Annelids are bilaterally symmetrical segmented animals.
Systematics. The type includes 5 classes, of which the most famous classes are Polychaeta (Polychaeta) - 13000 species, Olygochaeta - 3500 species and Leeches (Hirudinea) - about 400 species.
Body shape and size. The body of the rings is overwhelmingly worm-shaped, round or oval in cross section. The trunk has a pronounced both external and internal segmentation. In this case one speaks of true metamerism. At the same time, metamerism extends to the internal structure of worms. In leeches, external segmentation does not correspond to internal segmentation.
The sizes of annelids range from a few millimeters to 2 m (terrestrial forms) and even up to 3 m (marine species).
The external structure of the body. In polychaetes, the head section is well expressed, bearing organs for various purposes: tentacles, eyes, palps. In some species, the palps grow into a complex hunting apparatus. The last segment bears one or several pairs of sensory antennae. Each body segment on the sides bears parapodia - complex outgrowths of the body. The main function of these outgrowths is the movement of the worm. Each parapodia consists of two lobes, inside which are numerous setae. Of these, several are larger, they are called atsikuly. A pair of sensitive antennae is attached to the blades. The parapodia often includes the gill apparatus. Parapodia have a rather diverse structure.
In oligochaete worms, the head section is weakly expressed, lateral outgrowths (parapodia) are absent. Only relatively few setae are present. On the body, a “belt” is clearly visible, consisting of thickened segments.
Leeches have powerful suckers at the anterior and posterior ends of the body. Few species have gill outgrowths on the sides.
Skin-muscle bag. Outside, the body of annelids is covered with a thin cuticle, under which lie the cells of the skin epithelium. The skin of worms is rich in glandular cells. The secret of these cells has a protective value. In a number of species, skin secretions are used to build peculiar houses. The bristles of the worms are derivatives of the epithelium. Under the skin lies a layer of circular muscles, which allows the animal to change the transverse size of the body. Below are the longitudinal muscles that serve to change the length of the body. In leeches, between the layers of the annular and longitudinal muscles, there is a layer of diagonal muscles. Rings have special muscles that set in motion parapodia, palps, suckers, etc.
body cavity. The space between the wall of the body and the internal organs of the annulus represents the whole - the secondary cavity of the body. It differs from the primary one by the presence of its own epithelial walls, which are called the coelomic epithelium (the whole body). The coelothelium covers the longitudinal muscles of the body wall, intestines, muscle cords and other internal organs. On the walls of the intestine, the whole body is transformed into chloragogenic cells that perform an excretory function. At the same time, the coelomic sac of each body segment is isolated from the neighboring ones by partitions - dessepiments. Inside the coelomic sac is filled with a liquid containing various cellular elements. As a whole, it performs various functions - supporting, trophic, excretory, protective and others. In leeches, the whole has undergone a strong reduction and the space between the body wall and the internal organs is filled with a special tissue - mesenchyme, in which the whole is preserved only in the form of narrow channels.
The midgut is shaped like a simple tube that can become more complex. So, in leeches and some polychaetes, the intestine has lateral outgrowths. The oligochaetes have a longitudinal fold on the dorsal side of the intestine, which protrudes deeply into the intestinal cavity - typhlosol. These devices significantly increase the inner surface of the midgut, which allows the most complete assimilation of digested substances. The midgut is endodermic in origin. In small-bristle worms, on the border of the anterior and middle intestines, there is an extension - the stomach. It can be either ectodermal or endodermal.
The hindgut, which is a derivative of the ectoderm, is usually short and opens with an anus.
Circulatory system annelids is closed, that is, the blood moves everywhere through the vessels. The main vessels - longitudinal - dorsal and abdominal, connected by annular. The spinal vessel has the ability to pulsate and performs the function of the heart. In oligochaetes, this function is also performed by the annular vessels of the anterior part of the body. Blood moves from back to front along the dorsal vessel. Through the annular vessels located in each segment, the blood passes into the abdominal vessel and moves in it from front to back. Smaller vessels depart from the main vessels, and they, in turn, branch into the smallest capillaries that carry blood to all the tissues of the worms. In leeches, the system of blood vessels is significantly reduced. Blood moves through the system of sinuses - the remnants of the coelom.
The blood of most annelids contains hemoglobin. This allows them to exist in conditions with a low oxygen content.
Special respiratory organs usually not, so gas exchange occurs through the skin by diffusion. Polychaete worms and some leeches have well-developed gills.
excretory system most often represented by metanephridia, which are located metamerically, that is, in pairs in each segment. A typical metanephridium is represented by a long coiled tube. This tube begins with a funnel that opens as a whole (secondary body cavity) of the segment, then it penetrates the septum between the segments (dissepiment) and enters the glandular metanephridial body located in the next segment. In this gland, the tube winds strongly and then opens with an excretory pore on the lateral surface of the body. The funnel and tube are covered with cilia, with the help of which the cavity fluid is forced into the metanephridium. When moving through the tube through the gland, water and various salts are absorbed from the liquid, and only products to be removed from the body (urine) remain in the tube cavity. These products are excreted through the excretory pore. In many species, there is an extension in the posterior part of the metanephridial tube - the bladder, in which urine temporarily accumulates.
In primitive annelids, the excretory organs, like flatworms, are arranged according to the type of protonephridia.
Nervous system consists of the peripharyngeal ring and the ventral nerve cord. Above the pharynx lies a powerfully developed paired complex of ganglia, representing a kind of brain. A pair of ganglia also lie under the pharynx. The brain is connected to the subpharyngeal ganglia by nerve cords covering the pharynx from the sides. All this formation is called the peripharyngeal ring. Further, in each segment under the intestine there is a pair of nerve ganglia, which are connected both to each other and to the ganglia of neighboring segments. This system is called the ventral nerve cord. From all ganglia, nerves depart to various organs.
Sense organs. The head section of polychaete worms has well-developed sense organs: antennae and palps (organs of touch), eyes (sometimes quite complex), and olfactory pits. Some forms have developed organs of balance - statocysts. On the lateral outgrowths of the body (parapodia) there are antennae that perform a tactile function.
In oligochaete worms, the sense organs are much less developed than in polychaete worms. There are organs of chemical sense, sometimes - tentacles, statocysts, poorly developed eyes. A large number of light-sensitive and tactile cells are scattered in the skin. Some tactile cells have a pin.
In leeches, many sensitive cells are scattered in the skin, there are always eyes and chemical sense organs (taste buds).
reproductive system. Among annelids, there are both hermaphroditic and dioecious forms.
Polychaete worms are mostly dioecious. Sometimes there is sexual dimorphism. Sex glands (gonads) are formed in the coelomic epithelium. This process usually occurs in the posterior segments of the worm.
In small-bristle worms, hermaphroditism is more common. The sex glands are usually located in certain segments of the anterior part of the worm. Relatively small male gonads (testes) have excretory ducts, which are either modified metanephridia or canals isolated from them. Larger female sex glands (ovaries) have ducts, which are altered metanephridia. For example, when the ovary is located in the 13th segment, the female genital openings open on the 14th. There are also seminal receptacles, which are filled during mating with the spermatozoa of another worm. Leeches are mostly hermaphrodites. The testes are located metamerically, the ovaries are one pair. Fertilization in leeches occurs by the exchange of spermatophores between partners.
reproduction. Ringed worms are characterized by a wide variety of forms of reproduction.
Asexual reproduction is characteristic of some polychaete and oligochaete worms. In this case, either strobilation or lateral budding occurs. This is a rare example of asexual reproduction among highly organized animals in general.
During sexual reproduction, polychaete individuals containing mature gonads (epitocal) move from a crawling or sedentary lifestyle to a swimming one. And in some species, the sexual segments during the maturation of gametes can even break away from the body of the worm and lead an independent floating lifestyle. Gametes enter the water through breaks in the body wall. Fertilization takes place either in water or in the epitonic segments of the female.
Reproduction of oligochaetes begins with cross-fertilization. At this time, two partners are applied to each other by the abdominal sides and exchange sperm, which enters the seminiferous receptacles. After that, the partners disperse.
Subsequently, abundant mucus is secreted on the girdle, forming a sleeve around the girdle. The worm lays its eggs in this clutch. When the clutch is moved forward, it passes by the holes of the seed receptacles; at this point, fertilization of the eggs occurs. When the clutch with fertilized eggs slides off the head end of the worm, its edges close, and a cocoon is obtained in which further development takes place. The cocoon of earthworms usually contains 1-3 eggs.
In leeches, reproduction occurs in much the same way as in oligochaete worms. Leech cocoons are large, reaching 2 cm in length in some species. In a cocoon, there are from 1 to 200 eggs in different species.
Development. The zygote of annelids undergoes complete, usually uneven fragmentation. Gastrulation occurs by invagination or epiboly.
In polychaete worms, a larva called a trochophore is subsequently formed from the embryo. She has eyelashes and is quite mobile. Their larvae subsequently develop into an adult worm. Thus, in most polychaete worms, development proceeds with metamorphosis. Species with direct development are also known.
Small-bristle worms have direct development without a larval phase. Fully formed young worms emerge from the eggs.
In leeches, peculiar larvae form from eggs in a cocoon, which swim in the cocoon fluid with the help of a ciliary apparatus. Thus, an adult leech is formed by metamorphosis.
Regeneration. Many annelids are characterized by a developed ability to regenerate lost body parts. In some species, an entire organism can regenerate from just a few segments. However, in leeches, regeneration is very weak.
Nutrition. Among the polychaete worms, there are both predators and herbivorous species. There are also known cases of cannibalism. Some species feed on organic remains (detritivores). Small-bristle worms are mainly detritivores, but there are also predators.
Small-bristle worms are mostly soil inhabitants. In humus-rich soils, the number of, for example, enchitreid worms reaches 100-200 thousand per square meter. They also live in fresh, brackish and salty water bodies. Aquatic inhabitants inhabit mainly the surface layers of the soil and vegetation. Some of the species are cosmopolitan, and some are endemic.
Leeches inhabit fresh water bodies. Few species live in the seas. Some have switched to a terrestrial way of life. These worms either lead an ambush lifestyle or actively seek out their hosts. A single bloodsucking provides leeches with food for many months. There are no cosmopolitans among leeches; they are confined to certain geographic areas.
paleontological finds annelid worms are very few. Polychaetes are more diverse in this respect. Not only prints have been preserved from them, but also in many cases the remains of pipes. On this basis, it is assumed that all the main groups of this class were represented already in the Paleozoic. Reliable remains of oligochaete worms and leeches have not been found to date.
Origin. Currently, the most plausible hypothesis is the origin of annelids from parenchymal ancestors (ciliary worms). The most primitive group is considered to be polychaetes. It is from this group that the oligochaetes most likely originate, and from the latter a group of leeches emerged.
Meaning. In nature, annelids are of great importance. Inhabiting various biotopes, these worms are included in numerous food chains, serving as food for a huge number of animals. Terrestrial worms play a leading role in soil formation. By processing plant residues, they enrich the soil with mineral and organic substances. Their moves contribute to the improvement of soil gas exchange and its drainage.
In practical terms, a number of earthworm species are used as vermicompost producers. The worm - enchitreus is used as food for aquarium fish. Enchitreev breed in huge quantities. For the same purpose, the tubifex worm is mined in nature. Medicinal leeches are currently used to treat certain diseases. In some tropical countries they eat palolo- genital (epitocal) segments of worms that have separated from the front of the animals and floated to the surface of the water.
General characteristics of the type Arthropods.
Arthropods are bilaterally symmetrical segmented animals with metamerically arranged jointed limbs. This is the most species-rich and diverse group of animals.
Systematics. The type of arthropods is divided into several subtypes.
Subtype Gill-breathers (class Crustaceans)
Subphylum Trilobites (extinct group)
Subtype Cheliceraceae (class Merostomaceae, class Arachnids)
Subtype Primary tracheal
Subtype Tracheal breathing (class Millipedes, class Insects).
The Merostomaceae class includes modern horseshoe crabs and extinct shell scorpions. To subtype Primary tracheal small (up to 8 cm) tropical animals are included, which in structure occupy an intermediate position between annelids and arthropods. These groups of animals will not be considered here.
Body dimensions. The body length of arthropods ranges from 0.1 mm (some mites) to 90 cm (horsicle crabs). Terrestrial arthropods reach 15-30 cm. The wingspan of some butterflies exceeds 25 cm. Extinct crustaceans reached 1.5 m in length, and the wingspan of fossil dragonflies reached 90 cm.
External structure. The body of most arthropods consists of a head, thorax, and abdomen. The listed departments include a different number of segments.
Head, the segments of which are fixedly connected, bears the oral organs and sensory organs. The head is movably or immovably connected to the next section - the chest.
Thoracic bears walking limbs. Depending on the number of thoracic limb segments, there may be a different number. In insects, wings are also attached to the chest. The segments of the chest are connected to each other movably or motionless.
Abdomen contains most of the internal organs and most often consists of several segments, movably connected to each other. Limbs and other appendages can be located on the abdomen.
The oral apparatus of arthropods is very complex. Depending on the method of nutrition, it can have a very diverse structure. The parts of the mouth apparatus are for the most part highly modified limbs, adapted to eat almost any food. The apparatus may include 3-6 pairs of limbs.
Covers. The cuticle, consisting of chitin, is a derivative of the submerged epithelium - the hypodermis. Chitin performs a supporting and protective function. The cuticle can be impregnated with calcium carbonate, thus becoming a very strong shell, as happens, for example, in crustaceans. Thus, in arthropods, the integuments of the body represent the external skeleton. The mobile connection of the hard sections of the cuticle is provided by the presence of membranous sections. The cuticle of arthropods is not elastic and cannot be stretched during the growth of animals, so they periodically shed the old cuticle (molt) and, until the new cuticle has hardened, increase in size.
body cavity. In the process of embryonic development in arthropods, coelomic sacs are laid, but later they are torn and their cavity merges with the primary body cavity. Thus, a mixed body cavity is formed - a mixocoel.
musculature represented by separate muscle bundles that do not form a continuous muscle sac. Muscles are attached both directly to the inner wall of the body segments and to their inner processes that make up the internal skeleton. Musculature in arthropods striated.
Digestive system in arthropods, it generally consists of the anterior, middle, and posterior intestines. The anterior and posterior sections are lined from the inside with a thin chitinous cuticle. Depending on the type of nutrition, the structure of the intestine is extremely diverse. Salivary glands open into the oral cavity, which very often produce a number of enzymes, including digestive ones. The anal opening usually opens at the posterior end of the body.
excretory system in primary aquatic arthropods (crustaceans) it is represented by special glands located in the head of the body. The ducts of these glands open at the base of the antennae (antennae). In terrestrial arthropods, the excretory system is represented by the so-called malpighian vessels- tubes that are blindly closed at one end, and open at the other end into the intestine at the border of the middle and posterior sections. These tubules are located in the body cavity, and, being washed by hemolymph, they suck up decay products from it and bring them into the intestine.
Respiratory system arranged quite differently. Crustaceans have true gills. They are branched outgrowths on the limbs, covered with a thin chitinous cuticle, through which gas exchange occurs. Some crustaceans have adapted to live on land (for example, wood lice).
Spiders and scorpions have respiratory organs leaf-shaped lungs, which open outwards with holes (stigmas). Inside the lung sac has numerous folds. In addition to the lung sac, some spiders have a system of tracheal tubes that practically do not branch.
Ticks, centipedes, and insects have a respiratory system tracheae, which open outwards with holes (spiracles, stigmas). The tracheae branch strongly and penetrate into all organs and tissues. The trachea has a thin chitinous lining and is reinforced from the inside with a chitinous spiral, which does not allow the tube to fall off. In addition, flying insects have extensions - air sacs that fill with air and reduce the specific gravity of the animal. Ventilation in the tracheal system is both passive (diffusion) and active (change in the volume of the abdomen).
Some insect larvae have special respiratory organs - tracheal gills. Gas exchange in such arthropods proceeds by diffusion.
Some ticks do not have a respiratory system, and gas exchange occurs through the entire surface of the body.
Circulatory system in all arthropods open I, that is, not everywhere the blood flows through the vessels. Under the chitinous cover of the back there is a heart from which blood vessels depart. However, at some distance from the heart, the walls of the vessels disappear, and the blood makes its further path through the cracks between the internal organs. It then enters the heart through openings called ostia. Crustaceans and mites have a sac-like heart, while scorpions, spiders, and insects have a multi-chambered heart. Some ticks may not have a circulatory system.
The blood of the vast majority of arthropods is colorless and is commonly referred to as hemolymph. This is a rather complex fluid: it consists of both the blood itself and the cavity fluid. Due to the absence of special pigments, hemolymph practically cannot actively participate in the process of gas exchange. The hemolymph of some insects (leaf beetles, ladybugs) contains quite toxic substances and can play a protective role.
Fat body. Terrestrial arthropods have a storage organ - a fatty body located between the viscera. The fat body takes part in the regulation of water metabolism.
Nervous system. In general, in arthropods, the nervous system is built according to the type of annelids. It consists of a paired supraesophageal ganglion, peripharyngeal nerve ring and ventral nerve cord. Peripheral nerves depart from the ganglia of the chain. The supraesophageal ganglion in insects, in which the presence of a brain is usually said, reaches a special development. Often there is a concentration of ganglia of the abdominal nerve chain and the formation of large ganglions due to their fusion. Such a concentration is often associated with a decrease in the number of segments (merging them together). For example, in ticks that have lost segmentation, the abdominal chain turns into a common nerve mass. And in centipedes, whose body consists of many identical segments, the nerve chain is very typical.
sense organs most arthropods reach a high development.
organs of vision are located on the head and are often represented by complex (compound eyes), which occupy most of the surface of the head in some insects. Many crustaceans have compound eyes that sit on stalks. In addition, insects and arachnids have simple eyes. An unpaired frontal ocellus is characteristic of some crustaceans.
sense organs represented by various bristles and hairs located on the body and limbs.
Organs of smell and taste. Most of the olfactory endings are located on the antennae and jaw palps of insects, as well as on the antennulls of crustaceans. The sense of smell in insects is very well developed: 100 pheromone molecules per 1 cm 2 of air released by the female silkworm are enough for the male to start looking for a partner. The organs of taste in insects are located both on the mouth limbs and on the end segments of the legs.
Organs of balance. In crustaceans, in the main segment of the antennules, there is a statocyst - an invagination of the cuticle, seated from the inside with sensitive hairs. This cavity usually contains small grains of sand, which play the role of statoliths.
Hearing organs. Some insects have well-developed so-called tympanal organs that perceive sounds. For example, in grasshoppers, they are located on the bases of the shins of the front legs. As a rule, those insects that are able to perceive sounds are also able to make them. These include many orthoptera, some beetles, butterflies, etc. For this, insects have special devices located on the body, wings and limbs.
Spinning glands. Some arthropods are characterized by the presence of spinning glands. In spiders, they are located in the abdomen and open with arachnoid warts at the tip of the abdomen. Spiders use their webs most often for hunting and building shelters. This thread is one of the strongest in nature.
In the larvae of a number of insects, the spinning glands are located in the anterior part of the body and open near the mouth opening. Their cobweb goes mostly to build a shelter or cocoon.
Sexual system. Arthropods are dioecious animals that often have sexual dimorphism. Males differ from females in brighter coloration and often smaller size. In male insects, the antennae are much more developed.
reproductive system females consists of glands - ovaries, oviducts and vagina. This also includes accessory glands and seminal receptacles. Of the external organs, an ovipositor of various structures may be present.
At males reproductive organs are represented by testes, efferent ducts and accessory glands. A number of forms have differently arranged copulatory organs.
Polymorphism. In the colonies of social insects there are individuals that differ from each other in structure, physiology and behavior. In the nests of bees, ants and termites, there is usually only one female capable of laying eggs (womb or queen). Males in the colony are either constantly present, or appear as the sperm reserve in the uterus from the previous mating is depleted. All other individuals are called workers, which are females with depressed sexual function. In termites and ants, workers are divided into castes, each of which performs a specific function (gathering food, protecting the nest, etc.). The appearance of males and full-fledged females in the nest occurs only at a certain time.
Biology of reproduction. As already mentioned, arthropods are dioecious animals. However, among them, cases of parthenogenesis (aphids, daphnia) are not uncommon. Sometimes mating is preceded by a courtship ritual, and even fights between males for a female (in stag beetles). After mating, the female sometimes eats the male (mantises, some spiders).
Most often, eggs are laid in groups or one at a time. In some arthropods, the development of eggs and larvae occurs in the body of the female. In these cases, there is a live birth (scorpions, some flies). In the life of many species of arthropods, care for offspring takes place.
Fertility arthropod varies over a very wide range and depends very often on environmental conditions. In some aphids, for example, females lay only one overwintering egg. A honey bee queen can lay up to 3,000 eggs a day, and a termite queen can lay up to 30,000 eggs a day. These insects lay millions of eggs during their lifetime. On average, fertility is several tens or hundreds of eggs.
Development. In most arthropods, development occurs with metamorphosis, that is, with transformation. A larva emerges from the egg, which, after several molts, the larva turns into an adult animal (imago). Often the larva is very different from the adult both in structure and in lifestyle.
In the development cycle of a number of insects, there is pupal phase(butterflies, beetles, flies). In this case, one speaks of complete metamorphosis. Others (aphids, dragonflies, bugs) do not have such a phase, and the metamorphosis of these insects is called incomplete.
Some arthropods (spiders, scorpions) have direct development. In this case, fully formed young animals emerge from the eggs.
Lifespan arthropods is usually calculated in several weeks or months. In some cases, development is delayed for years. For example, the larvae of May beetles develop for about 3 years, deer beetles - up to 6 years. In cicadas, the larvae live in the soil for up to 16 years, and only after that they turn into adult cicadas. Mayfly larvae live in water bodies for 1-3 years, and an adult insect lives only a few hours, during which it manages to mate and lay eggs.
Distribution and ecology. Representatives of the arthropod type are found in almost any biotope. They are found on land, in fresh and salt water, and in the air. Among arthropods, there are both widespread and endemic species. The first include the cabbage white butterfly, crustaceans - daphnia, soil mites. Endemic species include, for example, a large and very beautiful butterfly brameya, which is found only in the Colchis lowland.
The distribution of individual species is limited by various environmental factors.
From abiotic factors the most important are temperature and humidity. The temperature limits of the active existence of arthropods lie in the range from 6 to 42°C. With a decrease or increase in temperature, animals fall into a state of stupor. Different phases of development of arthropods tolerate temperature fluctuations in different ways.
The humidity of the environment also largely determines the possibility of the existence of arthropods. Excessively low humidity of the environment, as well as high, can lead to death. For aquatic arthropods, the presence of liquid moisture is a necessary condition for active existence.
The distribution of arthropods is also greatly influenced by human activities ( anthropogenic influence). Changing environmental conditions lead to a change in species composition. As a result of human industrial and agricultural activities, some species disappear, while other species multiply extremely rapidly, becoming pests.
Origin. Most researchers agree that arthropods descended from ancestors close to annelids. Crustaceans, chelicerae, and extinct trilobites are thought to have evolved from annuli by one common root, and centipedes and insects by another.
Paleontological material on arthropods is very extensive. Thanks to the chitinous cuticle, their remains are quite well preserved in a petrified form. Terrestrial arthropods are exceptionally well preserved in amber as well. However, despite this, it is difficult to accurately trace the evolution of arthropods: the distant ancestors of arthropods in the geological layers have not been preserved. Therefore, the main methods of studying this issue are comparative anatomical and comparative embryological.
In practical human activity, it is customary to distinguish between beneficial and harmful species.
The type of annelids, or annelids, covers about 9 thousand species of worms, which have a much more complex organization than representatives of other types of worms.
Certain structural features of the larvae, which are very reminiscent of the larval forms of free-living flatworms (the body is not divided into segments and is covered with ciliated epithelium), suggest that the annulus, like roundworms, originated from primitive flatworms, similar in structure to modern ciliary worms. . This happened over 600 million years ago.
The body of most forms consists of separate rings - segments. Many rings are characterized by the presence of mobile lateral outgrowths of the body of parapodia and tufts of setae, which are the prototype of limbs. Some annelids have skin outgrowths - gills - on the dorsal part of the parapodia.
External segmentation corresponds to the division of the internal cavity of the body by partitions into separate sections and the segmental arrangement of a number of internal organs. The nerve ganglions, annular blood vessels, excretory organs - metanephridia, midgut pockets and genitals are correctly repeated. The skin-muscle sac consists of the cuticle, epithelium, annular and longitudinal muscles, as well as the inner lining of the body cavity.
The nervous system is represented by a near-pharyngeal nerve ring with a well-developed supra-esophageal and less pronounced sub-pharyngeal nerve nodes, as well as an abdominal nerve chain that forms nodes in each segment of the body. Numerous nerves depart from them. The sense organs are better developed in polychaete annelids and are represented by one or two pairs of eyes located on the dorsal side of the first segment.
The circulatory system is closed, consisting of vessels, some of which have contracting walls (“hearts”), which ensures blood circulation. Some groups do not have a circulatory system. The blood of a number of forms contains hemoglobin.
Breathing is carried out in most cases by the entire surface of the body, some have special outgrowths - skin gills.
The digestive system is end-to-end, complex, divided into the pharynx, esophagus, stomach and intestine, sometimes with lateral outgrowths; ends with an anus.
The excretory system is represented by segmentally arranged metanephridia. Their funnel faces the body cavity, and the other end opens outward.
Annelids reproduce sexually and asexually by budding. Among the rings there are dioecious species and hermaphrodites. Some rings have a rather complex reproductive system, while others do not have special genital organs - germ cells are formed from the inner lining of the body cavity and are brought out through the metanephridia.
The type combines several classes, of which the three main ones are Polychaete, Few-bristle and Leeches.
general characteristics
Type Annelids - an extensive group (12 thousand species). It includes deuterated animals, whose body consists of repeating segments, or rings. The circulatory system in annelids is closed. Compared with roundworms, annelids have a more advanced nervous system and sensory organs. The main features of this group need to be told in more detail.
The secondary cavity of the body, or the whole (from the Greek koiloma - “deepening”, “cavity”), develops in the embryo from the mesoderm layer. This is the space between the body wall and internal organs. Unlike the primary body cavity, the secondary cavity is lined from the inside with its own internal epithelium. The whole is filled with a liquid that creates a constancy of the internal environment of the body. Due to the pressure of the liquid, the secondary cavity maintains a certain shape of the body of the worm and serves as a support during movement. In other words, the whole serves as a hydroskeleton. Coelomic fluid is involved in metabolism: it carries nutrients, accumulates and removes harmful substances, and also removes reproductive products.
Annelids have a segmented body: it is divided into successive sections - segments, or rings (hence the name - annelids). There may be several or hundreds of such segments in different species. The body cavity is divided into segments by transverse partitions. Each segment is an independent compartment: it has its own external outgrowths, nodes of the nervous system, excretory organs and sex glands.
The phylum Annelids includes the Polychaete worms and the Olichaete worms.
Habitat, structure and activity of polychaete worms
About 7000 species of polychaete worms are known. Most of them live in the seas, a few live in fresh waters, in the litter of tropical forests. In the seas, polychaete worms live at the bottom, where they crawl among stones, corals, thickets of marine vegetation, and burrow into the silt. Among them are sessile forms that build a protective tube and never leave it (Fig. 62). There are planktonic species. Polychaete worms are found mainly in the coastal strip, but sometimes at a depth of up to 8000 m. In some places, up to 90 thousand polychaete worms live on 1 m2 of the seabed. They are eaten by crustaceans, fish, echinoderms, coelenterates, birds. Therefore, some polychaete worms were specially bred in the Caspian Sea as food for fish.
Rice. 62. Various polychaete annelids: 1 - sessile marine worm: 2 - nersis; 3 - sea mouse; 4 - sandstone
The body of polychaete worms is elongated, slightly flattened in the dorsal-abdominal direction, or cylindrical, from 2 mm to 3 m. Like all annelids, the body of polychaete consists of segments, the number of which varies from 5 to 800 in different species. there is a head section and an anal lobe.
On the head of these worms are a pair of palps, a pair of tentacles and antennae. These are the organs of touch and chemical sense (Fig. 63, A).
Rice. 63. Nersis: A - head department; B - parapodia (transverse section); B - larva; 1 - tentacle; 2 - palp; 3 - antennae; 4 - eye: 5 - bristles
On the sides of each segment of the body, skin-muscular outgrowths are noticeable - organs of movement, which are called parapodia (from the Greek para - “near” and podion - “leg”) (Fig. 63, B). Parapodia have a kind of reinforcement - bundles of bristles that contribute to the rigidity of the organs of movement. The worm rakes its parapodia from front to back, clinging to the irregularities of the substrate, and thus crawls forward.
In sessile forms of worms, a partial reduction (contraction) of the parapodia occurs: often they are preserved only in the anterior part of the body.
The body of polychaete worms is covered with a single-layered epithelium. In sessile forms of worms, the secretions of the epithelium can harden, forming a dense protective sheath around the body. The skin-muscular sac consists of a thin cuticle, skin epithelium and muscles (Fig. 64, A). Under the skin epithelium there are two layers of muscles: transverse, or annular, and longitudinal. Under the layer of muscles there is a single-layer internal epithelium, which lines the secondary cavity of the body from the inside and forms partitions between the segments.
Rice. 64. Transverse (A) and longitudinal (B) sections through the body of the Nereis (arrows show the movement of blood through the vessels): 1 - parapodim; 2 - longitudinal muscles; 3 - circular muscles: 4 - intestine; 5 - abdominal nerve chain; 6 - dorsal blood vessel; 7 - abdominal blood vessel; 8 - mouth opening; 9 - pharynx; 10 - brain
Digestive system begins with the mouth, which is located on the ventral side of the head lobe. In the section following the mouth, the muscular pharynx, many predatory worms have chitinous teeth that serve to grasp prey. The pharynx is followed by the esophagus and stomach. The intestine consists of three sections: the anterior, middle and hindgut (Fig. 64, B). The midgut looks like a straight tube. It digests and absorbs nutrients. Fecal masses are formed in the hindgut. The anal opening is located on the anal lobe. Stray polychaete worms are mainly predators, while sessile ones feed on small organic particles and plankton suspended in water.
Respiratory system. In polychaete worms, gas exchange (absorption of oxygen and release of carbon dioxide) is carried out either by the entire surface of the body, or by sections of parapodia, into which blood vessels enter. In some sessile forms, the corolla of tentacles on the head lobe performs the respiratory function.
The circulatory system of annelids is closed: in any part of the body of the worm, blood flows only through the vessels. There are two main vessels - dorsal and abdominal. One vessel passes over the intestine, the other - under it (see Fig. 64). They are connected to each other by numerous semicircular vessels. There is no heart, and the movement of blood is provided by contractions of the walls of the spinal vessel, in which blood flows from back to front, in the abdominal - from front to back.
excretory system represented by paired tubules located in each segment of the body. Each tubule begins with a wide funnel facing the body cavity. The edges of the funnel are covered with shimmering cilia. The opposite end of the tubule opens outward on the lateral side of the body. With the help of a system of excretory tubules, waste products that accumulate in the coelomic fluid are excreted outside.
Nervous system consists of paired supraesophageal, or cerebral, nodes (ganglia), connected by strands into the peripharyngeal ring, a paired abdominal nerve chain and nerves extending from them.
sense organs most developed in vagrant polychaete worms. Many of them have eyes. The organs of touch and chemical sense are located on the antennae, antennae and parapodia. There are organs of balance. Touch and other stimuli act on sensitive skin cells. The excitation that has arisen in them is transmitted along the nerves to the nerve nodes, from them along other nerves to the muscles, causing their contraction.
Reproduction. Most polychaete worms have separate sexes. Sex glands are present in almost every segment. Mature sex cells (in females - eggs, in males - spermatozoa) first enter the whole, and then through the tubules of the excretory system - into the water. Fertilization is external. A larva develops from the egg (see Fig. 63, B), which swims with the help of cilia. Then she settles to the bottom and turns into an adult worm. Some species also reproduce asexually. In some species, the worm divides across, and each half restores the missing part. In others, the daughter individuals do not diverge, and as a result, a chain is formed, including up to 30 individuals, but then it breaks up.
FEDERAL AGENCY FOR EDUCATION
TAMBOV STATE UNIVERSITY
NAMED AFTER G.R. DERZHAVIN
Test
in biology
on the topic: General characteristics and classification of annelids
Completed by a student
1st year of distance learning
Faculty of Geography
Petropavlovskaya Olesya Sergeevna
(To check Koryakin V.V.)
TYPE RINGED WORMS
( ANNELIDA )
Annelids include primary annulus, polychaete and oligochaete worms, leeches and echiurids. In the type of annelids, there are about 8 thousand species. The most primitive marine primary rings are archiannelids. Polychaete rings and echiurids are inhabitants of the sea. Small-bristle rings and leeches are mainly inhabitants of fresh water and soil.
Structure. Annelids are the most organized representatives of worms. The sizes of the rings range from fractions of a millimeter to 2.5 m. These are mainly free-living forms. The body of annuli is subdivided into three parts: the head, the body, consisting of rings, and the anal lobe. Such a clear division of the body into sections is not found in animals that are lower in their organization.
The head of the rings is equipped with various sense organs. Many ringlets have well developed eyes. Some have particularly sharp eyesight, and their lens is capable of accommodation. The eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. The rings also have developed taste sensations. On the head and tentacles of many of them there are special olfactory cells and ciliary pits that perceive various smells and the action of many chemical stimuli. The organs of hearing, arranged according to the type of locators, are well developed in the rings. Recently, auditory organs, very similar to those of the lateral line in fish, have been discovered in Echiurid marine rings. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds, which are heard much better in water than in air.
The body of the rings consists of rings, or segments. The number of rings can reach several hundred. Other rings consist of only a few segments. Each segment to some extent represents an independent unit of the whole organism. Each segment includes parts of vital organ systems.
Special organs of movement are very characteristic of rings. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "feet-like". Parapodia are lobe-shaped outgrowths of the body, from which bundles of bristles stick out. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all annulus. They are present in primary annulus and polychaete worms. In oligochaetes, only bristles remain. The primitive leech acanthobdella has bristles. The rest of the leeches do without parapodia and bristles in motion. Echiurids do not have parapodia, and have setae only at the posterior end of the body.
Parapodia, nodes of the nervous system, excretory organs, gonads, and, in some polychaetes, paired pockets of the intestine, are systematically repeated in each segment. This internal segmentation coincides with the external annulus. The repeated repetition of body segments is named by the Greek word "metamerism". Metamerism arose in the process of evolution in connection with the elongation of the body of the ancestors of the annulus. The elongation of the body necessitated repeated repetition, first of the organs of movement with their muscles and nervous system, and then of the internal organs.
The segmented secondary cavity of the body, or the whole, is extremely characteristic of the rings. This cavity is located between the intestines and the body wall. The body cavity is lined with a continuous layer of epithelial cells, or coelothelium. These cells form a layer that covers the intestines, muscles, and all other internal organs. The body cavity is divided into segments by transverse partitions - dissipations. A longitudinal septum passes along the midline of the body - the mesentery, which divides each compartment of the cavity into the right and left parts.
The body cavity is filled with a liquid that is chemically very similar to sea water. The fluid filling the body cavity is in continuous motion. The body cavity and cavity fluid perform important functions. The cavity fluid (like any fluid in general) does not compress and therefore serves as a good "hydraulic skeleton". The movement of the cavity fluid can transport various nutritious products, secretions of the endocrine glands, as well as oxygen and carbon dioxide involved in the breathing process inside the body of the rings.
Internal partitions protect the body in case of severe injuries and ruptures of the body wall. For example, an earthworm cut in half does not die. The partitions prevent the cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. But not all annelids have well-developed septa in the body cavity. For example, in Echiurids, the body cavity does not have partitions. A puncture of the body wall of an echiurida can lead to its death. In addition to the respiratory and protective role, the secondary cavity acts as a receptacle for the reproductive products, which mature there before being brought out.
Rings, with few exceptions, have a circulatory system. However, they have no heart. The walls of large vessels themselves contract and push blood through the thinnest capillaries. In leeches, the functions of the circulatory system and the secondary cavity coincide so much that these two systems are combined into a single network of lacunae through which blood flows. In some rings, the blood is colorless, in others it is colored green by a pigment called chlorcruorin. Often the rings have red blood, similar in composition to the blood of vertebrates. Red blood contains iron, which is part of the hemoglobin pigment. some rings, burrowing into the ground, experience an acute oxygen deficiency. Therefore, their blood is adapted to bind oxygen especially intensively. For example, the polychaete Magelonapapapillicornis has developed the pigment hemerythrin, which contains five times more iron than hemoglobin.
In annuli, compared with lower invertebrates, metabolism and respiration proceed much more intensively. Some polychaete rings develop special respiratory organs - gills. In the gills, a network of blood vessels branches, and through their wall oxygen penetrates into the blood, and then spreads throughout the body. The gills can be located on the head, on the paropodia and on the tail. The end-to-end intestine of the annulus consists of several sections. Each section of the intestine has its own specific function. The mouth leads to the throat. Some ringlets have strong horny jaws and denticles in the pharynx, helping to grasp live prey more firmly. In many predatory rings, the throat serves as a powerful weapon of attack and defense. The esophagus follows the pharynx. This department is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push the food into the following sections. In the wall of the esophagus there are glands, the enzyme of which serves for the primary processing of food. The esophagus is followed by the midgut. In some cases, goiter and stomach are developed. The wall of the midgut is formed by an epithelium very rich in glandular cells that produce a digestive enzyme. Other cells in the midgut absorb the digested food. In some annulus, the midgut is in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths from the sides of the intestine. The hindgut ends with an anus.
Special organs - metanifridia - serve to release the germ cells - sperm and eggs. Metanephridia begin as a funnel in the body cavity; a convoluted canal runs from the funnel, which opens outward in the next segment. Each segment contains two metanephridia.
Reproduction. Ringworms reproduce asexually and sexually. Aquatic rings frequently reproduce asexually. At the same time, their long body breaks into several parts. After a while, each part regenerates its head and tail. Sometimes a head with eyes, tentacles, and a brain forms in the middle of the worm's body before it splits apart. In this case, the detached parts already have a head with all the necessary sense organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These rings have lost their segmented body cavity. This is partly why, apparently, they do not have the ability to reproduce asexually and restore lost parts.
Fertilization of eggs in sea rings occurs most often outside the body of the mother's organism. In this case, males and females simultaneously release germ cells into the water, where fertilization takes place.
In marine polychaetes and echiurids, the crushing of fertilized eggs leads to the development of a larva that does not at all resemble adult animals and is called a trochophore. Trochophore lives for a short time in the surface layers of water, and then settles to the bottom and gradually turns into an adult organism. Freshwater and terrestrial rings are most often hermaphrodites and have direct development. Dense shells here protect the eggs from mechanical damage and from drying out under the scorching rays of the sun.
Practical value. In Russia, for the first time in the history of world science, acclimatization of some invertebrates was carried out to strengthen the food supply of the sea. For example, the polychaete Nereis, acclimatized in the Caspian Sea, has become the most important food item for sturgeon and other fish.
Earthworms not only serve as bait for fishing and food for birds. They bring great benefits to man, loosening the soil, making it more porous. This favors the free penetration of air and water to the roots of plants and increases crop yields. Rummaging in the ground, the worms swallow pieces of soil, crush them and throw them to the surface well mixed with organic matter. The amount of soil brought to the surface by worms is amazingly large. If the soil plowed by earthworms every 10 years were distributed over the entire surface of the land, then a layer of fertile earth 5 cm thick would be obtained.
The type of annelids, uniting about 12,000 species, is, as it were, a node of the genealogical tree of the animal world. According to existing theories, annelids originate from ancient ciliary worms (turbellar theory) or from forms close to ctenophores (trochophore theory). In turn, arthropods arose from annelids in the process of progressive evolution. Finally, in their origin, annelids are connected by a common ancestor with mollusks. All this shows the great importance that the type under consideration has for understanding the phylogeny of the animal world. Medically, annelides are of limited value. Only leeches are of some interest.
General characteristics of the type
The body of annelids consists of a head lobe, a segmented body, and a posterior lobe. Segments of the trunk throughout almost the entire body have external appendages similar to each other and a similar internal structure. Thus, the organization of annelids is characterized by structural repeatability, or metamerism.
On the sides of the body, each segment usually has external appendages in the form of muscular outgrowths equipped with bristles - parapodia - or in the form of setae. These appendages are important in the movement of the worm. Parapodia in the process of phylogenesis gave rise to the limbs of arthropods. At the head end of the body there are special appendages - tentacles and palygs.
A skin-muscular sac is developed, which consists of a cuticle, one layer of skin cells and several layers of muscles underlying it (see Table 1) and a secondary body cavity, or coelom, in which internal organs are located. The whole is lined with peritoneal epithelium and divided by septa into separate chambers. At the same time, each segment of the body has a pair of coelomic sacs (only the head and posterior lobes are devoid of the coelom).
The coelomic sacs in each segment are placed between the intestine and the body wall and are filled with a watery fluid in which the amoeboid cells float.
In general, it performs a supporting function. In addition, nutrients from the intestines enter the coelomic fluid, which are then distributed throughout the body. In general, harmful metabolic products accumulate, which are removed by the excretory organs. Male and female gonads develop in the walls of the coelom.
The central nervous system is represented by the supraesophageal ganglion and the ventral nerve cord. Nerves from the sense organs pass to the supraglottic node: eyes, balance organs, tentacles and palps. The abdominal nerve cord consists of nodes (one pair in each segment of the body) and trunks that connect the nodes to each other. Each node innervates all the organs of this segment.
The digestive system consists of the anterior, middle and hindgut. The foregut is usually divided into a number of sections: the pharynx, esophagus, crop and gizzard. The mouth is on the ventral side of the first body segment. The hindgut opens with an anus on the posterior lobe. In the wall of the intestine there is a musculature that ensures the movement of food.
The organs of excretion - metanephridia - are paired tubular organs, metamerically repeated in body segments. Unlike protonephridia, they have a through excretory canal. The latter begins with a funnel that opens into the body cavity. The cavity fluid enters the nephridium through the funnel. A tubule of nephridium departs from the funnel, sometimes opening outwards. Passing through the tubule, the liquid changes its composition; it concentrates the end products of dissimilation, which are ejected from the body through the outer pore of the nephridium.
For the first time in the phylogenesis of the animal kingdom, annelids have a circulatory system. The main blood vessels run along the dorsal and ventral sides. In the anterior segments they are connected by transverse vessels. The dorsal and anterior annular vessels are able to contract rhythmically and perform the function of the heart. In most species, the circulatory system is closed: blood circulates through a system of vessels, nowhere interrupted by cavities, lacunae or sinuses. In some species, the blood is colorless, in others it is red due to the presence of hemoglobin.
Most species of annelids breathe through skin rich in blood capillaries. A number of marine forms have specialized respiratory organs - gills. They usually develop on the parapodia or on the palps. Vessels carrying venous blood approach the gills; it is saturated with oxygen and enters the body of the worm in the form of arterial blood. Among annelids there are dioecious and hermaphroditic species. The sex glands are located in the body cavity.
Annelids have the highest organization in comparison with other types of worms (see Table 1); for the first time they have a secondary body cavity, a circulatory system, respiratory organs, and a more highly organized nervous system.
| Table 1. Characteristic features of various types of worms | ||||||
| Type | Skin-muscular sac | Digestive system | Circulatory system | reproductive system | Nervous system | body cavity |
| flatworms | Includes layers of longitudinal and circular muscles, as well as bundles of dorso-abdominal and diagonal muscles | From the ectodermal foregut and endodermal midgut | not developed | hermaphroditic | Paired brain ganglion and several pairs of nerve trunks | Absent, filled with parenchyma |
| roundworms | Only longitudinal muscles | From the ectodermal foregut and hindgut and endodermal midgut | Same | Dioecious | Periopharyngeal nerve ring and 6 longitudinal trunks | Primary |
| From external circular and internal longitudinal muscles | From the ectodermal foregut and hindgut and endodermal midgut | Well developed, closed | Dioecious or hermaphrodites | Paired brain ganglion, peripharyngeal nerve ring, ventral nerve cord | Secondary | |
|
Animals belonging to the type of annelids, or annelids, are characterized by:
Ringed worms live in fresh and marine waters, as well as in the soil. Several species live in the air. The main classes of the type of annelids are:
Class polychaetal ringsFrom the point of view of the phylogenesis of the animal world, polychaetes are the most important group of annelids, since the emergence of higher groups of invertebrates is associated with their progressive development. The body of polychaetes is segmented. There are parapodia, consisting of dorsal and ventral branches, each of which bears a tendril. The muscular wall of the parapodia has thick supporting setae, and tufts of thin setae protrude from the apex of both branches. The function of the parapodia is different. Usually these are locomotor organs involved in the movement of the worm. Sometimes the dorsal barnacle grows and turns into a gill. The circulatory system of polychaetes is well developed and always closed. There are species with cutaneous and gill respiration. Polychaetes are dioecious worms. They live in the seas, mainly in the coastal zone. Nereid (Nereis pelagica) can serve as a characteristic representative of the class. It is found in abundance in the seas of our country; leads a bottom way of life, being a predator, captures prey with its jaws. Another representative - sandworm (Arenicola marina) - lives in the seas, digs holes. It feeds by passing sea silt through its digestive tract. Breathe with gills. Class low-bristle ringsThe oligochaetes are descended from polychaetes. The external appendages of the body are setae, which sit directly in the wall of the body; no parapodia. The circulatory system is closed; skin breathing. Small-bristle rings are hermaphrodites. The vast majority of species are inhabitants of fresh water and soil. An earthworm (Lumbricus terrestris) can serve as a characteristic representative of the class. Earthworms live in the soil; during the day they sit in holes, and in the evening they often crawl out. Rummaging in the soil, they pass it through their intestines and feed on the plant residues contained in it. Earthworms play an important role in soil-forming processes; they loosen the soil and contribute to its aeration; leaves are dragged into holes, enriching the soil with organic substances; they extract deep layers of soil to the surface, and superficial ones carry them deeper. The structure and reproduction of the earthworm The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments or segments. In the front third of the body of the earthworm there is a thickening - a girdle (its cells function during the period of sexual reproduction and oviposition). On the sides of each segment, two pairs of short elastic bristles are developed, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side. A characteristic feature of the internal structure is that earthworms have developed real tissues. Outside, the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells. Under the skin there is a well-developed musculature, consisting of a layer of annular and a more powerful layer of longitudinal muscles located under it. With the contraction of the circular muscles, the body of the animal is stretched and becomes thinner; with the contraction of the longitudinal muscles, it thickens and pushes the soil particles apart.
The digestive system begins at the front end of the body with a mouth opening, from which food enters sequentially into the pharynx, esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves that animals feed on). Then the food passes into an enlarged goiter, and a small muscular stomach (the muscles in its walls contribute to the grinding of food). From the stomach almost to the rear end of the body stretches the middle intestine, in which, under the action of enzymes, food is digested and absorbed. Undigested residues enter the short hindgut and are thrown out through the anus. Earthworms feed on half-decayed plant remains, which they swallow along with the earth. When passing through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than ordinary soil. The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and under it - the abdominal one. In each segment, they are united by an annular vessel. In the anterior segments, some annular vessels are thickened, their walls contract and rhythmically pulsate, due to which blood is distilled from the dorsal vessel to the abdominal one. The red color of blood is due to the presence of hemoglobin in the plasma. For most annelids, including earthworms, skin respiration is characteristic, almost all gas exchange is provided by the body surface, therefore earthworms are very sensitive to soil moisture and are not found in dry sandy soils, where their skin dries out soon, and after rains, when in soil a lot of water, crawl to the surface. The excretory system is represented by metanephridia. Metanephridium begins in the body cavity with a funnel (nephrostome) from which a duct extends - a thin loop-shaped curved tube that opens outward as an excretory pore in the side wall of the body. Each segment of the worm has a pair of metanephridia - right and left. The funnel and duct are equipped with cilia that cause the movement of excretory fluid. The nervous system has a structure typical of annelids (see Table 1), two ventral nerve trunks, their nodes are interconnected and form an ventral nerve chain. The sense organs are very poorly developed. The earthworm does not have real organs of vision, their role is performed by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Like hydra, earthworms are capable of regeneration. Reproduction occurs only sexually. Earthworms are hermaphrodites. In front of their body are the testes and ovaries. Fertilization of earthworms is cross. During copulation and oviposition, the cells of the girdle on the 32-37th segment secrete mucus, which serves to form the egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous sleeve. The worm crawls out of it with its rear end forward, laying eggs in the mucus. The edges of the muff stick together and a cocoon is formed, which remains in the earthen burrow. Embryonic development of eggs occurs in a cocoon, young worms emerge from it. The passages of earthworms are mainly in the surface layer of the soil to a depth of 1 m, for the winter they descend to a depth of 2 m. Through the minks and passages of earthworms, atmospheric air and water penetrate the soil, which are necessary for the roots of plants and the vital activity of soil microorganisms. Through its intestines, the worm passes as much soil per day as its body weighs (an average of 4-5 g). On each hectare of land, earthworms daily process an average of 0.25 tons of soil, and annually they throw out to the surface in the form of excrement from 10 to 30 tons of the soil they have processed. In Japan, specially bred breeds of fast-reproducing earthworms are bred and their excrement is used for the biological method of tillage. Vegetables and fruits grown on such soil have an increased sugar content. Charles Darwin was the first to point out the important role of earthworms in soil formation processes. Annelids play a significant role in the nutrition of bottom fish, since in some places worms make up to 50-60% of the biomass of the bottom layers of water bodies. In 1939-1940. The nereis worm was moved from the Sea of Azov to the Caspian Sea, which now forms the basis of the diet of sturgeons in the Caspian Sea.
Leech classThe body is segmented. In addition to true metamerism, there is false ringing - several rings in one segment. Parapodia and setae absent. The secondary body cavity was reduced; instead, there are sinuses and gaps between the organs. The circulatory system is not closed; blood only part of its path passes through the vessels and pours out of them into the sinuses and lacunae. There are no respiratory organs. The reproductive system is hermaphrodite. Medical leeches are specially bred and then sent to hospitals. They are used, for example, in the treatment of eye diseases associated with an increase in intraocular pressure (glaucoma), with cerebral hemorrhage and hypertension. With thrombosis and thrombophlebitis, hirudin reduces blood clotting and promotes the dissolution of blood clots. |
