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General characteristics of annelids 7. On the topic: General characteristics and classification of annelids
Annelids- bilaterally symmetrical three-layer animals with a secondary cavity. They live in marine and fresh water bodies, in soil, and leaf litter.
The body of annelids is divided into a cephalic lobe, a segmented body, and an anal lobe. Metamerism (repetition) of internal organs is associated with external segmentation. The segments of some annelids (polychaetes) bear paired primary limbs - parapodia.
Annelids have a skin-muscular sac, which consists of a cuticle, single-layer epithelium and two layers of muscles (circular, with longitudinal ones underneath). The longitudinal muscles are lined internally by coelomic epithelium.
The secondary body cavity (coelom) is filled with coelomic fluid, which plays the role of the internal environment of the body. In most species, the coelom is metameric, with septa.
The circulatory system is a closed type.
The digestive system is divided into three functionally different sections: ectodermal anterior, endodermal middle and ectodermal posterior. In the anterior and middle sections of the intestine there are differentiated areas (crop, stomach).
Gas exchange occurs either through the body integument (oligochaete worms and leeches) or in the gills (polychaete worms).
The excretory organs of most species are metanephridia, located in pairs in each segment; in some species, protonephridia.
The nervous system includes paired suprapharyngeal and subpharyngeal ganglia and a double ventral nerve cord extending from the latter. The suprapharyngeal and subpharyngeal ganglia are “connected” by commissures into a peripharyngeal ring. The ventral nerve cord consists of ganglia that are metamerically repeated in each segment.
Annelids can be either dioecious or bisexual. The development of many species proceeds with metamorphosis. The larva of sea annelids is called a trochophore. Some species can reproduce asexually.
The phylum Annelids is divided into classes: Polychaeta, Oligochaeta, Leeches (Hirudinea), Echiurida.
The section Bilaterally symmetrical (Bilateria) subkingdom Multicellular also includes.
Number of species: about 75 thousand.
Habitat: in salt and fresh waters, found in soil. Aquatic creatures crawl along the bottom and burrow into the mud. Some of them lead a sedentary lifestyle - they build a protective tube and never leave it. There are also planktonic species.
Structure: bilaterally symmetrical worms with a secondary body cavity and a body divided into segments (rings). The body is divided into the head (head lobe), trunk and caudal (anal lobe) sections. The secondary cavity (coelom), unlike the primary cavity, is lined with its own internal epithelium, which separates the coelomic fluid from the muscles and internal organs. The fluid acts as a hydroskeleton and also participates in metabolism. Each segment is a compartment containing external outgrowths of the body, two coelomic sacs, nodes of the nervous system, excretory and genital organs. Annelids have a skin-muscular sac, consisting of one layer of skin epithelium and two layers of muscles: circular and longitudinal. The body may have muscular outgrowths - parapodia, which are organs of locomotion, as well as bristles.
Circulatory system first appeared during evolution in annelids. It is of a closed type: blood moves only through the vessels, without entering the body cavity. There are two main vessels: the dorsal (carries blood from back to front) and the abdominal (carries blood from front to back). In each segment they are connected by annular vessels. Blood moves due to the pulsation of the spinal vessel or “hearts” - annular vessels of 7-13 segments of the body.
There is no respiratory system. Annelids are aerobes. Gas exchange occurs across the entire surface of the body. Some polychaetes have developed dermal gills - outgrowths of parapodia.
For the first time in the course of evolution, multicellular organisms appeared excretory organs– metanephridia. They consist of a funnel with cilia and an excretory canal located in the next segment. The funnel faces the body cavity, the tubules open on the surface of the body with an excretory pore, through which decay products are removed from the body.
Nervous system formed by the peripharyngeal nerve ring, in which the paired suprapharyngeal (cerebral) ganglion is particularly developed, and by the abdominal nerve chain, consisting of pairwise contiguous abdominal nerve ganglia in each segment. From the “brain” ganglion and the nerve chain, nerves extend to the organs and skin.
Sense organs: eyes - organs of vision, palps, tentacles (antennae) and antennae - organs of touch and chemical sense are located on the head lobe of polychaetes. In oligochaetes, due to their underground lifestyle, the sense organs are poorly developed, but the skin contains light-sensitive cells, organs of touch and balance.
Reproduction and development
They reproduce sexually and asexually - by fragmentation (separation) of the body, due to a high degree of regeneration. Budding also occurs in polychaete worms.
Polychaetes are dioecious, while polychaetes and leeches are hermaphrodites. Fertilization is external; in hermaphrodites, it is cross fertilization, i.e. worms exchange seminal fluid. In freshwater and soil worms, development is direct, i.e. Young individuals emerge from the eggs. In marine forms, development is indirect: a larva, a trochophore, emerges from the egg.
Representatives
Type Annelids are divided into three classes: Polychaetes, Oligochaetes, Leeches.
Oligochaetes mainly live in soil, but there are also freshwater forms. A typical representative living in the soil is the earthworm. It has an elongated, cylindrical body. Small forms are about 0.5 mm, the largest representative reaches almost 3 m (giant earthworm from Australia). Each segment has 8 setae, arranged in four pairs on the lateral sides of the segments. Clinging to uneven soil, the worm moves forward with the help of the muscles of the skin-muscular sac. As a result of feeding on rotting plant remains and humus, digestive system has a number of features. Its anterior section is divided into the muscular pharynx, esophagus, crop and gizzard.
An earthworm breathes over the entire surface of its body due to the presence of a dense subcutaneous network of capillary blood vessels.
Earthworms are hermaphrodites. Cross fertilization. The worms attach themselves to each other with their ventral sides and exchange seminal fluid, which enters the seminal receptacles. After this, the worms disperse. In the anterior third of the body there is a belt that forms a mucous muff in which eggs are laid. As the coupling moves through the segments containing the spermatheca, the eggs are fertilized by sperm belonging to another individual. The muff is shed through the anterior end of the body, becomes compacted and turns into an egg cocoon, where young worms develop. Earthworms are characterized by a high ability to regenerate.
Longitudinal section of the body of an earthworm: 1 - mouth; 2 - pharynx; 3 - esophagus; 4 - goiter; 5 - stomach; 6 - intestine; 7 - peripharyngeal ring; 8 - abdominal nerve chain; 9 - “hearts”; 10 - dorsal blood vessel; 11 - abdominal blood vessel.
The importance of oligochaetes in soil formation. Even Charles Darwin noted their beneficial effect on soil fertility. By dragging the remains of plants into the burrows, they enrich it with humus. By making passages in the soil, they facilitate the penetration of air and water to the roots of plants and loosen the soil.
Polychaetes. Representatives of this class are also called polychaetes. They live mainly in the seas. The segmented body of polychaetes consists of three sections: the head lobe, the segmented body and the posterior anal lobe. The head lobe is armed with appendages - tentacles and carries small eyes. The next segment contains a mouth with a pharynx, which can turn outward and often has chitinous jaws. The body segments have two-branched parapodia, armed with setae and often having gill projections.
Among them there are active predators that can swim quite quickly, bending their bodies in waves (nereids); many of them lead a burrowing lifestyle, making long burrows in the sand or silt (peskozhil).
Fertilization is usually external, the embryo turns into a larva characteristic of polychaetes - a trochophore, which actively swims with the help of cilia.
Class Leeches unites about 400 species. Leeches have an elongated and dorso-ventrally flattened body. At the anterior end there is one oral sucker and at the rear end there is another sucker. They do not have parapodia or setae; they swim, bending their body in waves, or “walk” along the ground or leaves. The body of leeches is covered with a cuticle. Leeches are hermaphrodites and have direct development. They are used in medicine because... Thanks to their release of the protein hirudin, the development of blood clots that clog blood vessels is prevented.
Origin: Annelids evolved from primitive, flatworm-like, ciliated worms. From polychaetes came oligochaetes, and from them came leeches.
New concepts and terms:, polychaetes, oligochaetes, coelom, segments, parapodia, metanephridia, nephrostomy, closed circulatory system, cutaneous gills, trochophore, hirudin.
Questions for consolidation:
· Why did annelids get their name?
· Why are annelids also called secondary cavities?
· What structural features of annelids indicate their higher organization compared to flat and round worms? What organs and organ systems first appear in annelids?
· What is characteristic of the structure of each body segment?
· What is the significance of annelids in nature and human life?
· What are the structural features of annelids in connection with their lifestyle and habitat?
The Class Polychaetes includes free-living annelids, in which numerous long setae are collected in tufts and located on the sides of each segment. Polychaete worms are about \(7000\) species. Most of them live in the seas, where they crawl along the bottom, burrow into silt or swim in the water column.
Like all annelids, the body of Polychaetes consists of segments, the number of which in different species ranges from \(5\) to \(800\). Polychaete worms have a head section and an anal lobe.
On the sides of each body segment there are noticeable skin-muscular outgrowths - organs of movement, which are called parapodia. The worm rakes its parapodia from front to back, clinging to uneven surfaces of the substrate, and thus crawls forward.
Nereid
Sea mouse (Aphrodite)
Nereis
Among Polychaete worms there are sessile forms that build a protective tube and never leave it.
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.
Sessile form of sea worm
Skin-muscle bag
Skin-muscle bag consists of thin cuticles, skin epithelium And muscles. Located under the skin epithelium two layers of muscles: transverse (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.
Transverse and longitudinal sections through the body of Nereis
Digestive system
The digestive system begins mouth, which is located on the ventral side of the head lobe, continues with the muscular throat(many predatory worms have chitinous teeth in it, which serve to capture prey). The throat is followed esophagus And stomach.
The intestine consists of three sections: foregut, midgut and hindgut. Anal hole located on the anal blade.
Free-living polychaete worms are mainly predators, feeding on crustaceans, mollusks, coelenterates and worms. Sessile fish feed on small organic particles and plankton suspended in water.
Respiratory system
In polychaete worms, gas exchange (oxygen absorption and carbon dioxide release) occurs either the entire surface of the body, or 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.
Circulatory system
The circulatory system of annelids is closed: in any part of the worm’s body blood flowsonly by vessels.
There are two main vessels - dorsal and abdominal (one vessel passes above the intestine, the other below it), which are interconnected by numerous semi-circular vessels. 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
The excretory system is represented by paired tubes located in each segment of the body. 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 removed to the outside.
Nervous system
The nervous system consists of paired suprapharyngeal nodes (ganglia), connected by cords into a peripharyngeal ring, a paired abdominal nerve cord and nerves extending from them.
Sense organs are most developed in free-living polychaete worms. 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. Many of the polychaete worms have eyes. There are organs of balance.
Reproduction and development
Mostly polychaete worms 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.
External fertilization. The egg develops into a larva that swims using 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 diverge, and as a result, a chain is formed, including up to \(30\) individuals, but then it breaks up.
The most famous representatives of annelids for every person are leeches (subclass Hirudinea) and earthworms (suborder Lumbricina), which are also called earthworms. But in total there are more than 20 thousand species of these animals.
Taxonomy
Today, experts classify from 16 to 22 thousand modern animal species as annelids. There is no single approved classification of ringlets. The Soviet zoologist V.N. Beklemishev proposed a classification based on the division of all representatives of annelids into two superclasses: non-girdle worms, which includes polychaetes and echiurids, and girdle worms, which includes oligochaetes and leeches.
Below is the classification from the World Register of Marine Species website.
Table of biological taxonomy of annelids
| Class* | Subclass | Infraclass | Squad |
| Polychaete worms, or polychaetes (lat. Polychaeta) | |||
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| Polychaeta incertae sedis (disputed species) | |||
| Sedentaria | Canalipalpata |
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| Scolecida |
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| Palpata |
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| Errantia (sometimes called Aciculata) |
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| Belt class (Clitellata) | Leeches (Hirudinea) | Acanthobdellidea | |
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Oligochaete worms |
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Echiuridae |
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There is also a superclass Annelida incertae sedis, which includes the controversial species. There, according to the World Register of Marine Species, such a controversial group as Myzostomida, which other classifications classify as polychaete worms or even separate them into a separate class, was included as an order.
- Class Polychaetes(Polychaetes). Representatives of the class have connected lateral appendages (parapodia) bearing chitinous setae; The name of the group is determined by the presence of a large number of setae per segment. Head with or without appendages. In most cases - dioecious; gametes are discharged directly into the water, where fertilization and development occur; free floating and are called trochophores. Sometimes they reproduce by budding or fragmentation. The class includes more than 6,000 species, which are divided into free-living and sessile forms.
- Class Girdle (Clitellata). Representatives of the class have an insignificant amount or no bristles on their body. There are no parapodia. They are characterized by the presence of a unique reproductive organ - the girdle, which is formed from the remains of the cocoon and performs a protective function for fertilized eggs. The class has about 10,000 representatives.
- Subclass Oligochaetes(Oligochaetes). They live primarily in fresh water. They have setae that arise directly from the walls of the body, due to the small number of which (usually 4 on each segment) the subclass is called oligochaete. As a rule, they do not have appendages on the body. Hermaphrodites. Development is direct, there is no larval stage. There are about 3250 species.
- Leech subclass. They inhabit mainly freshwater bodies, but there are also terrestrial and marine forms. There is a small sucker at the anterior end of the body and a large sucker at the posterior end. The fixed number of body segments is 33. The body cavity is filled with connective tissue. Hermaphrodites. Fertilized eggs are laid in a cocoon. Development is direct, there is no larval stage. There are about 300 species of representatives.
Class Echiura. This is a small group with only about 170 known species, all of which are exclusively marine inhabitants. Echiurids were recently classified as annelids after DNA examinations, but previously it was a separate type. The reason is that their body is different - it does not have segmentation, like those of ringed animals. In some sources, the Echiurides are considered not as a separate class, but as a subclass of the Polychaetes.
Spreading
Annelids, depending on the species, live on land, in fresh and salt water.
Polychaete worms, as a rule, live in seawater (with the exception of some species that can also be found in freshwater bodies). They are food for fish, crayfish, as well as birds and mammals.
Oligochaete worms, to which the earthworm belongs, live in soil fertilized with humus or fresh water bodies.
Echiurids are distributed only in marine waters.
Morphology
The main characteristic of representatives of the Annelida phylum is considered to be the division of the body into a number of cylindrical segments, or metameres, the total number of which varies widely depending on the type of worm. Each metamer consists of a section of the body wall and a compartment of the body cavity with its internal organs. The number of outer rings of worms corresponds to the number of internal segments. The annelid body consists of a head region (prostomium); a body consisting of metameres; and a segmented posterior lobe called the pygidium. In some primitive representatives of this type the metamers are identical, or very similar to each other, each containing the same structures; in more advanced forms there is a tendency to consolidate certain segments and restrict certain organs to certain segments.
The outer shell of the annelid body (muscular sac) includes the epidermis surrounded by a cuticle, as well as well-developed, segmentally located muscles - circular and longitudinal. Most annelids have short external setae composed of chitin. In addition, on each metamere, some representatives of this type of animal may have primitive limbs called parapodia, on the surface of which bristles and sometimes gills are located. The spatial movement of worms is carried out either through muscle contraction or movements of parapodia.
The body length of annelids varies from 0.2 mm to 5 m.
Basic general anatomical features of annelids in cross section Digestive system annelids consists of an unsegmented gut that passes through the middle of the body from the mouth, located on the underside of the head, to the anus, located on the anal lobe. The intestine is separated from the body wall by a cavity called the coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue called septa, which perforate the intestine and blood vessels. With the exception of leeches, the whole of annelids is filled with fluid and functions as a skeleton, providing muscle movement, as well as transport, sexual, and excretory functions of the body. If the integrity of the worm's body is damaged, it loses the ability to move properly, since the functioning of the body's muscles depends on maintaining the volume of coelomic fluid in the body cavity. In primitive annelids, each compartment of the coelom is connected to the outside via channels for the release of germ cells and paired excretory organs (nephridia). In more complex species, both excretory and reproductive functions are sometimes served by one type of canals (and the canals may be absent in certain segments).
Circulatory system. Annelids developed a circulatory system for the first time in the process of evolution. Blood typically contains hemoglobin, a red respiratory pigment; however, some annelids contain chlorocruorin, a green respiratory pigment that gives the blood its corresponding color.
The circulatory system is usually closed, i.e. enclosed in well-developed blood vessels; in some species of polychaetes and leeches, an open-type circulatory system appears (blood and cavity fluid mix directly in the sinuses of the body cavity). The main vessels - abdominal and dorsal - are connected to each other by a network of annular vessels. Blood is distributed in each segment of the body along the lateral vessels. Some of them contain contractile elements and serve as a heart, i.e. play the role of pumping organs that move the blood.
Respiratory system. Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most representatives of this type of invertebrate do not have any special organs for gas exchange, and respiration occurs directly through the surface of the body.
Nervous system, as a rule, consists of a primitive brain, or ganglion, located in the head region, connected by a ring of nerves to the ventral nerve cord. In all metameres of the body there is a separate nerve ganglion.
The sense organs of ringed fish usually include eyes, taste buds, tactile tentacles and statocysts - organs responsible for balance.
Reproduction Annelids occur sexually or asexually. Asexual reproduction is possible through fragmentation, budding or fission. Among the worms that reproduce sexually, there are hermaphrodites, but most species are dioecious. Fertilized ringed eggs usually develop into free-swimming larvae. The eggs of terrestrial forms are enclosed in cocoons and larvae, like miniature versions of the adults.
The ability to restore lost body parts is highly developed in many multi- and oligochaete representatives of annelids.
Ecological significance
The earthworm has a very important to maintain soil condition Charles Darwin, in The Formation of Vegetable Mold through the Action of Worms (1881), presented the first scientific analysis of the effect of earthworms on soil fertility. Some of the worms dig burrows in the soil, while others live exclusively on the surface, usually in damp leaf litter. In the first case, the animal is able to loosen the soil so that oxygen and water can penetrate into it. Both surface and burrowing worms help improve soil in several ways:
- by mixing organic and mineral substances;
- by accelerating the decomposition of organic substances, which in turn makes them more accessible to other organisms;
- by concentrating minerals and converting them into forms that are more easily absorbed by plants.
Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers in some cases.
Terrestrial annelids in some cases can be invasive (brought into a certain area by people). In glacial areas of North America, for example, scientists believe that almost all native earthworms were killed off by glaciers and the worms currently found in these areas (such as Amynthas agrestis) were introduced from other areas, primarily Europe , and more recently, from Asia. Northern deciduous forests have been particularly negatively impacted by invasive worms through loss of leaf litter, decreased soil fertility, changes in soil chemistry, and loss of ecological diversity.
Marine annelids may account for more than one-third of benthic animal species around coral reefs and in intertidal zones. Burrowing annelid species increase the penetration of water and oxygen into seafloor sediment, which promotes the growth of populations of aerobic bacteria and small animals.
Human interaction
Anglers find that worms are more effective baits for fish than artificial fly baits. In this case, the worms can be stored for several days in a tin can filled with damp moss.
Scientists study aquatic annelids to monitor oxygen levels, salinity and pollution environment in fresh and sea water.
The jaws of polychaetes are very strong. These advantages attracted the attention of engineers. Research has shown that the jaws of this genus of worms are made of unusual proteins that bind strongly to zinc.
On the island of Samoa, catching and eating one of the representatives of annelids - the Palolo worm - is a national holiday, and the worm itself is considered a delicacy by local residents. In Korea and Japan, the worms Urechis unicinctus from the class Echiuridae are eaten.
Representatives of annelids that are eaten Cases of the use of leeches for medicinal purposes were known in China around 30 AD, India around 200 AD, ancient Rome around 50 AD, and then throughout Europe. In 19th-century medical practice, the use of leeches was so widespread that supplies in some areas of the world were depleted, and some regions imposed restrictions or bans on their export (with medicinal leeches themselves considered an endangered species). More recently, leeches have been used in microsurgery for transplantation of organs and their parts, and skin areas. In addition, scientists claim that the saliva of medicinal leeches has an anti-inflammatory effect, and some anticoagulants contained in it prevent the growth of malignant tumors.
About 17 species of leeches are dangerous to humans.
Medical leeches are used for hirudotherapy, and a valuable remedy, hirudin, is extracted from pharmaceuticals. Leeches can attach to a person’s skin from the outside, or penetrate internal organs (for example, the respiratory system or gastrointestinal tract). In this regard, two types of this disease are distinguished - internal and external hirudinosis. With external hirudinosis, leeches most often attach to human skin in the armpits, neck, shoulders, and calves.
Misostomidae on sea lily Even the farmers of Ancient Egypt saw earthworms as a guarantee of future harvests. Aristotle called them the intestines of the earth. And this is true: by passing earth and plant debris through their intestines, worms enrich the soil. In the 50s of our century, the question arose about breeding worms specifically as producers of very valuable, environmentally friendly fertilizer. The concept of “Vermiculture” arose - the culture of breeding worms. The red Californian worm was bred, which is used to create vermiculture. Biohumus can be grown both on an industrial scale and in an apartment, on a balcony and in a summer cottage. The Californian is a wonderful pet. It can be placed at home in a box made of wood or plywood, even in a cardboard box, but lined with polyethylene on the inside, in an old glass aquarium, in a plastic box.
Now the topic of annelids is of particular interest, thanks to the latest research by scientists who are revealing more and more amazing abilities of these animals. For example, it has recently become known that annelids are able to distinguish sharp angles. Another amazing ability is that most worms use their "photon installations" to disorient their opponents. Worms in the food pyramid of the ocean occupy one of the lower steps, serving as food for a wide variety of organisms - cephalopods, crayfish, crabs, fish and even aggressive relatives of polychaetes.
Ringed worm. Photo: Chanabun R, Sutcharit C, Tongkerd P, Panha S
When a predator attacks a polychaete and begins to tear and tear its body, the tail section of the worm flashes brightly, attracting the attention of the “aggressor”. He grabs the luminous part of the body, and the second (head) disappears in the darkness. Subsequently, the worm's tail grows back. It turns out that annelids, long before lizards, were the inventors of a clever trick with a discarded tail.
The object of study of this course work is the type of annelids. Given a brief description of This type of worm features the organization of annelids. In the practical part of the work, such classes of this type were considered as the leech class, the polychaete class, the oligochaete class, and the echiurida class. The systems of these worms and their features are described.
The first part of the work provides general information about the type of annelids. The practical part of the work contains information about some classes of worms of this type.
general characteristics type of annelids
Annelids are a large group of animals, including about 12 thousand species that live mainly in the seas, as well as in fresh waters and on land. This is a group of non-skeletal invertebrates, which for this reason are of particular importance in the nutrition of other animals, as they are digested without residue. At the same time, they all actively participate in the destruction of organic matter in biocenoses, contributing to the biogenic cycle. Marine forms are especially diverse, which are found at different depths down to the extreme (up to 10 - 11 kilometers) and in all latitudes of the World Ocean. They play a significant role in marine biocenoses and have high density settlements: up to 100 thousand specimens per 1 square meter of bottom surface. Sea rings are a favorite food of fish and occupy an important position in the trophic chains of marine ecosystems.
Earthworms, or, as we call them, earthworms, are the most numerous in the soil. Their density in forest and meadow soils can reach 600 specimens per 1 square meter. Earthworms participate in the process of soil formation and help increase crop yields and the productivity of natural biocenoses. Blood-sucking ringlets - leeches live mainly in fresh waters, and in tropical areas they are found in the soil and on trees. They are used in medicine to treat hypertension.
Let us consider the main features of the organization of the type of annelids as the first coelomic animals.
1. Metamerism of external and internal structure. Metamerism is the repetition of identical parts or rings along the main axis of the body (from the Latin words meta - repetition, mera - part). The body is worm-shaped, divided into segments or segments. Many organ systems are repeated in each segment. The body of annelids consists of a head lobe, a segmented body and an anal lobe.
2. There is a skin-muscular sac, consisting of skin epithelium, circular and longitudinal muscles, which are lined from the inside by coelomic epithelium.
3. The secondary body cavity (coelom) is filled with coelomic fluid, which acts as the internal environment of the body. In general, a relatively constant biochemical regime is maintained and many functions of the body are carried out (transport, excretory, sexual, musculoskeletal).
4. The intestine consists of three functionally different sections: the foregut, midgut and hindgut. Some species have salivary glands. The anterior and posterior sections are ectodermal, and the middle section of the digestive system is of endodermal origin.
5. Most ringlets have a closed circulatory system. This means that blood flows only through vessels and has a network of capillaries between arteries and veins.
6. The main excretory organs are metanephridia of ectodermal origin. Each pair of metanephridia begins in one segment with generally open funnels, from which the excretory canals continue into the next segment and open there outwards with paired openings. Metanephridia are not only excretory organs, but also the regulation of water balance in the body. In the metanephridia channels, the excretory products are thickened (ammonia is converted into uric acid), and water is absorbed back into the coelomic fluid. This saves moisture in the body and maintains a certain water-salt regime as a whole. Saving moisture is especially necessary for ground and soil rings.
7. The nervous system consists of paired dorsal ganglia and a ventral nerve cord with metamerically repeating paired ganglia in each segment. The appearance of the brain, located dorsally above the pharynx, significantly distinguishes annelids from flatworms. The paired dorsal lobes of the annular brain are divided into anterior, middle and posterior ganglia. This feature of the brain structure distinguishes ringworms from roundworms.
8. Annelids are usually dioecious, but simultaneous development of male and female gonads (hermaphroditism) is often observed.
9. Development often occurs with metamorphosis. The typical larva of sea ringnecks is a trochophore.
Thus, in the organization of annelids, progressive features of the organization of coelomic animals can be traced: the presence of a coelom, metamerism of structure, the appearance of a circulatory system, an excretory system such as metanephridia, a more highly organized nervous system and sensory organs. This is how ringworms differ from lower flat and round worms.
However, a number of features in the organization of ringworms indicate their relationship with lower worms. Thus, the ringlet larvae - trochophores - have a primary body cavity, protonephridia, an orthogonal nervous system and, in the early stages, a cecum. These features are sometimes found in adult ringlets from primitive groups.
The type of annelids is divided into classes:
Class Primary ringlets (Archiannelida),
Class Polychaetes (Polychaeta),
Class Oligochaeta,
Class Leeches (Hirudinea),
Class Echiurida,
Class Sipunculida.
Signs and variety of ringlets
About 200 years ago, the great French naturalist J. Cuvier, working on the creation of a system of the animal world, identified six types of animals, including the articulated type, in which he united all creatures whose body is divided into segments: insects, crayfish, spiders, woodlice, earthworms and leeches. modern science has more extensive information about leeches and earthworms, and therefore these worms are classified as a special type - ringworms.
Annelids are characterized by the appearance of the following organizational features: the presence of a secondary body cavity or cellome, a circulatory system, the presence of metamerism - segmented body
In addition to the above-mentioned features, which play an important role in the evolution of animals, annelids are also characterized by the presence of special organs of movement - parapodia, significant development of the central nervous system, consisting of the suprapharyngeal ganglion and the abdominal nerve cord with nerve ganglia; the presence of a closed circulatory system, metanephridial structure of the excretory system.
1 Primary rings
2 Polychaetes
3 Oligochaetes
5 Echiurides
6 Sipunculidae
External structure of annelids
Annelids are the most highly organized representatives of the group of worms. The sizes of the rings range from fractions of a millimeter to two and a half meters. These are predominantly free-living forms. The body of the ringlets is divided into three parts: the head, the body, consisting of rings, and the anal lobe. Animals that are lower in their organization do not have such a clear division of the body into sections.
The ringlet's head is equipped with various sensory organs. Many ringlets have well-developed eyes. Some species have particularly acute vision, and their lens is capable of accommodation. True, eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. Ringworms also have developed senses of taste. On the head and tentacles, many of them have special olfactory cells and ciliary fossae, which perceive various odors and the actions of many chemical irritants. The ringed birds have well-developed hearing organs, arranged like locators. Recently, the sea ringed echiruids have discovered hearing organs that are very similar to the lateral line organs of fish. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds, which are heard much better than in the air.
Internal structure of the rings
Digestive system consists of three sections: foregut, midgut and hindgut. The foregut is highly differentiated into a number of organs: mouth, pharynx, esophagus, crop, stomach.
Circulatory system closed. It consists of large longitudinal vessels - dorsal and abdominal, connected in each segment by annular vessels. The movement of blood is carried out due to the pumping activity of the contractile areas of the spinal cord, and less commonly of the annular vessels. Blood plasma contains respiratory pigments similar to hemoglobin, thanks to which ringworms have populated habitats with very different oxygen content. Many annelids have red blood, like humans. It is colored this way, naturally, due to the presence of iron. But at the same time, iron is part of a completely different pigment, not similar to hemoglobin - hemerythrin. It is capable of capturing 5 times more oxygen than hemoglobin. The choice of pigment is determined by the lifestyle characteristics of such worms. These are bottom-dwelling creatures that spend most of their time in the soil, where they experience an acute lack of oxygen.
Respiratory system in polychaete worms, the gills are thin-walled, leaf-shaped, feathery or bushy outer outgrowths of part of the dorsal lobes of the parapodium, penetrated by blood vessels. Oligochaete worms breathe over the entire surface of their body.
Excretory organs- metanephridia located in pairs in each segment, removing the final waste products from the cavity fluid. The funnel of metanephridium is located in the coelom of one segment, and the short tubule extending from it opens outward in the next segment.
Nervous system ganglion type. It consists of paired suprapharyngeal and subpharyngeal ganglia, connected by nerve trunks into the peripharyngeal nerve ring, and many pairs of ganglia of the ventral nerve cord, one pair in each segment.
Sense organs. A number of ringlets have well-developed sensory organs, primarily the eyes. Unlike humans and other warm-blooded animals, worms sometimes have a significant number of eyes, which can be located on the head, at the rear end of the body, on the sides (including on each segment) and even on the tail. Marine polychaetes not only react sensitively to light, but are also capable of independently emitting it.
Waste products of worms. Photo: Chanabun R, Sutcharit C, Tongkerd P, Panha S
Reproduction of ringlets. The vast majority of ringlets are dioecious animals, less often hermaphrodites. The gonads develop either under the coelomic epithelium in all body segments (in polychaete worms), or only in some (in oligochaete worms). In polychaete worms, germ cells enter the coelomal fluid through breaks in the coelomic epithelium, from where they are released into the water by special sex funnels or metanephridia. In most aquatic ringlets, fertilization is external, while in soil forms it is internal. Development with metamorphosis (in polychaete worms) or direct (in polychaete worms, leeches). Some types of ringworms, in addition to sexual reproduction, also reproduce asexually (by fragmentation of the body with subsequent regeneration of the missing parts). The phylum Annelids are divided into three classes - Polychaetes, Oligochaetes and Leeches.
Peculiarities of reproduction of annelids
Annelids can reproduce either sexually or asexually. The first is most typical of aquatic species, especially some marine polychaetes. Asexual reproduction comes down to either dividing the body into parts or budding. When dividing, the worm's body splits into halves, each of which subsequently restores the missing end.
It is curious that the tail end, after separation, is an independent creature and is capable of growing a new head. Sometimes this head grows back long before the worm has split in half. In the middle of the body of such a ringlet, preparing to prolong the race, there is a second head. After some time, the two-headed creature disintegrates to give birth to two new worms.
The cocoon of the medicinal leech easily feeds its many young ones before they have their heads.
Fertilization in marine worms that reproduce sexually is external. Females and males release reproductive cells into the water, where sperm merge with eggs. Subsequently, the eggs hatch into larvae - trochophores, which are not similar to adult individuals. Terrestrial and freshwater ringers, including leeches, have direct development, with young individuals copying adults almost exactly. Young leeches develop from cocoons containing eggs.
Glow plays an important role in the reproduction of annelids. The glow of the worms is ensured by the presence in the body of a special substance called luciferin. Under the action of a special enzyme, luciferase, luciferin is oxidized by oxygen to form carbon dioxide. In this case, the released chemical energy goes towards the release of light particles - photons - by excited atoms. Luciferin is contained in worms in granules that float in the liquid cellular substance, where they are oxidized. Therefore, it seems as if the body tissues of polychaetes glow.
Worms are endowed with a relatively efficient oxidation mechanism, the efficiency of which is from 10 to 20 percent and perhaps even more. This means that marine polychaetes have learned to convert over 10 percent of the chemical energy of luciferin into light, and the rest is useless losses. Compared to incandescent lamps, polychaete cells are extremely economical and compact biological devices. Thus, for every 3 molecules of luciferin there are 3 oxygen molecules, and as a result of the reaction 3 molecules of carbon dioxide and 2 photons are released.
The biological significance of the glow may be different. There are cases when invertebrates use their illumination for the purpose of communicating with relatives, primarily with representatives of the opposite sex. Once a year, many tropical worms leave their bottom shelters and swim to the surface of the ocean to swarm. Here females meet males.
Polychaetes of the Bermuda Triangle use flashlights during swarming. Females attract males with an intense glow, causing them to dance, during which the gentlemen must encourage their ladies to throw sexual products into the water. If the female does not glow, then she has already performed her “dance of love.” Males are not interested in her. Probably, the “moving candles” that Columbus observed in the waters of the Caribbean Sea were such swarming worms.
