Type of annelids habitat. Habitat of annelids

general characteristics

Type Annelids are a large group (12 thousand species). It includes secondary cavity animals, whose body consists of repeating segments, or rings. The circulatory system of annelids is closed. Compared to roundworms, annelids have a more advanced nervous system and sensory organs. The main features of this group need to be described in more detail.

The secondary body cavity, or coelom (from the Greek koiloma - “recess”, “cavity”), develops in the embryo from the mesoderm layer. This is the space between the body wall and the 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 liquid, creating a constant internal environment of the body. Thanks to fluid pressure, the secondary cavity maintains a certain shape of the worm's body and serves as a support when moving. In other words, the whole serves as a hydroskeleton. Coelomic fluid is involved in metabolism: it transports 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 internally 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 gonads.

The phylum Annelids include Polychaete worms and Oligochaete worms.

Habitats, structure and life activity of polychaete worms

About 7,000 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 on the bottom, where they crawl among stones, corals, thickets of marine vegetation, and burrow into silt. Among them there 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 zone, but sometimes at a depth of up to 8000 m. In some places, up to 90 thousand polychaete worms live per 1 m2 of the seabed. They are eaten by crustaceans, fish, echinoderms, intestinal cavities, and birds. Therefore, some polychaete worms were specially bred in the Caspian Sea as food for fish.

Rice. 62. Various polychaete annelids: 1 - sessile form of the sea worm: 2 - nersis; 3 - sea mouse; 4 - sand core

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 polychaetes consists of segments, the number of which in different species ranges from 5 to 800. In addition to many body segments there is a head section and an anal lobe.

On the head of these worms there 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 section; B - parapodya (cross section); B - larva; 1 - tentacle; 2 - palp; 3 - antennae; 4 - eyes: 5 - bristles

On the sides of each body segment, dermal-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 within them - bundles of bristles that contribute to the rigidity of the organs of movement. The worm rakes its parapodia from front to back, clinging to uneven surfaces of the substrate, and thus crawls forward.

In sessile forms of worms, a partial reduction (shortening) of the parapodia occurs: they are often preserved only in the anterior part of the body.

The body of polychaete worms is covered with a single-layer epithelium. In sessile forms of worms, epithelial secretions can harden, forming a dense protective cover around the body. The skin-muscle 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 circular, and longitudinal. Under the muscle layer there is a single-layer internal epithelium, which lines the secondary body cavity from the inside and forms partitions between the segments.

Rice. 64. Transverse (A) and longitudinal (B) sections through the body of 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 next to 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 foregut, middle and hind intestine (Fig. 64, B). The midgut looks like a straight tube. Digestion and absorption of nutrients occurs in it. Fecal matter forms in the hindgut. The anal opening is located on the anal blade. Vagrant 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 (oxygen absorption and carbon dioxide release) is carried out either over the entire surface of the body or through areas of parapodia into which blood vessels extend. In some sessile forms, the respiratory function is performed by the corolla of the tentacles on the head lobe.

The circulatory system of annelids is closed: in any part of the worm’s body, blood flows only through the vessels. There are two main vessels - dorsal and abdominal. One vessel passes above the intestine, the other - under it (see Fig. 64). They are connected to each other by numerous semi-circular vessels. There is no heart, and the movement of blood is ensured 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 tubes located in each body segment. Each tube begins with a wide funnel facing the body cavity. The edges of the funnel are lined with flickering cilia. The opposite end of the tube opens outward on the side of the body. With the help of the system of excretory tubules, waste products that accumulate in the coelomic fluid are excreted outside.

Nervous system consists of paired suprapharyngeal, or cerebral, nodes (ganglia), connected by cords into a peripharyngeal ring, a paired abdominal nerve cord and nerves extending from them.

Sense organs most developed in wandering 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 irritants act on sensitive skin cells. The excitation that arises in them is transmitted along the nerves to the nerve nodes, from them through other nerves to the muscles, causing them to contract.

Reproduction. Most polychaete worms are dioecious. Gonads are present in almost every segment. Mature germ cells (in females - eggs, in males - sperm) enter first as a 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 it settles to the bottom and turns into an adult worm. Some species also reproduce asexually. In some species, the worm is divided crosswise, and each half restores the missing part. In others, the daughter individuals do not disperse, and as a result, a chain is formed, including up to 30 individuals, but then it breaks up.

Annelids, a very large group, are the evolutionary descendants of flatworms. The most studied of them are polychaete worms living in the seas - polychaetes and oligochaete worms - Oligochaetes. The most famous representatives of oligochaetes are the earthworm and the leech. A characteristic feature of the structure of annelids is external and internal metamerism: their body consists of several, mostly identical, segments, each of which contains a set of internal organs, in particular a pair of symmetrically located ganglia with nerve commissures. As a result, the nervous system of annelids has the appearance of a “nervous ladder.”

A special place is occupied by representatives of the class of oligochaetes - earthworms, on which the main experiments were carried out related to the study of their reactions to various environmental agents and the development of conditioned reflexes. The nervous system of earthworms is presented in the form of nerve nodes - ganglia, located along the entire body in the form of a symmetrical chain. Each node consists of pear-shaped cells and a dense plexus of nerve fibers. Motor nerve fibers extend from these cells to the muscles and internal organs. Under the skin of the worm there are sensitive cells that are connected by their processes - sensory fibers - to the nerve ganglia. This type of nervous system is called chain, or ganglionic. The body of an earthworm consists of a number of segments. Each segment has its own nerve node and can respond to stimulation, being completely separated from the rest of the body, but all nodes are interconnected by jumpers, and the body acts as a single whole. The head node of the nervous system, located in the upper part of the head, receives and processes the greatest amount of irritation. It is much more complex than all other nodes of the worm's nervous system.

Movements of annelids

The locomotor activity of annelids is highly diverse and quite complex. This is ensured by highly developed muscles, consisting of two layers: the outer layer, consisting of circular fibers, and the inner layer, made of powerful longitudinal muscles. The latter extend, despite segmentation, from the anterior to the posterior end of the body. Rhythmic contractions of the longitudinal and circular muscles of the musculocutaneous sac provide movement. The worm crawls, stretching and contracting, expanding and contracting individual parts of its body. In an earthworm, the front part of the body stretches and narrows, then the same thing happens sequentially with the following segments. As a result, “waves” of muscle contractions and relaxations run through the worm’s body.

For the first time in the evolution of the animal world, annelids have true paired limbs: each segment has a pair of outgrowths called parapodia. They serve as organs of locomotion and are equipped with special muscles that move them forward or backward. Often parapodia have a branched structure. Each branch is equipped with a supporting seta and, in addition, a corolla of setae, which have different shapes in different species. Tentacle-shaped organs of tactile and chemical sensitivity also extend from the parapodia. The latter are especially long and numerous at the head end, where the eyes (one or two pairs) are located on the dorsal side, and the jaws are located in the oral cavity or on a special protruding proboscis. Thread-like tentacles at the head end of the worm can also participate in the capture of food objects.

Annelid behavior

Annelids live in seas and freshwater bodies, but some also lead a terrestrial lifestyle, crawling along the substrate or burrowing in loose soil. Marine worms are partly carried passively by water currents as part of plankton, but the majority of them lead a bottom-dwelling lifestyle in coastal zones, where they settle among colonies of other marine organisms or in rock crevices. Many species live temporarily or permanently in tubes, which in the first case are periodically abandoned by their inhabitants and then found again. Particularly predatory species regularly leave these refuges to “hunt”. The tubes are built from grains of sand and other small particles, which are held together by the secretions of special glands, thereby achieving greater strength of the buildings. Animals sitting motionless in tubes catch their prey (small organisms) by pushing and filtering water with the help of a corolla of tentacles that protrudes from the tube, or by driving a stream of water through it (in this case, the tube is open at both ends).

In contrast to sessile forms, free-living worms actively search for their food, moving along the seabed: predatory species attack other worms, mollusks, crustaceans and other relatively large animals, which they grab with their jaws and swallow; herbivores tear off pieces of algae with their jaws; other worms (the majority of them) crawl and rummage in the bottom mud, swallow it along with organic remains or collect small living and dead organisms from the bottom surface.

Oligochaete worms crawl and burrow in soft soil or bottom silt; some species are able to swim. In tropical rainforests, some oligochaetes even crawl onto trees. The bulk of oligochaete worms feed on deuterium, sucking up slimy silt or gnawing through the soil. But there are also species that eat small organisms from the surface of the ground, filter water or bite off pieces of plants. Several species lead a predatory lifestyle and capture small aquatic animals by sharply opening the mouth. As a result, the prey is sucked in with the flow of water.

Leeches swim well, making wave-like movements with their bodies, crawl, dig tunnels in soft soil, and some move on land. In addition to blood-sucking leeches, there are also leeches that attack aquatic invertebrates and swallow them whole. Terrestrial leeches that live in tropical rainforests lie in wait for their victims on land, in the grass or on the branches of trees and shrubs. They can move quite quickly. In the movement of terrestrial leeches along the substrate, suckers play an important role: the animal extends its body, then sticks to the substrate with the head sucker and pulls the rear end of the body to it, simultaneously contracting it, then sucks with the rear sucker, etc.

Experimental study of the behavior of annelids

Earthworms or earthworms are widespread throughout the globe. These animals play a huge role in soil formation, so they have long attracted the close attention of scientists of various profiles. Their behavior has also been studied quite well. Thus, the life activity of earthworms was described in detail by Charles Darwin. During his experiments, it turned out that they react differently to visual, tactile, olfactory and temperature stimuli. R. Yerkes and a number of other scientists studied the ability of earthworms to form simple skills. For this purpose, the method of developing defensive conditioned reactions in the T-shaped maze. The worms were trained to turn into the right or left arm of the maze. The unconditioned stimulus was an alternating current of varying intensity, and the conditioned stimulus was the maze itself, the elements of which were probably perceived by proprioceptive and tactile afferentation. The criterion for the development of the reflex was an increase in the number of turns into the arm of the maze, where the animals were not subjected to electrical stimulation. In the experiments of R. Yerkes, worms learned to correctly choose a side after 80–100 combinations (Fig. 15.3).

The presence of sensory organs helps earthworms distinguish between the simplest forms. So, in the process of storing food, they grab double pine needles by the base, and fallen leaves by the tops, by which they pull them into their burrow.

Even clearer conditioned reflexes manages to produce polychaete worms - polychaetes. Yes, y Nereis managed to develop stable conditioned reflexes to tactile stimulation, food, light and vibration. Analysis of the results showed that polychaetes develop reactions that have all the basic properties of true conditioned reflexes: an increase in the number of positive responses from experiment to experiment, a high maximum percentage of positive reactions (up to 80– 100) and the duration of their storage (up to 6–15 days).

It is very significant that the developed reaction faded away in the absence of reinforcement and was restored spontaneously.

Rice. 15.3

The revealed patterns of conditioned reflex activity of polychaetes correlate with the relatively differentiated brain of animals. Thus, true conditioned reflexes, as one of the sufficiently perfect mechanisms that determine acquired behavior, apparently appear for the first time in evolution in annelids.

• Tushmalova N. A. Basic patterns of the evolution of invertebrate behavior.

Annelids belong to the subsection of coelomic animals Coelomata), a group (superphylum) of protostomes (Protostomia). For primary stomates it is characteristic:

• The primary mouth (blastopore) of the embryo (gastrula) passes into the adult animal or the definitive mouth is formed in place
• primary mouth.
• Mesoderm is formed, as a rule, by a teloblastic method.
• The covers are single-layered.
• External skeleton.
• Protostomes are the following types of animals: annelids (Annelida), mollusks (Mollusca), arthropods (Arthropoda), onychophorans (Onychophora).
• Annelids are a large group of animals, about 12 thousand species are known. They are inhabitants of the seas, fresh water bodies, and inhabit land.

Polychaete annelids Polychaetes

Main characteristics of the type:

• The body consists of a head lobe (prostomium), a segmented trunk and an anal lobe (pygidium). Characterized by metamerism of external and internal structure.
• The body cavity is secondary, well developed in most animals. The blades lack a coelom.
• The skin-muscular sac is developed, represented by epithelium and circular and longitudinal muscles.
• The intestine consists of three sections; salivary glands are developed.
• The excretory system is of the nephridial type.
• The circulatory system is a closed type, absent in some groups.
• The respiratory system is either absent, animals breathe with the entire surface of the body, some representatives have gills.
• The nervous system consists of a paired brain and a ventral nerve cord or scala.
• Annelids are dioecious or hermaphrodites.
• Crushing of eggs according to a spiral type, deterministic.
• Development with metamorphosis or direct.

Annelids General characteristics

Latin name Annelida

Type annelids, or rings, is a very important group for understanding the evolution of higher invertebrate animals. It includes about 8,700 species. Compared with the considered flat and roundworms and even with nemerteans, annelids are significantly more highly organized animals.

The main feature of the external structure of the rings is metamerism, or body segmentation. The body consists of a more or less significant number of segments, or metameres. The metamerism of the rings is expressed not only in the external, but also in the internal organization, in the repeatability of many internal organs.

They have a secondary body cavity - generally absent in lower worms. The body cavity of the ringlets is also segmented, that is, divided by partitions in greater or lesser accordance with the external segmentation.

U ringlets there is a well-developed closed circulatory system. The excretory organs - metanephridia - are located segment by segment, and therefore are called segmental organs.

Nervous system consists of a paired suprapharyngeal ganglion, called the brain, connected by peripharyngeal connectives to the ventral nerve cord. The latter consists of a pair of longitudinally contiguous trunks in each segment, forming ganglia, or nerve ganglia.

Internal structure

Musculature

Under the epithelium there is a muscular sac. It consists of external circular and internal longitudinal muscles. Longitudinal muscles in the form of a continuous layer or divided into ribbons.
Leeches have a layer of diagonal muscles, which are located between the circular and longitudinal ones. The dorso-abdominal muscles are well developed in leeches. In wandering polychaetes, flexors and extensors of parapodia are developed - derivatives of the ring muscles. The ring muscles of oligochaetes are more developed in the anterior eight segments, which is associated with the way of life.

Body cavity

Secondary or whole. The body cavity is lined with coelomic or perinoneal epithelium, which separates the cavity fluid from tissues and organs. Each body segment of polychaetes and oligochaetes has two coelomic sacs. The walls of the sacs on one side are adjacent to the muscles, forming a somatopleura, on the other side to the intestines and to each other, a splanchnopleura (intestinal leaf) is formed. The splanchnopleura of the right and left sacs forms the mesentery (mesentery) - a two-layer longitudinal septum. Either two or one septum is developed. The walls of the sacs facing adjacent segments form dissepiments. Dissepiments disappear in some polychaetes. Coelom absent from prostomium and pygidium. In almost all leeches (with the exception of bristle-bearing ones), the parenchyma between the organs is generally preserved in the form of lacunae.

The functions of the coelom are: supporting, distributive, excretory and, in polychaetes, reproductive.

Origin of the coelom. There are 4 known hypotheses: myocoel, gonocoel, enterocoel and schizocoel.

Digestive system

Represented by three departments. Cavity digestion. The pharynx of predatory polychaetes is armed with chitinous jaws. The ducts of the salivary glands open into the pharynx of annelids. Leech glands contain the anticoagulant hirudin. In earthworms, ducts of calcareous (morrain) glands flow into the esophagus. The foregut of earthworms includes, in addition to the pharynx and esophagus, a crop and a muscular stomach. The absorption surface of the midgut increases due to outgrowths - diverticulum (leeches, part of the polychaetes) or typhlosol (oligochaetes).

Excretory system

Nephridial type. As a rule, each segment has two excretory canals; they begin in one segment and open with an excretory pore in the next segment of the body. The excretory organs of polychaetes are the most diverse. Polychaete worms have the following types of excretory systems: protonephridia, metanephridia, nephromyxia and myxonephridia. Protonephridia are developed in larvae; they begin with club-shaped terminal cells with a flagellum (solenocytes), then the nephridia canal. Metanephridia begins with a funnel with a nephrostomy, inside
the funnels contain the cilia, followed by the duct and nephropore. Protonephridia and metanephridia are ectodermal in origin. Nephromyxia and myxonephridia are the fusion of the ducts of the protonephridia or metanephridia with the coelomoduct - the genital funnel. Coelomoducts of mesodermal origin. The excretory organs of oligochaetes and leeches are metanephridia. In leeches, their number is significantly less than that of body segments (medicinal leeches have 17 pairs), and the funnel is separated from the canal. In the excretory canals of the nephridia, ammonia is converted into high molecular weight compounds, and water is absorbed as a whole. Annelids also have storage “buds”: chloragogenous tissue (polychaetes, oligochaetes) and botryodenic tissue (leeches). They accumulate guanine and uric acid salts, which are removed from the coelom through nephridia.

Circulatory system of annelids

Most annelids have a closed circulatory system. It is represented by two main vessels (dorsal and abdominal) and a network of capillaries. Blood movement is carried out due to contraction of the walls of the dorsal vessel; in oligochaetes, the annular hearts also contract. The direction of blood movement through the spinal vessel is from back to front, and in the abdominal vessel - in the opposite direction. The circulatory system is developed in bristle-bearing and proboscis leeches. In jaw leeches there are no vessels; the function of the circulatory system is performed by the lacunar system. The process of functional replacement of one organ with another, different in origin, is called organ substitution. The blood of annelids is often colored red due to the presence of hemoglobin. Primitive polychaetes do not have a circulatory system.

Respiratory system

Most breathe over the entire surface of the body; some polychaetes and some leeches have gills. Respiratory organs are evaginated. The gills of polychaetes are in origin a modified dorsal antennae of parapodia, while those of leeches are skin outgrowths.

Nervous system and sensory organs

The nervous system includes: the paired medullary (suprapharyngeal) ganglion, connectives, subpharyngeal ganglia and the ventral nerve cord or scalene nervous system. The abdominal trunks are connected by commissures. The evolution of the nervous system went in the direction of transforming the ladder-type nervous system into a chain, immersing the system in the body cavity. The nerves that arise from the central system make up the peripheral system. There are varying degrees of development of the suprapharyngeal ganglion; the brain is either monolithic or divided into sections. Leeches are characterized by the fusion of ganglion segments that make up the suckers. Sense organs. Polychaetes: epithelial sensory cells, antennae, nuchal organs, antennae of parapodia, statocysts, organs of vision (goblet or bubble type eyes). Sense organs of oligochaetes: light-sensitive cells, some water inhabitants have eyes, chemical sense organs, tactile cells. Leeches: goblet organs – chemical sense organs, eyes.

Classification

The type of rings is divided into several classes, of which we will consider four:

1. Polychaeta ringlets

2. Echiurida

Echiurids are a highly modified group of ringlets, the internal organization of which differs from that of polychaetes by an unsegmented coelom and the presence of one pair of metanephrpdia.
The trochophore larva of echiurids is of greatest importance for establishing the unity of origin of echiurids with polychaetes.

At the bottom of the sea, among stones in silt and sand, there are peculiar animals, but in appearance they bear very little resemblance to annelids, primarily due to their lack of segmentation. This includes such forms as Bonellia, Echiurus and some others, about 150 species in total. The body of the female Bonellia, which lives in rock crevices, has the shape of a cucumber and carries a long, non-retractable trunk, forked at the end. The length of the trunk can be several times greater than the length of the body. A groove lined with cilia runs along the trunk, and at the base of the trunk there is a mouth. With the flow of water, small food particles are brought to the mouth along the groove. On the ventral side of the anterior part of Bonellia's body there are two large setae, and in other echiurids there is also a corolla of small setae at the posterior end. The presence of setae brings them closer to the ringlets.

3. Oligochaeta

The oligochaetes, or oligochaetes, are a large group of annelids, including about 3,100 species. They undoubtedly descend from polychaetes, but differ from them in many significant features.
Oligochaetes overwhelmingly live in the soil and at the bottom of fresh water bodies, where they often burrow into muddy soil. The Tubifex worm can be found in almost every freshwater body, sometimes in huge quantities. The worm lives in silt, and sits with its head end buried in the ground, and its back end constantly makes oscillatory movements.
Soil oligochaetes include a large group of earthworms, an example of which is the common earthworm (Lumbricus terrestris).
Oligochaetes feed mainly on plant foods, mainly on decaying parts of plants, which they find in the soil and silt.
When considering the characteristics of oligochaetes, we will mainly have in mind the common earthworm.

4. Leeches (Hirudinea) >> >>

Phylogeny

The problem of the origin of rings is very controversial; there are various hypotheses on this issue. One of the most widespread hypotheses to date was put forward by E. Meyer and A. Lang. It is called the turbellar theory, since its authors believed that polychaete ringlets originate from turbellarian-like ancestors, i.e., they associated the origin of ringlets with flatworms. At the same time, supporters of this hypothesis point to the phenomenon of so-called pseudometamerism, observed in some turbellarians and expressed in the repeatability of some organs along the length of the body (intestinal outgrowths, metameric arrangement of the gonads). They also point out the similarity of the ringlet trochophore larvae with the Müllerian turbellarian larva and the possible origin of metanephridia by changing the protonephridial system, especially since the ringlet larvae - trochophores - and the lower ringlets have typical protonephridia.

However, other zoologists believe that annelids are closer to nemerteans in a number of ways and that they descend from nemertean ancestors. This point of view is developed by N. A. Livanov.

The third hypothesis is called the trochophore theory. Its proponents produce ringlets from a hypothetical ancestor of Trochozoon, which has a trochophore-like structure and originates from ctenophores.

As for the phylogenetic relationships within the four classes of annelids considered, they currently seem quite clear.

Thus, annelids, which are highly organized protostomes, apparently originate from ancient protostomes.

Undoubtedly, not only modern polychaetes, but also other groups of annelids originated from ancient polychaetes. But it is especially important that polychaetes are a key group in the evolution of higher protostomes. Mollusks and arthropods originate from them.

The meaning of annelids

Polychaete worms.

 Food for fish and other animals. Mass species play the greatest role. Introduction of the polychaete Azov nereid into the Caspian Sea.
 Human food (palolo and other species).
 Purification of sea water, processing of organic matter.
 Settlement on the bottoms of ships (serpulids) – reduction in movement speed.

Oligochaete worms.

 Oligochaetes, inhabitants of water bodies, provide food for many animals and participate in the processing of organic matter.
 Earthworms are animal food and human food.Gallery

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 nervous 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.

The phylum Annelids unites about 12 thousand species of segmented secondary cavities. It includes both free-living freshwater and marine organisms, as well as soil and woody ones up to 3 m long.

Annelids have pronounced head and posterior ends of the body, between which there is a segmented body (Fig. 4.134). At the head end there are sensory organs: eyes, organs of touch and chemical sense. Subsequent body segments may have paired body extensions - parapodia with setae, which is the basis for the classification of annelids: polychaetes have parapodia and long setae, oligochaetes do not have pronounced parapodia, but are equipped with short setae, and leeches lack both parapodia and setae. The body of the ringlets is covered with a thin cuticle, under which there is a single-layer epithelium, as well as circular and longitudinal muscles that form a skin-muscular sac.

The body cavity of ringlets is secondary, differing from the primary in that it is limited by epithelium. The body cavity contains fluid that allows these worms to maintain a constant internal environment (Fig. 4.135).

Digestive system The rings are formed by the foregut, middle and hindgut. Through the mouth, food enters the pharynx, esophagus, and then into the intestines. The mouth of some predatory worms may be equipped with chitinous jaws, others may have salivary or calcareous glands that neutralize the acidity of the soil, and a number of species have a stomach of larger or smaller sizes (Fig. 4.136).

Respiratory system most representatives of the type are absent; only some species of marine polychaete worms have gills. Oxygen enters through the entire surface of the body.

Appears for the first time in ringlets circulatory system, which is formed by large dorsal and abdominal vessels connected by annular bridges. Blood flows forward through the abdominal vessel, to the head section; through the annular vessels in the anterior segments, it flows into the dorsal vessel, which carries blood backward. In the posterior segments of the body, blood flows backward. Smaller vessels branch off from large vessels, carrying blood to the organs. The blood of ringlets can be red or other colors, and it performs the respiratory function of carrying oxygen and removing carbon dioxide.

Selection they carry out using paired pairs located in each segment metanephridia, which are tubules, on one side opening into the body cavity with funnel-shaped extensions with cilia, and on the other end - outward in the next segment. Metanephridia not only remove metabolic products, but also maintain water-salt balance in the body.

Nervous system annelids consists of a paired suprapharyngeal nerve ganglion and a ventral nerve cord formed by paired ganglia in each body segment. Sense organs - eyes, organs of smell and balance.

Reproduction of annelids occurs asexually or sexually. During asexual reproduction, the worm's body is divided into several parts, which then grow to their original size. Annelids can be dioecious or hermaphrodite, but they undergo cross-fertilization. For most, development is indirect, since larvae emerge from the fertilized egg, which are not similar to adults.

Classification of annelids. This type includes the classes Polychaetes, Oligochaetes and Leeches.

Class Oligochaete worms unites freshwater and soil ringlets, occasionally found in the seas. Their head and tail sections are much smaller than those of polychaetes. On There are no parapodia in body segments; only tufts of short setae are located on the sides of the body. The sense organs are usually poorly developed. Hermaphrodites. Fertilization is external. Development is direct.

They participate in soil formation processes and are a link in the food chains of water bodies.

Representatives: earthworm, Californian worm, tubifex.

Class Polychaete worms mainly represented by free-living marine animals that live on the bottom or in the water column. Unlike other ringlets, they have a well-separated head section with relatively highly developed sensory organs and parapodia with numerous setae. Among them there are both swimming and burrowing species. Respiration in polychaetes is mainly cutaneous, but some have gills. Most polychaetes are dioecious and undergo external fertilization. Development is indirect.

Representatives: Pacific palolo, nereid, sandworm, serpula.

Leech class consists mainly of blood-sucking, less often - predatory annelids, which have a flattened body with two suckers (perioral and posterior). Parapodia and setae on body segments are usually absent. Leech saliva contains a substance that prevents blood clotting. The nervous and muscular systems are well developed. Hermaphrodites. Fertilization is internal.

Representatives: medical leech (Fig. 4.137), horse leech.