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.
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.
- 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. Nerves coming from central system, constitute 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, internal organization which differs from those 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 the stones in the silt and sand, there are peculiar animals, but appearance extremely little reminiscent of 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 fresh body of water, sometimes in a huge number. 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 changes in 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. Play the greatest role mass species. Introduction of the polychaete Azov nereid into the Caspian Sea.
Human food (palolo and other species).
Cleaning 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
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 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). Such segments have different types there may be several or hundreds. The body cavity inside is divided into segments by transverse partitions. Each segment is an independent compartment: it has its own external outgrowths, nodes of the nervous system, excretory organs and 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 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 per 1 m2 seabed up to 90 thousand polychaete worms live. 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 take place in it nutrients. 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 organs. 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 a 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 are invertebrate animals, among which scientists identify approximately 12 thousand species of oligochaetes, polychaetes, mysostomids and leeches.
Description of annelids
The body length of different types of annelids varies from a few millimeters to 6 meters. Body ringworm has bilateral symmetry. It is divided into the tail, head and midsection, which consist of numerous repeating segments. All body segments are separated by partitions. Each of them contains a complete set of organs.
The mouth is in the first segment. The annelid's body is filled with fluid, which creates hydrostatic pressure and gives the body shape. The outer layer is formed by two layers of muscles. The fibers of one layer are arranged in the longitudinal direction, and in the second layer they work in a circular pattern. Movement is accomplished through the action of muscles located throughout the body.
The muscles of annelids can work in such a way that parts of the body can alternate between lengthening and becoming thick.
Lifestyle of annelids
Annelids are distributed throughout the world. They primarily live in land and water, but some species of annelids are blood-sucking. Among annelids there are predators, filter feeders and scavengers. The biggest ecological significance have annelids that recycle the soil. Annelids include not only oligochaete worms, but also leeches. On 1 square meter soil may contain 50-500 worms.
The most diverse marine forms are annelids. They live in all latitudes of the World Ocean and can be found at different depths, up to 10 kilometers. They have a high population density: there are about 500-600 marine annelids per 1 square meter. Annelids are very important in the marine ecosystem.
Annelids are dioecious animals, some are hermaphrodites. Reproduction of annelids
Many species of annelids reproduce asexually, but there are species that reproduce sexually. Most species develop from larvae.
Polychaetes and oligochaetes are characterized by the ability to regenerate, so they reproduce vegetatively. In some species, for example, in Aulophorus, in the presence of a sufficient amount of food, additional oral openings are formed on the body segments, through which, over time, the separation and formation of new individuals - daughter clones - occurs.
Feeding of annelids
Classification of annelids
Annelids are considered close relatives of arthropods. They have common features: segmented body and structure of the nervous system. Polychaete worms are most similar to arthropods. They also have developed lateral appendages - parapodia, which are considered the rudiments of legs.
By the type of crushing and structure of the larvae, annelids are similar to mollusks and sipunculids.
It is believed that the closest relatives of annelids are brachiopods, nemerteans and phoronids, mollusks are more distant relatives, and the most distant relatives are flatworms.
Different classifications distinguish different numbers of classes of annelids. But traditionally they are divided into 3 classes: oligochaetes, polychaetes and leeches. There is also another taxonomy:
Polychaete worms - this class is the most numerous, and it consists mainly of marine forms;
Misostomidae;
Belted worms with a characteristic belt on the body.
Evolution of annelids
There are several versions about the origin of annelids. They are generally believed to have descended from lower flatworms. Certain features indicate that annelids have a general resemblance to lower worms.
It is assumed that polychaete worms arose first, and from them freshwater and terrestrial forms were formed - polychaete worms and leeches.
The phylum annelids, or ringworms, covers about 9,000 species of higher worms. This group of animals has great value to understand the phylogenetic pathways of higher invertebrates. Annelids have a higher organization compared to flatworms and roundworms. They live in sea and fresh waters, as well as in soil. The type is divided into several classes. Let's get acquainted with a representative of the class of oligochaetes (earthworm).
General characteristics
The body of the ringlets consists of segments. The body segments are externally identical. Each segment, except the anterior one, which bears the oral opening, is equipped with small bristles. These are the last remnants of the disappeared pair of podia.
Annelids have a well-developed skin-muscle sac, consisting of one layer of epithelium and two layers of muscles: an outer layer of circular muscles and an inner layer formed by longitudinal muscle fibers.
Between the skin-muscle sac and the intestines there is a secondary body cavity, or coelom, which is formed during embryogenesis inside the growing mesodermal sacs.
Morphologically, the secondary cavity differs from the primary cavity in the presence of an epithelial lining adjacent to the body wall on one side and to the walls of the digestive tube on the other. The lining leaves grow together above and below the intestines, and the mesentery formed from them divides the whole into right and left side. Transverse partitions divide the body cavities into chambers corresponding to the boundaries of the outer rings. Entirely filled with liquid.
Organ systems
The appearance of a secondary body cavity provides annelids with a higher level of vital processes than other worms. Coelomic fluid, washing the organs of the body, along with the circulatory system, supplies them with oxygen, and also promotes the removal of waste products and the movement of phagocytes.
excretory
Each segment of the earthworm has a paired organ of the excretory system, consisting of a funnel and a convoluted tubule. Waste products from the body cavity enter the funnel. A canaliculus extends from the funnel, which enters the adjacent segment, forms several loops and opens outwards with an excretory pore in the lateral wall of the body. Both the funnel and the tubule are equipped with cilia, causing the movement of the secreted fluid. Such excretory organs are called metanephridia.
Circulatory and respiratory systems
In most annelids it is closed, consisting of abdominal and dorsal vessels, which pass into each other at the anterior and posterior ends of the body. In each segment, an annular vessel connects the dorsal and ventral vessels. Blood moves through the vessels due to rhythmic contractions of the dorsal and anterior annular vessels.
In earthworms, gas exchange occurs through the skin, rich in blood vessels, and some ringworms have gills.
Digestive
It begins with the oral opening at the anterior end of the body and ends with the anal opening at the rear. The intestine consists of three sections:
- Anterior (ectodermal);
- average ( endodermal, unlike other departments);
- posterior (ectodermal).
The foregut is often represented by several sections; oral cavity and muscular pharynx. The so-called salivary glands are located in the wall of the pharynx.
Some predatory annelids have cuticular “teeth” that are used to grasp prey. A layer of muscle appears in the intestinal wall, which ensures its independent peristalsis. The midgut passes into a short posterior intestine, ending in the anus.
Nervous system
Significantly more complicated compared to flat and roundworms. Around the pharynx there is a peripharyngeal nerve ring, consisting of the suprapharyngeal and subpharyngeal nodes, connected by jumpers.
On the abdominal side there are two nerve trunks, which have thickenings in each segment - ganglia, which are connected to each other by jumpers. In many types of ringlets, the right and left nerve trunks come together, resulting in the formation of the ventral nerve cord.
Among the sense organs, annelids have antennae, eyes, and balance organs, which are often located on the head lobe.
Regeneration
An earthworm, like hydra and ciliated worms, is capable of regeneration, that is, restoration of lost body parts. If an earthworm is cut into two parts, then the missing organs will be restored in each of them.
The reproductive system consists of female gonads (ovaries), which are a complex of germ cells surrounded by epithelium, and male gonads (testes), lying inside voluminous seminal sacs.
Reproduction of annelids: 1 - copulation, 2 - egg laying, 3 - egg fertilization, 4 - cocoon laying Earthworms are hermaphrodites, but among the ringworms there are also dioecious forms. The earthworm has a girdle on its body that produces mucus, from which the cocoon is formed. Eggs are laid in it and their development occurs there.
Development
In earthworms, development is direct, but in some ringworms a larva develops from a fertilized egg, i.e. development occurs with metamorphosis.
Thus, annelids have a number of progressive characteristics, which include the appearance of segmentation, coelom, circulatory and respiratory systems, as well as increased organization of the excretory and nervous systems.
The importance of annelids in nature
Many of the polychaete worms serve as the main food for fish, and therefore are of great importance in the cycle of substances in nature.
For example, one of the species of annelids, Nereis, living in the Sea of Azov, serves as food for commercial fish. It was acclimatized by Soviet zoologists in the Caspian Sea, where it multiplied intensively and is now an important part of the diet. sturgeon fish. The polychaete worm, called "palolo" by the natives of Polynesia, is used by them as food.
Earthworms feed on plant debris found in the soil, which is passed through their guts, leaving piles of excrement consisting of soil on the surface. By doing this, they contribute to mixing and, consequently, loosening the soil, as well as enriching it with organic substances, improving the water and gas balance of the soil. Even Charles Darwin noted beneficial influence annelids on soil fertility.
Annelids(Annelida) is a type of the most highly organized worms with a coelom. Their sizes range from a few millimeters to 3 m.
The elongated body is divided by internal annular partitions into segments; sometimes there are several hundred such segments. Each segment may have lateral outgrowths with primitive limbs - parapodia, armed with setae.
Musculature consists of several layers of longitudinal and circular muscles.
Breath carried out by the skin; excretory organs - paired nephridia located segment by segment.
Nervous system consists of a “brain” formed by paired ganglia and a ventral nerve cord.
Closed circulatory system consists of abdominal and dorsal vessels, connected in each segment by small annular vessels. Several of the thickest vessels in the front of the body have thick muscular walls and act as “hearts.” In each segment, blood vessels branch, forming a dense capillary network.
Some annelids hermaphrodites, others differentiate between 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 large number 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.
Polychaete worms. Top row, from left to right: green Nereis, brown Bispira, multilegged Chaetopterus, funnel-shaped Mixicola. Bottom row, from left to right: Dumeril's platinereis, chloe, giant spirobrachus, magnificent protula
Oligochaetes (Oligochaeta) - 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 segments of the body. 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.
Oligochaete worms. From left to right: common earthworm, Aporrectoda longus, Eisenia, tubifex
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.