Organs of annelids. Type annelids classes taxonomy
Type annelids unites about 9,000 species that have the most perfect organization among other worms. Their body consists of a large number of segments; many have bristles on the sides of each segment, which play an important role in movement. Internal organs are located in a body cavity called coelom. There is a circulatory system. In the anterior part there is a cluster of nerve cells that form the subpharyngeal and suprapharyngeal nerve nodes. Annelids live in fresh water bodies, seas and soil.
Most of the representatives of annelids belong to the classes: oligochaetes, polychaetes and leeches.
Class oligochaetes
Representative of the class oligochaetes - earthworm lives in burrows in moist humus soil. The worm crawls to the surface in damp weather, at dusk and at night. In an earthworm, the anterior and abdominal parts of the body can be easily distinguished. In the anterior part there is a thickening of the girdle; on the ventral and lateral sides of the body, elastic and short bristles are developed.
The body of the worm is covered with skin made of integumentary tissue, in which the cells adhere tightly to each other. The skin contains glandular cells that secrete mucus. Under the skin there are circular and deeper - longitudinal muscles, thanks to the contraction of which the body of the worm can lengthen or shorten, thereby moving through the soil.
The skin and muscle layers form skin-muscle sac, inside which there is a body cavity where the internal organs are located. Earthworms feed on rotting plant debris. Through the mouth and pharynx, food enters the crop and muscular stomach, where it is ground and enters the intestine and is digested there. Digested substances are absorbed into the blood, and undigested substances are excreted along with the soil through the anus.
Circulatory system of an earthworm closed and consists of dorsal and abdominal blood vessels, connected to each other by ring vessels from each segment. Around the esophagus there are larger annular vessels that act as the “hearts” of the large vessels; side branches branch off, forming a network of capillaries. The blood does not mix anywhere with the fluid of the body cavity, which is why the system is called closed.
The excretory organs are represented by convoluted tubes through which liquid and harmful substances are removed from the body.
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. The earthworm does not have specialized sensory organs. There are only different kinds of sensitive cells that perceive external stimuli (light, smell, etc.).
Earthworms are hermaphrodites. However, they have cross-insemination; two individuals participate in this process. When eggs are laid on the worm's belt, copious mucus is formed, into which the eggs fall, after which the mucus darkens and hardens, forming a cocoon. Then the cocoon is thrown off the worm through the head end of the body. Inside the cocoon, young worms develop from fertilized eggs.
Among the oligochaetes there are dwarfs, whose body length does not exceed a few millimeters, but there are also giants: Australian earthworm 2.5-3 m long.
Characteristic of earthworms regenerative ability. Earthworms are called soil formers, since they, by making passages in the soil, loosen it and promote aeration, that is, the entry of air into the soil.
Class polychaetes
This includes a variety of marine worms. Among them nereid. Her body consists of a large number of segments. The anterior segments form the head section, on which the mouth and sensory organs are located: touch - tentacles, vision - eyes. On the sides of the body, each segment has lobes on which numerous bristles sit in tufts. With the help of blades and bristles, Nereids swim or move along the bottom of the sea. They feed on algae and small animals. Breathe with the entire surface of the body. Some polychaetes have gills- primitive respiratory organs.
Refers to polychaetes sandstone, living in burrows, in the sand, or building itself a gypsum turtle, which is attached to algae. Many marine fish feed on nereids and other annelids.
Leech class
The most famous representative of this class is medical leech, which was already used in ancient times to treat people. Leeches are characterized by the presence of two suckers: the front one, at the bottom of which the mouth is located, and the back one.
The posterior sucker is large, its diameter exceeding half the greatest width of the body. Leeches bite through the skin with three jaws lined with sharp teeth along the edges (up to 100 on each jaw). Strong bloodsucker. In medicine, it is used for diseases of blood vessels (formation of blood clots), hypertension, and pre-stroke conditions. Leeches are applied to a certain part of a sick person to suck out blood; as a result, blood clots dissolve, blood pressure decreases, and the person’s condition improves. In addition, the salivary glands of the medicinal leech produce a valuable substance - hirudin, - preventing blood clotting. Therefore, after leech injections, the wound bleeds for a long time. While in the leech's stomach, the blood, under the influence of hirudin, is preserved for months without being subject to coagulation and rotting.
The leech's digestive system is designed in such a way that it can accumulate large reserves of blood, preserved with the help of hirudin. The size of a leech that has sucked blood increases significantly. Thanks to this feature, leeches can starve for a long time (from several months to 1 year). A leech lives up to 5 years. Leeches are hermaphrodites. In nature I achieve! They reach sexual maturity only in the third year of life and lay cocoons once a year in the summer.
Leeches are characterized by a straight development. Leeches include the non-blood-sucking predatory leech - large pseudokonskaya. It eats worms (including leeches), soft-bodied animals, larvae of aquatic insects, small vertebrates (tadpoles), which it can overcome.
Annelides are bilaterally symmetrical segmented animals.
Taxonomy. The phylum includes 5 classes, of which the most famous classes are Polychaeta - 13,000 species, Olygochaeta - 3,500 species and Leeches (Hirudinea) - about 400 species.
Body shape and size. The body of ringlets is overwhelmingly worm-shaped, round or oval in cross section. The body has pronounced both external and internal segmentation. In this case they talk about true metamerism. In this case, metamerism also 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).
External body structure. Polychaetes have a well-defined head section, bearing organs for various purposes: tentacles, ocelli, palps. In some species, the palps grow into a complex trapping apparatus. The last segment contains one or more pairs of sensory antennae. Each body segment bears parapodia on the sides - complex outgrowths of the body. The main function of these outgrowths is the movement of the worm. Each parapodia consists of two lobes, inside of which there are numerous setae. Of these, several are larger, they are called aciculi. A pair of sensitive antennae are attached to the blades. The parapodia often includes the gill apparatus. Parapodia have a fairly diverse structure.
In oligochaete worms, the head section is weakly expressed, and there are no lateral projections (parapodia). There are only relatively few setae. A “belt” consisting of thickened segments is clearly visible on the body.
Leeches have powerful suckers at the front and rear ends of their bodies. Few species have gill projections on the sides.
Skin-muscle bag. On the outside, the body of annelids is covered with a thin cuticle, under which lie skin epithelial cells. The skin of worms is rich in glandular cells. The secretion of these cells has a protective value. In a number of species, skin secretions are used to build unique houses. Worm bristles 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, which serve to change the length of the body. In leeches, between the layers of circular and longitudinal muscles there is a layer of diagonal muscles. The ringlets have special muscles that move parapodia, palps, suckers, etc.
Body cavity. The space between the body wall and the internal organs of the rings represents the coelom - the secondary body cavity. It differs from the primary one by the presence of its own epithelial walls, called coelomic epithelium (coelothelium). The coelothelium covers the longitudinal muscles of the body wall, intestines, muscle cords and other internal organs. On the walls of the intestine, the coelothelium is transformed into chloragogenic cells that perform an excretory function. In this case, the coelomic sac of each body segment is isolated from neighboring ones by partitions - dessepiments. Inside, the coelomic sac is filled with fluid containing various cellular elements. In general, it performs different functions - supporting, trophic, excretory, protective and others. In leeches, the coelom 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 coelom is preserved only in the form of narrow canals.
The midgut is shaped like a simple tube that can become more complex. Thus, in leeches and some polychaetes the intestine has lateral projections. In oligochaetes, on the dorsal side of the intestine there is a longitudinal fold that protrudes deeply into the intestinal cavity - typhlosol. These devices significantly increase the internal surface of the midgut, which allows for the most complete absorption of digested substances. The midgut is of endodermic origin. In oligochaete worms, at the border of the foregut and midgut 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 into the anus.
Circulatory system annelids are closed, that is, blood moves everywhere through the vessels. The main vessels are longitudinal - dorsal and abdominal, connected by circular ones. 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 through the spinal 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 tiny capillaries that carry blood to all the tissues of the worms. In leeches, the blood vessel system is significantly reduced. Blood moves through the system of sinuses - remnants of the coelom.
The blood of most annelids contains hemoglobin. This allows them to exist in conditions with little oxygen.
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 convoluted tube. This tube begins as a funnel, which opens into the 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 twists 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 driven into the metanephridium. As it moves through the tube through the gland, water and various salts are absorbed from the liquid, and only products that need to be removed from the body (urine) remain in the cavity of the tube. These products are excreted through the excretory pore. In many species, in the posterior part of the metanephridial tube there is an extension - the bladder, in which urine temporarily accumulates.
In primitive annelids, the excretory organs, like flatworms, are structured like 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 lies under the pharynx. The brain is connected to the subpharyngeal ganglia by nerve cords that cover the pharynx from the sides. This entire formation is called the peripharyngeal ring. Further, in each segment under the intestine there is a pair of nerve ganglia that are connected both to each other and to the ganglia of neighboring segments. This system is called the ventral nerve cord. Nerves extend from all ganglia to various organs.
Sense organs. The head section of polychaete worms has well-developed sensory organs: antennae and palps (organs of touch), eyes (sometimes quite complex), and olfactory pits. Some forms have developed balance organs - statocysts. On the lateral outgrowths of the body (parapodia) there are antennae that perform a tactile function.
In polychaete worms, the sensory organs are much less developed than in polychaete worms. There are chemical sense organs, sometimes tentacles, statocysts, and poorly developed eyes. The skin contains a large number of light-sensitive and tactile cells. Some tactile cells have a pin.
Leeches have many sensitive cells scattered throughout their skin; they also always have eyes and chemical sense organs (taste buds).
Reproductive system. Among annelids there are both hermaphroditic and dioecious forms.
Polychaete worms are mostly dioecious. Sometimes sexual dimorphism occurs. The sex glands (gonads) are formed in the coelomic epithelium. This process usually occurs in the posterior segments of the worm.
In oligochaete worms, hermaphroditism is more common. The gonads 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 separated from them. The larger female gonads (ovaries) have ducts that are modified 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 sperm of another worm. Leeches are mostly hermaphrodites. The testes are located metamerically, there is one pair of ovaries. Fertilization in leeches occurs through the exchange of spermatophores between partners.
Reproduction. Annelids have 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 of polychaetes, individuals containing mature gonads (epitocenes) switch from a crawling or sessile lifestyle to a swimming one. And in some species, the sexual segments, when the gametes mature, can even tear off from the body of the worm and lead an independent swimming lifestyle. Gametes enter the water through breaks in the body wall. Fertilization occurs either in water or in the epitocine segments of the female.
Reproduction of oligochaetes begins with cross-fertilization. At this time, the two partners touch each other with their ventral sides and exchange sperm, which enters the seminal receptacles. After which the partners separate.
Subsequently, abundant mucus is secreted on the girdle, forming a muff around the girdle. The worm lays eggs in this muff. When the coupling is moved forward, it passes past the openings of the seminal receptacles; At this moment, fertilization of the eggs occurs. When the sleeve with fertilized eggs slides off the head end of the worm, its edges close, and a cocoon is obtained in which further development occurs. An earthworm cocoon usually contains 1-3 eggs.
In leeches, reproduction occurs in approximately the same way as in oligochaete worms. Leech cocoons are large, reaching 2 cm in length in some species. Different species have from 1 to 200 eggs in the cocoon.
Development. The zygote of annelids undergoes complete, usually uneven, fragmentation. Gastrulation occurs by intussusception or epiboly.
In polychaete worms, a larva called a trochophore is subsequently formed from the embryo. She has eyelashes and is quite mobile. From this larva the adult worm develops. Thus, in most polychaete worms, development occurs with metamorphosis. Species with direct development are also known.
Oligochaete worms have direct development without a larval phase. Fully formed young worms emerge from the eggs.
In leeches, the eggs in the cocoon form peculiar larvae that swim in the cocoon liquid using the 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 weakly expressed.
Nutrition. Among polychaete worms there are both predators and herbivorous species. There are also known facts of cannibalism. Some species feed on organic debris (detritivores). Oligochaete worms are primarily detritivores, but predators are also found.
Oligochaete worms are mostly soil dwellers. In soils rich in humus, the number of, for example, enchytraeid worms reaches 100-200 thousand per square meter. They also live in fresh, brackish and salt water bodies. Aquatic inhabitants inhabit mainly surface layers of soil and vegetation. Some species are cosmopolitan, but there are also endemics.
Leeches inhabit fresh water bodies. Few species live in the seas. Some switched to a terrestrial lifestyle. These worms either lead an ambush lifestyle or actively seek out their hosts. A single blood sucking provides leeches with food for many months. There are no cosmopolitans among leeches; they are confined to certain geographical areas.
Paleontological finds annelids are very few in number. Polychaetes represent greater diversity in this regard. Not only prints have been preserved from them, but also, in many cases, remains of pipes. On this basis, it is assumed that all the main groups of this class were already represented in the Paleozoic. To date, no reliable remains of oligochaete worms and leeches have been found.
Origin. At present, the most plausible hypothesis is the origin of annelids from parenchymal ancestors (ciliated worms). Polychaetes are considered to be the most primitive group. It is from this group that the oligochaetes most likely originate, and from the latter the 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. Land worms play a leading role in soil formation. By processing plant residues, they enrich the soil with mineral and organic substances. Their passages help improve soil gas exchange and drainage.
In practical terms, a number of species of earthworms are used as vermicompost producers. The worm - enchytraea is used as food for aquarium fish. Enchitraevs are bred in huge quantities. For the same purposes, the tubifex worm is harvested from nature. Medicinal leeches are currently used to treat certain diseases. In some tropical countries they eat palolo– reproductive (epitocene) segments of worms that have separated from the front part of the animal 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 richest and most diverse group of animals.
Taxonomy. The phylum arthropods are divided into several subtypes.
Subtype Gill-breathing (class Crustaceans)
Subphylum Trilobites (extinct group)
Subphylum Cheliceraceae (class Merostomaceae, class Arachnidae)
Subtype Primary tracheal
Subtype Tracheine-breathing (class Centipedes, class Insects).
The class Merostomaceae includes modern horseshoe crabs and extinct Cancerscorpios. To subtype Primary tracheal These include small (up to 8 cm) tropical animals, 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 (horseshoe crabs). Terrestrial arthropods reach 15-30 cm. The wingspan of some butterflies exceeds 25 cm. Extinct crustacean scorpions 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 connected motionlessly, bears the oral organs and sensory organs. The head is movably or immovably connected to the next section - the chest.
Thoracic region carries walking limbs. Depending on the number of thoracic limb segments, there may be a different number. Insects also have wings attached to their chest. The breast segments are connected to each other either movably or immovably.
Abdomen contains most of the internal organs and most often consists of several segments, movably connected to each other. Limbs and other appendages may 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. Parts of the oral apparatus for the most part are highly modified limbs, adapted for eating almost any food. The apparatus may include 3-6 pairs of limbs.
Veils. The cuticle, consisting of chitin, is a derivative of the submerged epithelium - the hypodermis. Chitin performs a supporting and protective function. The cuticle can become saturated with calcium carbonate, thereby becoming a very strong shell, as happens, for example, in crustaceans. Thus, in arthropods, the body integument is an exoskeleton. The movable connection of the hard sections of the cuticle is ensured by the presence of membranous sections. The cuticle of arthropods is not elastic and cannot stretch as animals grow, so they periodically shed the old cuticle (molt) and, until the new cuticle hardens, increase in size.
Body cavity. During the process of embryonic development, coelomic sacs are formed in arthropods, but later they rupture and their cavity merges with the primary body cavity. This is how a mixed body cavity is formed - a mixocoel.
Musculature It is represented by separate muscle bundles that do not form a continuous muscle bag. The muscles are attached both directly to the inner wall of the body segments and to their internal processes that make up the internal skeleton. Musculature in arthropods striated.
Digestive system in arthropods, in general, it consists of the anterior, middle and posterior parts of the intestine. 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. The salivary glands open into the oral cavity, which very often produce a number of enzymes, including digestive ones. The anus usually opens at the posterior end of the body.
Excretory system in proto-aquatic arthropods (crustaceans) it is represented by special glands located in the head part 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 tubes are located in the body cavity, and, washed by the hemolymph, absorb decay products from it and remove them into the intestine.
Respiratory system arranged quite diversely. Crustaceans have real 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, woodlice).
Spiders and scorpions have respiratory organs leaf-shaped lungs, which open outwards with holes (stigmas). Inside the pulmonary sac has numerous folds. In addition to the pulmonary sac, some spiders have a system of tracheal tubes that have practically no branches.
In ticks, centipedes and insects, the respiratory system is represented by trachea, which open outward with openings (spiracles, stigma). The tracheae are highly branched 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 collapse. 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 occurs both passively (diffusion) and actively (change in abdominal volume).
Some insect larvae have special respiratory organs - tracheal gills. Gas exchange in such arthropods occurs 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, blood does not flow through the vessels everywhere. Under the chitinous covering of the back there is a heart from which blood vessels extend. However, at some distance from the heart, the walls of the blood vessels disappear, and the blood makes its further journey through the cracks between the internal organs. It then enters the heart through openings called ostia. Crustaceans and mites have a sac-shaped 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 usually called hemolymph. This is a rather complex liquid: it consists of both blood itself and cavity fluid. Due to the lack 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 fat body, located between the viscera. The fat body takes part in the regulation of water metabolism.
Nervous system. In general, arthropods have a nervous system similar to that of annelids. It consists of the paired suprapharyngeal ganglion, the peripharyngeal nerve ring and the ventral nerve cord. Peripheral nerves arise from the chain ganglia. The suprapharyngeal ganglion reaches particular development in insects, which are usually said to have a brain. Often there is a concentration of ganglia of the abdominal nerve chain and the formation of large nerve ganglia due to their fusion. This 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 in most arthropods they reach high development.
Organs of vision located on the head and are often represented by complex (faceted 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.
Organs of touch 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 maxillary palps of insects, as well as on the antennullae of crustaceans. The sense of smell in insects is very well developed: 100 pheromone molecules per 1 cm 2 of air secreted by a female silkworm are enough for the male to begin searching for a partner. The taste organs of insects are located both on the oral 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, lined with sensitive hairs from the inside. This cavity usually contains small grains of sand that act as statoliths.
Organs of hearing. Some insects have well-developed so-called tympanic organs that perceive sounds. For example, in grasshoppers they are located on the bases of the tibia of the front legs. As a rule, those insects that are able to perceive sounds are also able to produce 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 web is mostly used to build a shelter or cocoon.
Reproductive system. Arthropods are dioecious animals, which are often characterized by sexual dimorphism. Males differ from females in being brighter in color and often smaller in size. Male insects have much more developed antennae.
Reproductive system females consists of glands - ovaries, oviducts and vagina. This also includes accessory glands and spermatic receptacles. The external organs may contain an ovipositor of various structures.
U males reproductive organs are represented by testes, efferent ducts and accessory glands. A number of forms have differently arranged copulatory organs.
Polymorphism. In 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, as a rule, only one female capable of laying eggs (queen or queen). Males in the colony are either constantly present or appear as the queen’s supply of sperm 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 (collecting 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, cases of parthenogenesis (aphids, daphnia) are not uncommon among them. Sometimes mating is preceded by a courtship ritual, and even fights between males for the 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, viviparity occurs (scorpions, some flies). In the life of many arthropod species, care for offspring takes place.
Fertility arthropods fluctuates within very wide limits and very often depends on environmental conditions. In some aphids, for example, females lay only one overwintering egg. A honeybee queen can lay up to 3,000 eggs per day, while a termite queen can lay up to 30,000 eggs per day. During their lifetime, these insects lay millions of eggs. 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, and after several molts the larva turns into an adult animal (imago). Often the larva is very different from the imago 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 they talk about complete metamorphosis. Others (aphids, dragonflies, bedbugs) do not have such a phase, and the metamorphosis of these insects is called incomplete.
In some arthropods (spiders, scorpions) development is direct. In this case, fully formed young animals emerge from the eggs.
Lifespan arthropod life is usually calculated over several weeks or months. In some cases, development is delayed for years. For example, the larvae of May beetles develop for about 3 years, and for stag 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 reservoirs for 1-3 years, and the adult insect lives only a few hours, during which time it manages to mate and lay eggs.
Distribution and ecology. Representatives of the phylum arthropods are found in almost any biotope. They are found on land, in fresh and salt water bodies, and also in the air. Among arthropods there are both widespread species and endemics. The first include the cabbage white butterfly, crustaceans - daphnia, and soil mites. Endemic species include, for example, a large and very beautiful butterfly frame, 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 for the active existence of arthropods range from 6 to 42°C. When the temperature drops or rises, animals fall into a state of torpor. Different phases of arthropod development tolerate temperature fluctuations differently.
The humidity of the environment also largely determines the possibility of the existence of arthropods. Excessively low humidity, as well as high humidity, 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 activity ( anthropogenic influence). Changes in environmental conditions lead to changes 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 evolved from ancestors close to annelids. It is assumed that crustaceans, chelicerates and extinct trilobites descended from ringlets 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 fossilized form. Terrestrial arthropods are also exceptionally well preserved in amber. However, despite this, it is difficult to accurately trace the evolution of arthropods: the distant ancestors of arthropods have not been preserved in geological layers. Therefore, the main methods for studying this issue are comparative anatomical and comparative embryological.
In practical human activities, it is customary to distinguish between useful and harmful types.
As is known, invertebrate animals are a large group of animals that do not have an internal axial skeleton - a notochord or a vertebral column replacing it. These include unicellular animals, or protozoa, and multicellular animals (coelenterates, flat, round and annelid worms, mollusks, arthropods).
Origin of single-celled animals
The first living creatures arose in the sea and looked like tiny mucous lumps. They had neither nuclei, nor vacuoles, nor other formed parts of cells, but they could grow, absorbing nutrients from the environment, and multiply. As a result of natural selection, these primary organisms gradually became more complex, and subsequently the first single-celled organisms with nuclei arose from them. At the earliest stages of the evolution of living nature, they, in turn, gave rise to single-celled animals and primitive fungi. Most biologists consider their ancestors to be the oldest single-celled organisms - the simplest flagellates.
So, the first animals to appear on Earth were single-celled animals belonging to the protozoa. Among them there are not only unicellular, but also colonial forms.
Origin of multicellular animals
You know how multicellular animals differ from unicellular animals. How did multicellular animals originate? There are no exact data, since this happened hundreds of millions of years ago. Scientists believe that most likely some ancient colonial protozoa had their constituent cells arranged not in one, but in two layers. The cells of these layers found themselves in different conditions in relation to the external environment. Therefore, in the process of long historical development, under the influence of natural selection, they began to differ more and more from each other in structure and function. Some cells directly connected with the environment retained the functions of movement, protection, and food capture. Other cells, less connected with the environment, still performed the functions of digestion, which was carried out in the same way as in the hydra.
Origin of coelenterates
Of all multicellular organisms, the coelenterates have the simplest structure. They have no tissues; germ cells are very similar to whole single-celled organisms. It is believed that under the influence of evolutionary factors they originated from ancient colonial protozoa. The most ancient coelenterates lacked a skeleton and therefore were not preserved in fossil form.
Origin of flatworms
Origin of roundworms
The main feature that distinguishes roundworms from flatworms is the body shape, round in cross section, and the presence of a cavity in it. It is believed that roundworms evolved from ancient flatworms. Under the influence of variability, heredity and natural selection, they developed a body cavity and at the same time an anus, through which undigested food remains are removed from the body.
Origin of annelids
From the common ancestors of worms, under the influence of evolutionary factors, annelids also evolved. An important point in their evolution is the division of the body into segments (rings). Due to active movement, annelids have developed a circulatory system that supplies the body with nutrients and oxygen. Ancient annelids had a more complex structure compared to other worms.
Origin of shellfish
Mollusks are not similar to annelids either in external or internal structure. However, the development of embryos in the early stages occurs in exactly the same way: many species of marine gastropods have a larva very similar to the larva of marine polychaete worms. Thus, the origin of mollusks and annelids from common ancestors is evidenced by embryological data.
Origin of arthropods
Ancient arthropods - trilobites - resembled marine polychaete worms, but unlike them they had one pair of limbs on each ring of the body, similar to the legs of arthropods. They occupy an intermediate position between modern arthropods and ancient annelids.
The body of the ringlets is divided into the head section ( prostomium), the following rings (or segments, or metamers), the number of which is usually large (several dozen), and the posterior section (anal lobe, or pygidium). The head section of marine worms, called polychaetes, is well defined and bears various appendages: wide, narrow, etc. (Fig. 61). In freshwater and terrestrial ringlets, the head section is weakly expressed (Fig. 61). Several anterior rings may be fused with the prostomium. Body segments are usually similar in structure. This kind of division is called homonomic segmentation or homonomy metamerism. It is not only external, but deeply internal, since each segment is separated from neighboring segments by partitions and has a set of organs.
The skin consists of a single-layer epithelium and a thin cuticle secreted by it (Fig. 62). There are many glands in the skin that secrete mucus, which facilitates the movement of worms, and other secretions (for example, substances that help attract females to males in dioecious ringworms, poisonous to other animals, etc.).
Nervous system. This system is much better developed than that of other worms, and its structure very clearly reflects the division of the ringlet body into segments. Its central section consists, as a rule, of two head nodes lying on the dorsal side, peripharyngeal cords, which pass on the ventral side into a chain, usually very long and forming a node in each segment (Fig. 63, B), which explains its name. Thus, the abdominal chain was formed from two cords. In the lower forms of the type, the cords remain separated along their entire length and are connected by bridges, which resembles a ladder (Fig. 63, A). Such a system is less centralized, it is similar to the central nervous system of lower worms - flat and primitive (see Fig. 31, B, and 54).
The nodes and cords of typical annelids are much better developed and their structure is more complex than those of the latter. The entire central system of ringlets is separated from the epidermis, while in lower worms it is still connected to the epidermis. Each node of the abdominal chain innervates and affects the functioning of organs located in the ring where the node is located. The head nodes, better developed than the nodes of the chain, coordinate the work of the latter and, through them, the activity of the whole body. In addition, they innervate the eyes and other sensory organs located in the head of the body.
The senses are varied. Tactile cells are scattered in the skin, which are especially numerous on the appendages of the body. There are organs that perceive chemical irritations. All annelids have light-sensitive organs. The simplest of them are represented by special cells scattered throughout the skin. Therefore, almost all ringworms have skin that is sensitive to light stimulation. At the anterior end of the body, and in a number of leeches at the rear, the light-sensitive organs become more complex and turn into eyes. A number of forms have balance organs that are similar in structure to similar organs of jellyfish and other lower animals.
The progressive development of the nervous system of annelids provides more complex and energetic movements of their body, active work of all organ systems, better coordination of the functions of all parts of the body, more complex behavior and makes possible a more subtle adaptation of these animals in the environment.
Propulsion system. This system in annelids is more advanced than in previously studied worms. Ciliary movement is characteristic only of larvae; in adult forms, with rare exceptions, it is absent, and their movement is accomplished only through the work of muscles. The skin-muscle sac is developed much better than in flatworms and protocavitary worms (cf. Fig. 32, 53 and 62). Under the epidermis lies a well-developed layer of circular muscles (Fig. 62), consisting of long fibers with nuclei. When these muscles contract, the body of the worm becomes thinner and longer. Behind the circular muscles there is a much thicker layer of longitudinal muscles, the contraction of which shortens the body and makes it thicker. Unilateral contraction of the longitudinal and some other muscles leads to bending of the body and a change in the direction of movement. In addition, there are muscles running from the dorsal side to the abdominal side: muscles passing through the septa separating the rings; muscles of various appendages of the body, which play a supporting role in the movement of worms, etc. The strength of the muscles of the skin-muscular sac is great and allows worms to quickly penetrate deep into the ground. Many annelids can swim. The support for the muscles is mainly the hydroskeleton formed by the fluid of the body cavity, as well as border formations.
The movement of annelids is facilitated by auxiliary appendages (see Fig. 61, 62, 64): bristles(available in the vast majority of species) and parapodia(available in most sea worms). The bristles (see Fig. 62, 64, A, B) are solid formations of organic matter, a very complex carbohydrate - chitin, of different shapes, thickness and length. The bristles are formed and driven by special muscle bundles. The setae are arranged (singly or in tufts) in regular longitudinal rows on almost all rings of the worms. Parapodia (Fig. 64, B) are powerful lateral outgrowths of the body with well-developed muscles. The parapodia are movably connected to the body, and these appendages act like a simple lever. Each parapodia usually consists of two lobes: dorsal and ventral, which, in turn, can be divided into second-order lobes. Inside each of the main blades there is a supporting bristle. The parapodia bear tufts of bristles that extend far beyond the body. The parapodium has two palps - dorsal and ventral, in the epidermis of which there are various sensory organs that perceive mechanical and other irritations. The movement of annelids is greatly facilitated by their division into rings, as a result of which the flexibility of the body increases.
The body of the rings contains compacted plates called border entities, which underlie the epidermis, separate the muscles, are highly developed in the partitions between the rings. They give strength to the entire body, serve as a support for the musculoskeletal system, are important for the functioning of the circulatory and digestive systems, and play a protective role.
Circulatory system. In annelids, due to the significant complication of the structure of their body and the sharply increased activity of their vital functions, a more advanced system of transporting substances has developed - the circulatory system. It consists of two main vessels - dorsal and ventral(Fig. 62 and 65). The first passes over the intestine, coming close to its walls, the second - under the intestine. In each segment both vessels are connected circular vessels. In addition, there are smaller vessels - there are especially many of them in the walls of the intestine, in the muscles, in the skin (through which gases are exchanged), in the partitions separating the segments of the body, etc. Blood moves due to the contraction of the vessels themselves, mainly the spinal and anterior annular ones, in the walls of which muscle elements are well developed.
Blood consists of a liquid part - plasma in which blood cells float - blood cells. Plasma contains respiratory pigments, i.e. special complex organic compounds. They absorb oxygen in the respiratory organs and release it to the tissues of the body. Some ringlets in the plasma contain one of the most advanced respiratory pigments - hemoglobin; these rings have a reddish blood color. For the most part, the blood of annelids contains other pigments and its color can be greenish, yellowish, etc. Blood cells are quite diverse. Among them there are phagocytes, which, like amoebas, release pseudopods that capture bacteria, all sorts of foreign bodies, dying body cells and digest them. As noted earlier, all animals have phagocytes. Thus, the circulatory system not only ensures the transport of various substances, but also performs other functions.
Body cavity. The body cavity of the ringlets differs in structure from the primary cavity. The latter does not have its own walls: on the outside it is limited by the muscles of the skin-muscular sac, on the inside by the intestinal wall (see Fig. 53). The body cavity of annelids, called secondary or coelom, is surrounded by a single-layer epithelium, which, on the one hand, is adjacent to the skin-muscular sac, and on the other, to the intestine (see Fig. 62). Consequently, the intestinal wall becomes double. The whole is filled with a watery fluid, constantly in motion, in which cells similar to blood cells (phagocytes, cells with respiratory pigments, etc.) float. Thus, the secondary body cavity, in addition to the role of the hydroskeleton, performs functions similar to those of the blood (transfer of substances, protection from pathogens, etc.). However, it should be emphasized that the coelomic fluid moves slower than blood and it cannot come into such close contact with all parts of the body as a branched network of capillaries.
Respiratory system. In annelids, the exchange of gases mainly occurs through the skin, but the respiratory processes in connection with the appearance of the circulatory system and coelom are more advanced in them than in the previously considered worms. Many ringlets, mainly marine ones, have branched appendages that play the role of gills (see Fig. 61, B). The respiratory surface also increases due to the presence of various outgrowths of the body. Improving respiratory processes is of great importance for annelids due to the activation of their lifestyle.
Excretory system. The main excretory organs are metanephridia(Fig. 66, B). A typical metanephridia consists of a funnel and a long convoluted tube, in the walls of which blood vessels branch. In each segment, with the exception of some, there are two of these organs, to the left and to the right of the intestine (see Fig. 65). The funnel faces the cavity of one segment, and the tube pierces the septum, passes into the other segment and opens outward on the ventral side of the body. Dissimilation products are extracted by metanephridia from the coelomic fluid and from the blood vessels entwining them.
In a number of annelids, metanephridia are associated with tubes of the protonephridial type, closed at the ends facing the body cavity by flame cells. It is possible that metanephridia arose from protonephridia, which connected with funnels that developed on the partitions between the rings (Fig. 66, A). It is believed that these funnels, called coelomoducts, originally served for the exit of reproductive products from the body cavity.
On the walls of the coelom there are numerous cells that absorb decay products from the cavity fluid. There are especially many of these cells called chloragogenous, is present on the walls of the middle part of the intestine. Decay products removed from the coelomic fluid and contained in these cells can no longer have a harmful effect on the body. Cells loaded with such products can escape through metanephridia or through pores in the walls of the body.
Digestive system. The digestive system of ringlets (see Fig. 65), due to a more active lifestyle than that of the previously considered groups of animals and the progress of the entire organization, is also more perfect. In ringlets: 1) the division of the digestive system into various sections is more pronounced, each of which performs its own function; 2) the structure of the walls of the digestive tube is more complex (digestive glands, muscles, etc. are more developed), as a result of which food is processed better; 3) the intestine is connected to the circulatory system, due to which the digestion of nutrients and their absorption is more intense and the supply of substances necessary for the work it performs is improved.
The digestive tube is usually straight and divided into the following sections: oral cavity, pharynx, esophagus, which can expand into a crop, muscular stomach (present in a number of species, such as earthworms), midgut (usually very long), hindgut (relatively short), opening outward through the anus. Gland ducts flow into the pharynx and esophagus, the secretion of which is important in processing food. In many predatory polychaete ringlets, the pharynx is armed with jaws; the front part of the digestive tube can turn out in the form of a trunk, which helps to take possession of the prey and penetrate its body. The midgut in a number of species has a deep invagination ( typhlosol), stretching along the entire dorsal side of this intestine (see Fig. 62). Typhlosol increases the surface of the intestines, which speeds up the digestion and absorption of food.
Reproduction. Some ringlets reproduce asexually and sexually, while others exhibit only sexual reproduction. Asexual reproduction occurs by division. Often, as a result of division, a chain of worms may result that have not yet had time to disperse.
The structure of the reproductive apparatus is different. Polychaete ringlets (they live in the seas) are dioecious and have a simply constructed reproductive apparatus. Their gonads develop on the walls of the coelom, the germ cells enter the water through breaks in the body walls or through metanephridia, and fertilization of the eggs occurs in water. Ringlets living in fresh water and damp soil (oligochaetes), as well as all leeches are hermaphrodites, their reproductive apparatus has a complex structure, fertilization is internal.
Development. The crushing of the fertilized egg, as a result of which the resulting blastomeres are arranged in a spiral (Fig. 67), resembles the same processes in ciliated worms. Polychaete ringlets develop with transformation: larvae are formed from their eggs trochophores(Fig. 68), completely different from adult worms and turning into the latter only after complex transformations. Trochophore is a planktonic organism. It is very small, transparent, and there are usually two belts of cilia along the equator of its body: one, upper, above the mouth, the other, lower, under the mouth. Consequently, the trochophore consists of two parts: the upper, or anterior, and the lower, or posterior, ending in the anal lobe. Trochophores of some species may have several belts of cilia. At the upper end there is a tuft of cilia attached to the parietal plate (the larval sensory organ). Under the plate is the nerve center, from which the nerves extend. The muscular system consists of fibers running in different directions. There is no circulatory system. The space between the body walls and the intestines is the primary body cavity. Excretory organs are protonephridia. The digestive apparatus consists of three sections: anterior, middle and posterior, ending with the anus. Thanks to the work of the cilia, the larva moves and food, consisting of microscopic organisms and organic pieces, enters the mouth. Some trochophores actively capture small animals with their mouths. In its structure, the trochophore resembles protocavitary worms, but in some respects it is also similar to the larvae of marine ciliated worms. The walls of the body, the nervous system, protonephridia, the beginning and end of the digestive apparatus, trochophores, were formed from the ectoderm, most of the intestine - from the endoderm, muscle fibers - from cells called mesenchymal and originating from both layers.
When a trochophore transforms into an adult worm, it undergoes a number of significant changes. In these changes, the most important role is played by the rudiments of the third germ layer - mesoderm. Some rudiments of mesoderm are still present in the larva before the onset of metamorphosis; they lie on each side between the walls of the body and the posterior part of the intestine (Fig. 68, B, 12). Other rudiments of mesoderm are formed later from the anterior edge of the anal lobe, which turns into growth zone worm (Fig. 68, B, 13). Metamorphosis of the larva begins with the fact that its rear part lengthens and the constrictions of the body walls are divided into 3, 7, and rarely more segments. After this, the rudiments of the mesoderm, lying between the walls of the body and the posterior part of the intestine, also lengthen and are divided into as many sections as the number of segments formed as a result of external constrictions. There are two of them in each ring (Fig. 68, D, 14). The segments formed from the back of the trochophore are called larval or larval, they are characteristic of the later stages of trochophore development, when it already begins to look a little like an adult worm, but still has few segments. In the process of further development, segments are formed by the growth zone mentioned above. These segments are called postlarval, or postlarval(Fig. 68, D). There are as many of them formed as the number of segments an adult worm of a given species has. In the postlarval segments, the mesodermal rudiments are first divided into sections (two in each ring), and then the outer integument.
The main organ systems of an adult worm are formed as follows (Fig. 69, A). From the ectoderm the epidermis, the nervous system, and the anterior and posterior ends of the digestive tube develop. Mesodermal primordia in each ring grow and displace the primary cavity. Eventually the right and left rudiments converge above and below the intestine, so that along it, above and below, dorsal and abdominal blood vessels are formed. Consequently, the walls of the vessels are formed from the mesoderm, and their cavity represents the remains of the primary body cavity. In the middle of the rudiments, the cells move apart, a coelomic body cavity appears and grows, which is surrounded on all sides by cells of mesodermal origin. This method of coelom formation is called teloblastic. Each mesodermal rudiment, growing, converges in front and behind with neighboring rudiments (Fig. 69, B) and septa appear between them, and the mesodermal cells surrounding the remains of the primary cavity between the septa form ring blood vessels. The outer layer of mesodermal primordia, adjacent to the ectoderm, gives rise to muscles, the inner layer surrounds the digestive tube. Consequently, the intestinal walls now become double: the inner layer (with the exception of the anterior and posterior ends, originating from the ectoderm) developed from the endoderm, the outer layer from the mesoderm. The metanephridia funnels are formed from the cells of the mesodermal layer, and their tubes (representing the remains of protonephridia) are from the ectoderm.
Gradually, all parts of the body of an adult worm develop; layers of muscles are differentiated, the number of blood vessels increases, the intestine is divided into sections, glandular cells, muscle fibers, blood vessels, etc. develop in its walls. The head lobe (prostomium) of an adult worm is formed from the upper part of the trochophore, the body ring from larval and postlarval segments, and the pygidium is from the anal lobe of the larva.
Origin. Various hypotheses have been put forward about the origin of annelids. Proponents of one hypothesis believe that annelids evolved from turbellarians. Indeed, there are similarities in the embryonic development of both groups of animals. The central nervous system of the ringlets (i.e., the cephalic nodes and the abdominal chain) could have formed from the same system of more complex turbellarians, in which the nodes moved to the anterior end of the body and two main ones remained from the longitudinal cords, and thus a central nervous system of the scalene type arose, preserved in lower annelids. The dermal-muscular sac of flatworms could develop into a similar ring system, and metanephridia could arise from protonephridia. However, from an evolutionary point of view, it is impossible to assume that the most highly organized worms descended directly from the lowest worms, in which the nervous and muscular systems were still poorly developed, there was no body cavity, the intestine was not differentiated into three more sections and digestion mainly remained intracellular, etc. d. Obviously, the ancestors of higher worms were worms with a more complex structure than turbellarians.
According to another hypothesis, ringlets began with nemerteans, i.e. worms, undoubtedly descended from turbellarians, but having a much more complex structure than the latter (significant development of the nervous and muscular systems, the appearance of a circulatory system, a through intestine, etc.). The author of this hypothesis, the outstanding Soviet zoologist N.A. Livanov, suggested that in the most progressive group of nemerteans, metamerically located cavities arose in the skin-muscle sac, which served as a support for the muscles and later turned into coelomic cavities, as a result of which the movement of animals sharply improved. Opponents of this hypothesis believe that nemerteans, in which one of the main features is a trunk, which is absent in ringlets, could not be the ancestors of the latter. However, it must be assumed that the trunk developed in nemerteans after a long evolution, when they had stronger rivals than before in hunting animals. Annelids could have evolved from unspecialized nemerteans, whose organization was already complex, but the trunk was not developed. Another objection to the hypothesis under consideration is more serious. From this hypothesis it follows that the circulatory system arose before the coelom, and the latter developed from the very beginning in the form of metameric formations. Meanwhile, worms are known, undoubtedly related to annelids, in which metamerism is not yet expressed, the whole is continuous and there is no circulatory system. It was previously believed that the worms mentioned were simplified due to adaptation to a sedentary lifestyle, but new research confirms the original primitiveness of the coelomic worms in question.
The authors of the third hypothesis believe that the ancestors of ringworms were protocavitary worms, but not as specialized as rotifers and roundworms, but closer to the ancestors of this type. This hypothesis is based mainly on the structure of the trochophore, which, as shown above, has important similarities (primary body cavity, protonephridia, through intestine) with protocavitary worms, but still lacks the features of annelids. Having accepted this hypothesis, it should be assumed that the coelom arose as a result of the development of epithelium on the walls of the primary body cavity, and body metamerism and the circulatory system appeared later. From the same hypothesis it follows that nemerteans, despite the progressive features of their organization, were not related to the emergence of more highly organized types of animals. On the contrary, the nonmertean hypothesis of the origin of annelids rejects the importance of protocavitary worms for the formation of new types of animals.
It is impossible to consider here in sufficient detail the various objections to each of the mentioned hypotheses, since this requires more detailed information about the structure and development of all types of worms, but there is no doubt that coelomic worms could not arise directly from the lowest worms.
Annelida are the most highly organized worms with a coelom. Their sizes range from a few millimeters to 3 m. The elongated body is divided into segments by internal annular partitions; sometimes there are several hundred such segments. Each segment may have lateral outgrowths with primitive limbs - parapodia, armed with setae. The musculature consists of several layers of longitudinal and circular muscles. Breathing is carried out through the skin; excretory organs - paired nephridia, located segment by segment. The nervous system consists of a “brain” formed by paired ganglia and a ventral nerve cord.
The closed circulatory system consists of abdominal and dorsal vessels connected in each segment by small annular vessels. Several of the thickest vessels in the anterior part of the body have thick muscular walls and act as “hearts.” In each segment, blood vessels branch, forming a dense capillary network.
Some annelids are hermaphrodites, while others have different males and females. Development is direct or with metamorphosis. Asexual reproduction (by budding) also occurs.
Annelids are divided into 3 classes: polychaetes, oligochaetes and leeches.
Polychaetes(Polychaeta) have primitive limbs (parapodia) with numerous setae on each segment. Bilobed parapodia are often associated with branched appendages - gills, with the help of which gas exchange is carried out. On the clearly distinct head there are eyes (in some species even capable of accommodation), tactile antennae and balance organs (statocysts). Some species are capable of luminescence.
During the breeding season, males release sperm into the water, and females release a large number of eggs. In some species, mating games and competition for territory have been observed. Fertilization is external; the parents then die. Development occurs with metamorphosis (free-swimming larva). Asexual reproduction is rare.
Oligochaeta are predominantly soil worms. Among them there are both giant earthworms up to 2.5 m long and dwarf forms. All segments, except the oral one, have bristles arranged in tufts. Parapodia are not pronounced, the head is poorly separated. The thin cuticle is constantly moistened by secreted mucus; Gas exchange occurs through the cuticle by diffusion.
Oligochaete worms are predominantly hermaphrodites with cross-fertilization; the genitals are distributed over several body segments. The complex structure of these organs is an adaptation to a terrestrial lifestyle. Parthenogenesis is known in some species. There is no metamorphosis; A dozen young worms emerge from the cocoons formed during the copulation process after a few weeks.
Leeches (Hirudinea) have a flattened body, usually colored brown or green. There are suckers on the anterior and posterior ends of the body. The body length is from 0.2 to 15 cm. Tentacles, parapodia and, as a rule, setae are absent. The muscles are well developed. The secondary body cavity is reduced. Breathing is cutaneous, some have gills. Most leeches have 1–5 pairs of eyes.
The lifespan of leeches is several years. They are all hermaphrodites. Eggs are laid in cocoons; there is no larval stage. Most leeches suck blood from various animals, including humans. Leeches pierce the skin with their proboscis or teeth on their jaws, and a special substance - hirudin - prevents blood clotting. Sucking blood from one victim can continue for months. Blood in the intestines does not deteriorate for a very long time: leeches can live without food for even two years. Some leeches are predators, swallowing their prey whole.
Leeches live in fresh water bodies and are also found in seas and soil. Leeches serve as food for fish. Medical leech used by humans for medicinal purposes. 400–500 species.
Annelids evolved from primitive flatworms in the Cambrian. The first annelids were polychaetes, which gave rise to oligochaetes, and through them, leeches.