Freshwater oligochaetes are an important component in fish nutrition.

Students can become familiar with oligochaete worms using living objects. Among freshwater oligochaetes, naids and tubifex are especially accessible, and from soil inhabitants- various earthworms and enchytraeids (pot worms). In addition to observations, a number of elementary experiments can be carried out in a corner of living nature, in particular, on regeneration, which is quite pronounced in oligochaetes.

Earthworms

The zoology textbook describes the common earthworm, one of the representatives of the Lumbricidae family. However, in fact, when working with students, the teacher will have to deal with that specific species, individuals of which will be extracted from the soil of a school plot or obtained on an excursion to study the soil fauna of a certain biocenosis (fields, meadows, forests, etc.). And although in basic features all these worms are similar, they differ from each other in details depending on their species.

It is important that children learn about the existence of many species of earthworms, adapted to different living conditions in nature, and not be limited to a one-sided idea of ​​them only on the basis of textbook materials. In the family Lumbricidae, for example, there are about 200 species, grouped into several genera. The species identification of worms is based on a number of characteristics: size and color of the body, number of segments, arrangement of setae, shape and position of the girdle and other external and internal features buildings. Students should also be informed that in favorable landscapes the biomass of earthworms can reach 200-300 kg per 1 hectare of land.

When becoming familiar with the external structure of earthworms, students should pay attention to the weak development of the bristles, which, however, play a significant role in the movement of worms in the soil. During the excursion it is easy to make sure that the body is firmly fixed earthworm in a mink. You can tell students that at the base of each bristle there are small bristles that replace the old ones as they wear off.

While observing the behavior of a worm in a corner of wildlife as it burrows into the ground, students should become familiar with the “mechanics” of this process and clarify the role of bristles in it. The earthworm acts with the front end of its body like a battering ram. It pushes soil particles to the sides while swelling the front part of the body, where fluid is pumped by muscle contraction. At this moment, the bristles of the head section rest against the walls of the stroke, creating an “anchoring”, i.e., an emphasis for pulling up the rear sections, and the bristles of these latter are pressed against the body, reducing friction on the soil during movement. When the head section begins to move forward again, the bristles of the rest of the body rest against the ground and provide support for the extension of the head.

Due to life in the soil, earthworms, compared with free-living oligochaetes, have underdeveloped bristles, and the receptor apparatus has also become simpler. The outer layer contains various sensitive cells. Some of them perceive light stimulation, others - chemical, others - tactile, etc. The head end is the most sensitive, and the rear end is less sensitive. The weakest sensitivity is observed in the middle part of the body. These differences are due to the unequal distribution density of sensitive cells.

Any harmful or unpleasant external influence; factor causes a defensive reaction in the earthworm: burrowing into the ground, contracting the body, secreting mucus on the surface of the skin. It is necessary to conduct elementary experiments that would show the attitude of worms to various stimuli. For example, tapping on the wall of the cage causes negative vibrotaxis (the worm hides in a hole). Bright light causes the worm to crawl into the shadows or hide in a hole (negative phototaxis). However, the worm reacts positively to weak light (heads towards the light source). Exposure of the head end to even a very weak solution of acetic acid causes negative chemotaxis (contraction of the anterior part of the body). If you place a worm on filter paper or glass, it tends to crawl to the ground. Negative thigmotaxis (avoidance of a foreign substrate from which unusual irritation emanates) operates here. A strong touch to the rear end entails pulling out the front end - the worm seems to run away. If you touch it from the front, the movement of the head end stops, and the tail end produces a backward movement. These experiments cannot be carried out directly on the surface of the earth, since the worms will burrow into the soil (defensive reaction).

When keeping worms in cages, you can observe them pulling leaves into the burrow. If the leaf is fixed in place, not allowing it to move, then the worm, after 10-12 unsuccessful attempts to bring the prey closer to the hole, leaves it alone and captures another leaf. This indicates the ability of worms to vary stereotypic behavior in accordance with specific circumstances. According to Darwin, the worms each time grab the leaves so that they are dragged into the hole more or less freely, for which they give them the appropriate orientation. However, recent observations have shown that worms achieve the desired results through trial and error.

Some scientists, following Darwin, believed that worms could distinguish the shape of objects and thus find leaves, but in reality it turned out that earthworms (like many other invertebrates) tend to find food using chemoreceptors. Thus, in the experiments of Mangold (1924), worms distinguished the petiole from the top of the blade in the foliage not by shape, but by the unequal smell of these parts of the leaf. It is now recognized that earthworms, while crawling on the ground, can perceive the outlines and placement of objects around them based on tactile and kinesthetic sensations.

In earthworms, activity varies throughout the day. About 1/3 of the day they are more active, and the rest of the time their activity decreases almost three times. In addition to the daily rhythm, earthworms also have a seasonal rhythm of activity. For example, during the winter, worms go deeper into the ground and remain there in burrows in a state of suspended animation. There are known cases of living worms being found inside pieces of ice, which indicates their great endurance and ability to withstand adverse conditions.

Studies conducted in Russia and abroad have shown the positive role of earthworms in improving soil structure and increasing their fertility.

Life in the soil, movement in the ground and contact with coarse particles of earth entail mechanical damage to the delicate skin of the earthworm, and sometimes tearing their body into pieces. However, all these injuries do not lead to their death, since the worms have developed protective devices that ensure their survival in their natural habitat. For example, the mucus secreted by the skin glands has properties that protect the body from infection by pathogenic microbes and fungi that penetrate wounds and scratches. In addition, mucus moisturizes the surface of the body, preventing it from drying out, and serves as a lubricant during movement. In addition to mucous secretions, regenerative processes play an important role in preserving the life of worms, which are especially important during mechanical dismemberment of the body into pieces.

IN school corner In living nature, it is not difficult to conduct experiments on the regeneration of earthworms and observe the progress of the restoration of lost parts. The cephalic ganglia play an important role in these processes, which is why in some species of worms (for example, the dung earthworm), cut in half, the anterior end regenerates better and faster.

The adaptability of worms to existence in the soil is also expressed in the presence of strong cocoons, inside which they develop small quantity eggs Cocoons can lie in the ground for up to 3 years, preserving the viability of the young. Adult worms also live for several years (from 4 to 10) in cages, where their life expectancy was determined. IN natural conditions many worms do not live to their natural end, since they are eaten by moles in underground passages, and on the surface of the earth they are attacked and destroyed by ground beetles, large centipedes, frogs, toads, and birds. In particular, many worms die after heavy rains, when water floods their passages and burrows, displacing them and forcing the worms to crawl out to breathe.

Under experimental conditions, earthworms are capable of changing their innate behavior based on the development of conditioned reflexes. This was clearly shown in the classic experiments of R. Yerkes (1912). He forced an earthworm to crawl through a T-shaped labyrinth consisting of two tubes connected at right angles. At one end of the transverse tube (right) there was an exit to a box with wet soil and leaves, and at the other (left) there was a strip of glass skin and battery electrodes. The worm crawled in the longitudinal tube until it entered the transverse one and then turned either to the right or to the left. In the first case, he found himself in a favorable environment, and in the second he experienced unpleasant sensations: irritation from the glass skin and an electric injection when his body connected the electrodes. After 120-180 trips, the worm began to prefer the path leading to the box. He developed a conditioned reflex to a biologically useful direction of movement. If the electrodes and the box were swapped, then after about 65 sessions the worm acquired a new conditioned reflex.

Class Leeches (Hirudinea)

Medicinal leech (Hirudo medicinalis) is used in medicine for diseases of blood vessels, blood clots, hypertension, sclerosis, etc.

TYPE RINGED WORMS

Annelids are animals that have a long, segmented body. The body segments look like rings. They live in seas, fresh waters, and soil.

Annelids have 3 aromorphoses:

· Metamerism (organ systems are repeated in different segments of the body).

· Overall(secondary body cavity, own epithelial lining).

· Lateral outgrowths of the body ( parapodia) - organs of movement, primitive limbs.

The sizes of ringed fish range from fractions of a millimeter to 3 m. The body is divided into three sections: the head. Trunk and anal lobe. The head was formed by the fusion of several body segments. The head contains the mouth opening, eyes, organs of touch (antennae, palps, etc.). The body consists of homogeneous segments, covered with a skin-muscular sac consisting of a thin cuticle, single-layer epithelium and two layers of muscles - external circular and internal longitudinal. In the anterior and middle sections of the intestine there are differentiated areas (crop, stomach). The circulatory system is closed. Blood moves only through blood vessels. Respiration is carried out either over the entire surface of the body (oligochaetes and leeches), or with the help of gills located on the parapodia (polychaetes). The excretory system is presented metanephridia. The nervous system is represented by a peripharyngeal nerve ring, consisting of the suprapharyngeal and subpharyngeal nodes connected by nerve cords, and two nerve trunks with ganglia, connected to each other by jumpers. Sense organs are more developed in movable rings.

The type of annelids is divided into classes:

1. Polychaetes

2. Oligochaetes

3. Leeches

Class polychaetes (polychaetes)

Representatives lead a free-swimming and attached lifestyle. Movement is carried out by parapodia equipped with tufts of bristles. Parapodia are prototypes of arthropod limbs. In some polychaetes, parapodia have gill apparatus that ensures gas exchange in aquatic environment. Representatives of the class have a well-separated head section, where the sensory organs are located: tentacles, light-sensitive eyes, olfactory fossa. In the structure of the nervous, circulatory, excretory and digestive systems, polychaetes repeat the characteristics of their type. Dioecious, development proceeds with metamorphosis (there is a larval stage).

Polychaetes are a progressive branch of animals from which arthropods descend. Serve as food for marine animals. Nereids are specially acclimatized in the Caspian Sea as food for sturgeon. Palolo, which lives in the tropical waters of the Pacific Ocean, is of commercial importance.

Class oligochaetes

Representatives live in soil or fresh water. The head end is not expressed. Sense organs are poorly developed. There are no parapodia and few setae. Hermaphrodites, direct development.

Class representative - earthworm. Earthworms live deep in burrows, which they dig by swallowing soil. As a result, the head section is very weakly expressed, there are no parapodia, tentacles and ocelli. The skin is permeated with blood capillaries and moistened with mucus, which makes gas exchange easier, movement in the soil easier, and mucus also has small bactericidal properties. Earthworms have a girdle that is lighter than the rest of the body.

The digestive system consists of the mouth opening, oral cavity, pharynx, long esophagus, which has an extension - goiter, muscular stomach, and intestines. It all ends with the anus. Earthworms feed on rotting plant debris, passing a mass of earth through the digestive tract.

There is no respiratory system as such; gas exchange occurs over the entire surface of the body.

Closed circulatory system. Blood moves through blood vessels, of which two are especially developed - the dorsal and abdominal. They communicate with each other through annular vessels located in each segment. There is an extensive network of capillaries. The movement of blood is determined by the rhythmic contractions of blood vessels from the 7th to the 11th segment. Blood plasma contains respiratory pigments similar to hemoglobin.

The excretory system consists of paired convoluted tubes (metanephridia), which begin as a funnel with ciliated cells on the walls of the body cavity, and end with an excretory pore that opens outward.

The nervous system is represented by a peripharyngeal nerve ring, consisting of 2 suprapharyngeal and 2 subpharyngeal nodes, connected by nerve cords. Two nerve trunks depart from the subpharyngeal node, having thickenings in each segment - ganglia, which are connected to each other by jumpers.

The earthworm is a hermaphrodite. Fertilization is cross, development is direct. One worm has both male and female reproductive organs: female oviducts, male testes, vas deferens and spermatheca. The girdle forms a special mucus from which the muff is formed. The coupling begins to move towards the head ring, passing the ducts of the oviducts, where the eggs enter. The muff then passes through the seminal receptacles, where the sperm is released.

Ringed animals are capable of regeneration. Earthworms influence the properties of the soil; By digging numerous burrows, they improve its structure, loosen it, and enrich it with organic matter.