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The meaning of polychaete worms. Class Polychaeta - full description What some sessile forms of polychaete worms breathe

Polychaete worms got their name for the large number of hard hairs - bristles. Most species belong specifically to polychaetes, which are also called polychaetes. In some species, the bristles sit on soft outgrowths - “legs”. Polychaete worms live mainly in the sea and use these “legs” (parapodia) when moving along the bottom or digging in sand and mud. Some polychaetes live in tubes. Compared to earthworms, most polychaete worms have pronounced head and tail ends. The head usually bears finger-like tentacles, which are arranged either in circles or collected in a corolla. Some species of polychaete worms even have eyes that can recognize the shape of objects. At the end of the tail they may have two long thread-like processes. Nereis and sandworms live on the sea coast. The former usually live in burrows lined with their own mucus, in dirty sediments that accumulate at the bottom of rocky reservoirs. During low tide, Nereis hide. With the onset of tide, they again leave their shelters to look for food on the surface of the silt. They are able to move like snakes or swim by paddling with parapodia. Nereis eat the remains of dead animals, small creatures like shrimp, and seaweed. Their strong jaws easily bite off pieces of flesh, and they are able to suck blood even through human skin. In addition, they can shed their mucous membrane, eating the microbes contained in the mucus along with it.

Body fan worms hidden inside a protective tube, tripled on the seabed from particles of silt and sand, glued together with mucus. These worms got their name from the hard feathery feeding tentacles that they release from the top of the tube, spreading them out like a fan. One of the most beautiful representatives of this group is the Sabella peacock worm. Its tube reaches 25 cm in length. At favorable conditions It releases tentacles fluttering in the water, covered with a thin layer of mucus, which capture food particles floating by. In addition, they perform the function of gills. If a fish or other predator swims by, the sabella retracts its tentacles and hides in the tube. Sometimes sabella and other fan worms form groups that resemble a flower carpet. At the moment of danger, they disappear inside the tubes, and the seabed becomes empty and lifeless.
At the bottom of the sea, near underwater volcanic springs that emit bubbling water rich in hydrogen sulfide, there live amazing Pompeian worms. They can withstand heating above 100 degrees and cooling down to +2 degrees. Their bristles harbor bacteria that process hydrogen sulfide and nutrients, which they share with the worms.
Some polychaete worms form hard and durable tubes of limestone in sand and on rocks. During high tide, the worms spread the crowns of their feeding tentacles over the tubes in search of food.
Miniature mounds of sand and mud on the coast - traces of activity sand-dwellers. These soft-bodied worms with a thickening at the front end reach a length of 15-20 cm. They feed like earthworms, eating sand and dirt, digesting the nutrients and then throwing out the remains in mounds. Sandworms hide in the sand in ∪-shaped burrows. As the worm moves, it creates a current of water through the hole, thus gaining the ability to breathe.

sea mouse- this is not a rodent, but one of the polychaete worms. It grows up to 20 cm in length. Its thick, lumpy body is covered with grey-brown hairs, very similar to the fur of a real mouse. The sea mouse lives mainly on sandy bottoms in the Atlantic Ocean and Mediterranean Sea. Some worms have dorsal scales overgrown with algae, which hide the sea mouse like a modern camouflage uniform. Small crustaceans and other animals live among the algae. As a result, the worm becomes like a mobile sea forest, which can motionlessly approach its prey.

Polychaete worms:
- 8500 species
- Mainly maritime
- There are parapodia - something like legs
- Dioecious
- Representatives: sandworm, nereis, sea mouse, sabella, Pompeian worm

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). This primitive limbs, with the help of 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 parapodia, 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, driving 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 parts 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 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 - nephridia funnel, 9 - nephridia canal, 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 of mesenchymal cells cover the inside of the muscle bands and the outer surface of the intestine. Some of these cells are capable of contraction, while others are able to turn into germ cells that mature in the cavity, only conventionally called secondary B 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 mesentery, 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 of 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 there is transport nutrients 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 sexual 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. 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, capable of firmly holding, wounding and crushing 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 system. 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 ring 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 contains 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 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 relationship 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 is 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 variants life cycle. 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 the 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. They can be found in different types of polychaetes different number: 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 completion of metamorphosis, an adult animal develops with a certain number segment for each type. 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 The development of polychaetes is spiral, determinate fragmentation, teloblastic anlage of mesoderm, metamorphosis with the formation of trochophore larvae, metatrochophores, nektochaetes and juvenile forms. 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.

The consistent change in the phases of individual development of polychaetes from oligomeric to polymeric reflects 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).

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

Classification

Annelids are a type of invertebrate animal. Their group is called protostomes. Biologists distinguish 5 classes of annelids:

Belt, or leeches;

Oligochaetes (the most famous representative of this class is the earthworm);

Polychaetes (peskozhil and nereid);

Misostomidae;

Dinophylids.

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

Annelids are one of the main links in the food chains of ecosystems both on land and in the oceans. They feed on fish, turtles, birds and other animals. Even people use them as a supplement when breeding commercial fish species in both fresh and sea waters. Fishermen use worms as bait on a hook when catching fish with a fishing rod.

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

Origin

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

Scientists believe that the main group of polychaete worms appeared first. In the process of evolution, when this type of animal moved to life on the surface and in fresh water bodies, oligochaetes, later called leeches, appeared.

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

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

general characteristics

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

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

Structure of the head blade

Annelids have sensory organs located at the front. These are eyes, olfactory cells, which are also present on the tentacles. Ciliary fossae are organs that distinguish between the effects of various odors and chemical irritants. There are also hearing organs that have a structure reminiscent of locators. And, of course, the main organ is the mouth.

Segmented part

This part represents the same general characteristic of the type of annelids. The central region of the body consists of rings, each of which represents a complete independent part body. This area is called the coelom. It is divided into segments by partitions. They are noticeable when looking at the appearance. The outer rings of the worm correspond to the internal partitions. It is on this basis that the worms received their main name - annelids, or ringworms.

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

Secondary body cavity, or coelom

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

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

Worm food system

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

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

Circulatory system

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

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

The movement of blood through the vessels is carried out due to contractions of some sections of the spinal and, less commonly, annular vessels. After all, they don’t. The rings contain special contractile elements in these vessels.

Excretory and respiratory systems

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

The excretory system consists of metanephridia, protonephridia and myxonephridia, located in pairs in each segment of the worm. Myxonephridia are the prototype of the kidneys. Metanephridia have the shape of a funnel located in the coelom, from which a thin and short channel brings the excretory products out in each segment.

Nervous system

If we compare the general characteristics of roundworms and annelids, the latter have a more advanced nervous system and sensory organs. They have a cluster of nerve cells above the peripharyngeal ring of the anterior lobe of the body. The nervous system consists of ganglia. These are suprapharyngeal and subpharyngeal formations connected by nerve trunks into a peripharyngeal ring. In each segment you can see a pair of such ganglia of the ventral chain of the nervous system.

You can see them in the figure above. They are marked yellow. Large ganglia in the pharynx play the role of the brain, from which impulses diverge along the abdominal chain. Also to nervous system also include the worm's sense organs. He has a lot of them. These are the eyes, the organs of touch on the skin, and the chemical senses. Sensitive cells are located throughout the body.

Reproduction

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

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

But there is another type of reproduction. In conditions favorable for life, when there is a lot of food, individuals begin to grow individual body parts. For example, several mouths may appear. Subsequently, the rest grows. The worm breaks down into several separate parts. This is an asexual type of reproduction, when a certain part of the body appears, and the rest are regenerated later. An example is the ability of Aulophorus for this type of reproduction.

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

Let's consider general form, 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 form. 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, in the event of an accident in one of the compartments, the ship does not sink.

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 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 dissolved in water chemical substances. 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 replaceable 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 it is impossible to distinguish males and females by their appearance.

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 the 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.

Polychaete polychaete worms are the largest group of organisms. Scientists count about 10 thousand species of the annelid class. Common representatives: sandworm, living in the Arctic and Arctic Ocean.

A distinctive feature is the numerous bristles collected in tufts located on the sides of each segment.

Appearance

The body of a polychaete worm is divided into a large number of divisions, ranging from five to eight hundred pieces, but sometimes there are exceptions.

Description

Like similar worms, the body of polychaete worms is divided into several parts:

head
long
torso
anal blade

located at the rear of the mill.

They are inhabitants of the water depths; they are covered with skin-muscular processes - organs of movement, which are called parapodia, it is with the help of them that movement forward is possible.

The entire carcass of the worm is dressed in a muscle sac. The outside of the body consists of a thin cuticle covering the epithelium. Under the skin of the polychaete there is a musculature consisting of longitudinal and circular muscles. The ringlets are from two millimeters to three meters long, which is quite large for invertebrates.

Habitat

Polychaetes mainly live in salty waters and lead a bottom-dwelling lifestyle. However, there are individuals that vegetated in a zone not located in close proximity to the bottom; these individuals include the Tomopterid family. There are also polychaetes that have adapted to fresh water and woody soil.

Nutrition

The diet of the polychaete polychaete worm is relatively varied. Most feed on detritus - dead organic substances, this choice is associated with a sedentary lifestyle. But there are also species that eat mollusks, coelenterates, and ampictinids.

Enemies

Fish and some types of crustaceans love to feast on polychaete worms, because they are tasty and healthy food. Let's talk about people's use of worms for fishing, since this activity sharply reduces their numbers.

Reproduction

Polychaete worms are heterosexual, with the exception of some hermaphrodites. Both females and males have gonads. The female has eggs, and the male has sperm. Due to external fertilization, a larva is formed from the eggs - trophora.

The trophora moves through outgrowths, sinking to the bottom, where metamorphosis into an adult takes place. Some families of Polychaete worms also reproduce asexually. There are a couple of varieties asexual reproduction: archetomy and paratomy .

In the first case, the body is divided into dozens of segments, which later grow to a normal state, and in the second variation everything happens exactly the opposite.

Digestive system

Worms and their system are very curious; the system responsible for receiving energy is represented by the mouth, pharynx, which has chitinous teeth, esophagus and stomach. These unusual creatures have an intestine divided into three sections:

front
average
rear

The last part contains the anal ring.

Circulatory system

Polychaetes have a closed circulatory system, each representative of annelids has a closed circulatory system, that is, blood always flows through the vessels.

There are two main vessels in the camp, connected by semi-circular formations: dorsal and abdominal. There is no heart, but its duties are performed by the folding of the walls of the spinal vessel and other rather large capillaries.

Nervous system

Freely moving polychaete worms have developed sensory organs, expressed by two tentacles and antennae. The smaller part for polychaetes has vision and balance organs. And all this is achievable thanks to the nerve nodes and nerves that permeate the entire body.