According to the classification, annelids belong to the group of invertebrate animals, the type of protostomes, which have a secondary body cavity (coelom).
The type of annelids (or annelids) includes 5 classes: belt worms (leeches), polychaetes (earthworm), polychaetes (nereid, sandworm) worms, mysostomids, dinophylids. This type includes about 18 thousand species of worms. Free-living ringworms are distributed throughout our planet; they live in freshwater and saltwater bodies of water and soil.
This group includes characteristic representatives of ringworms - oligochaete worms and leeches. Aeration and loosening of 1 sq.m of soil is carried out on average from 50 to 500 rings. Marine forms of annelids are distinguished by their diversity, which are found at different depths and throughout the World Ocean. They play an important role in the food chains of marine ecosystems.
Annelids have been known since the Middle Cambrian period.
It is believed that they descended from lower flatworms, since certain features of their structure indicate the similarity of these groups of animals. Polychaete worms are distinguished as the main class of the annelid type. Later in the course of evolution, in connection with the transition to a terrestrial and freshwater lifestyle, oligochaetes evolved from them, which gave rise to leeches.
All annelids have a characteristic structure.
Main characteristic: their bilaterally symmetrical body can be divided into a head lobe, a segmented body and a posterior (anal) lobe. The number of body segments can range from tens to several hundred. Dimensions vary from 0.25 mm to 5 m. At the head end of the rings there are sensory organs: eyes, olfactory cells and ciliary fossae, which react to the action of various chemical stimuli and perceive odors, as well as hearing organs, which have a structure similar to locators.
Sensory organs can also be located on the tentacles. The body of annelids is divided into segments in the form of rings. Each segment, in a certain sense, represents an independent part of the whole organism, since the coelom (secondary body cavity) is divided by partitions into segments in accordance with the outer rings.
Therefore, this type is given the name “ringed worms.” The significance of this division of the body is enormous. When damaged, the worm loses the contents of several segments, the rest remain intact, and the animal quickly regenerates.
Metamerism (segmentation) of internal organs, and, accordingly, organ systems of annelids is due to the segmentation of their bodies. The internal environment of the annular organism is coelomic fluid, which fills the coelom in the skin-muscular sac, consisting of the cuticle, skin epithelium and two groups of muscles - circular and longitudinal. In the body cavity, the biochemical constancy of the internal environment is maintained, and the transport, sexual, excretory, and musculoskeletal functions of the body can be realized.
More ancient polychaete worms have parapodia (paired primitive limbs with bristles) on each body segment. Some types of worms move by contracting muscles, while others use parapodia.
The oral opening is located on the ventral side of the first segment. Digestive system of annelids 
end-to-end
The intestine is divided into the foregut, midgut and hindgut. The circulatory system of annelids is closed, consisting of two main vessels - dorsal and abdominal, which are connected to each other by ring vessels like arteries and veins. The blood of this type of worm can be different colors among different species: red, green or clear. This depends on the chemical structure of the respiratory pigment in the blood. The respiration process is carried out over the entire surface of the worm's body, but some types of worms already have gills.
The excretory system is represented by paired protonephridia, metanephridia or myxonephridia (prototypes of the kidneys), present in each segment. The nervous system of annelids includes a large nerve ganglion (the prototype of the brain) and a ventral nerve cord of smaller ganglia in each segment. Most annelids are dioecious, but some have secondarily developed hermaphroditism (as in the earthworm and leech).
Fertilization occurs inside the body or in the external environment.
The importance of annelids is very great. It is worth noting their important role in food chains in their natural habitat. On the farm, people began to use marine species of ringed fish as a food source for growing valuable commercial fish species, for example sturgeon.
The earthworm has long been used as fishing bait and as food for birds. The benefits of earthworms are enormous, as they aerate and loosen the soil, which increases crop yields. In medicine, leeches are widely used for hypertension and increased blood clotting, as they secrete a special substance (hirudin) that has the property of reducing blood clotting and dilating blood vessels.
Related articles:
Worms
2. Flatworms
3. Roundworms
4. Oligochaetes
Structural features of annelids
Annelids are the most highly organized worms. They are the most advanced type of worms. Features that distinguish this type of worm from other types are the presence of cellome and metamerism of the structure. Based on this, annelids can be called coelomic animals with a high organization.
In addition, annelids play a very important role in the biocenosis.
They are widespread everywhere. The most diverse are the marine forms of ringlets. An important role is played by annelids that live in the ground and decompose complex organic compounds.
Also, ringlets play an important role not only in the biocenosis of nature, but also for human health. For example, leeches, on which hirudotherapy is based, help cure patients from quite complex diseases without the use of medications.
If we dwell in more detail on the structure of annelids, we can find that some annelids have enhanced vision, and the eyes can be located not only on the head, but also on the body and tentacles.
This type of worm also has developed taste sensations, and, based on research by biologists, they have the rudiments of logical thinking. This is due to the fact that worms can find sharp corners.
If we consider the internal structure, we can also note many features indicating the progressive structure of annelids.
An example of this is that most annelids are dioecious, only a small part are hermaphrodites. Development with metamorphosis occurs in polychaete worms and without metamorphosis in oligochaetes and leeches.
The circulatory system, like annelids, also has a special structure, because blood is pumped through blood vessels. In addition, the circulatory system is closed, which also in turn indicates the progressive structural features of annelids.
Also, the most important difference between annelids and all main types of worms is the appearance of the brain, located dorsally above the pharynx.
Of particular interest is the reproduction of annelids and methods of attracting individuals of the opposite sex. One of these methods is glow. Worms use it not only for reproduction, but also for protection. They lure predators to themselves and, with the help of glow, teach them to eat parts of the body that are unimportant for the worm, which it can easily restore without damage to the body.
If we consider the classes of worms, some of which are described in detail in the coursework, we can also highlight certain features of each class.
Polychaete worms are the most diverse in shape and color, most of which live in the seas.
Most of them lead a burrowing lifestyle, burrowing into the substrate or attaching to it. Sessile polychaetes and crawling polychaetes are also known. They carry out movement due to bristles, which often have bright colors of all colors of the rainbow.
When considering the next group, you can also see structural features associated with the lifestyle of worms.
And if in the previous case, polychaetes were characterized by a large number of setae for swimming and burrowing in silt, then the oligochaetes are characterized by a non-separated head section, a streamlined body, a small number of setae, all this is associated with a burrowing lifestyle, because many oligochaetes live in the ground and water and isolated individuals in the sea.
Leeches have adaptations for feeding on the blood of various animals: chitinous serrated plates, a large number of glands that secrete mucus, as well as the presence in the body of an enzyme that anesthetizes the bite and liquefies the blood of the victim.
Echiurids are marine burrowing worms.
Their body, unlike all other classes of worms, is not segmented and is often equipped with a proboscis.
Features of the organization of the earthworm
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Body structure |
The body is elongated, round, segmented. The symmetry is bilateral, the ventral and dorsal sides of the body, the anterior and posterior ends are distinguished. There is a secondary body cavity, lined with epithelium and filled with fluid. Locomotion using a skin-muscle bag. |
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Digestive system |
Digestive system - mouth, pharynx, esophagus, goiter, stomach, midgut, hindgut, anus, glands. |
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Respiratory system. Circulatory system. Excretory system |
The circulatory system is closed and consists of vessels. There are larger vessels - the hearts - that push blood through. Blood contains hemoglobin. Cavity fluid provides communication between the circulatory system and cells. Breathing through the entire surface of the body. The excretory system contains a pair of nephridia in each segment. |
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Nervous system, sensory organs |
Nodal type: paired cephalic ganglion, paired peripharyngeal cords connecting to the abdominal ones. Many annelids have sensory organs: eyes, olfactory pits, organs of touch. In earthworms (due to their underground lifestyle), the sense organs are represented by tactile and photosensitive cells over the entire surface of the body. |
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Reproduction |
Dioecious or secondary hermaphrodites. Fertilization is cross-fertilization, internal (in aquatic forms in water). Development is direct. Some marine annelids undergo metamorphosis and have a floating larva. Capable of regeneration. |
Question 1. What features of ringed worms allowed them to populate most of the planet?
Annelids have acquired a number of features in structure and physiology that allowed them to survive in a variety of environmental conditions.
Firstly, annelids developed specialized organs of locomotion, which gave them relative independence from the physical properties of their habitat.
These are parapodia in polychaetes, which ensure movement in the water column and along the bottom, and bristles in oligochaetes, which help with movement in the soil.
Secondly, in annelids, the nervous system and sensory organs have achieved significant development. Which allows you to increase the activity of your lifestyle.
Thirdly, annelids have mechanisms that make it possible to tolerate unfavorable environmental conditions.
For example, soil species of oligochaetes are characterized by diapause (see answer to the question
2), and some types of leeches are capable of falling into suspended animation (see answer to question 2).
Question 2. What adaptations do annelids have to endure unfavorable conditions?
How does this happen?
In soil species, in the event of unfavorable conditions, worms crawl to depths, curl up into a ball and, secreting mucus, form a protective capsule; they enter diapause - a condition in which the processes of metabolism, growth and development slow down.
Leeches living in cold waters can fall into suspended animation in winter - a state of the body in which life processes are so slow that all visible manifestations of life are absent.
Question 3.
What allows scientists to classify polychaetes, oligochaetes and leeches as one phylum?
All named animals have a number of characteristics that characterize their belonging to one type - annelids. All of them are multicellular animals with an elongated worm-like body, which has bilateral symmetry and consists of individual rings (segmental structure).
The internal cavity of these worms is divided by partitions into separate segments, inside of which there is liquid.
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- what features of annelids allowed them to colonize large
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Encyclopedia "Animal Life" (1970)
To the beginning of the encyclopedia
By first letter
BINANDTONABOUTPRWITHTFSCH
TYPE RINGED WORMS (ANNELIDES)
TO ringworms belong primary ringlets, polychaete and oligochaete worms, leeches and echiurids.
In the phylum of annelids there are about 8 thousand species. These are the most highly organized representatives of the group of worms. The sizes of the rings range from fractions of a millimeter to 2.5 m. 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 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 effects of many chemical irritants.
The ringed birds have well-developed hearing organs, arranged like locators. Recently, hearing organs have been discovered in sea ringed echiurids, 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 in water than in air.
The body of the ringlets consists of rings, or segments. The number of rings can reach several hundred. Other ringlets consist of only a few segments. Each segment to some extent represents an independent unit of the whole organism.
Each segment includes parts of vital organ systems.
Special organs of movement are very characteristic of ringlets. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "foot-like". Parapodia are lobe-shaped outgrowths of the body from which tufts of bristles protrude outward. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all ringlets. They are found in primary ringworms and polychaete worms.
In oligochaetes only the setae remain. Primitive leech acanthobdella has bristles. Other leeches move without parapodia and setae. U ehiurid there is no parapodia, and setae are present only at the posterior end of the body.
Parapodia, nodes of the nervous system, excretory organs, gonads and, in some polychaetes, paired intestinal pouches are systematically repeated in each segment. This internal segmentation coincides with the external annulation. The repeated repetition of body segments is called the Greek word “metamerism”.
Metamerism arose in the process of evolution in connection with the elongation of the body of the ancestors of ringlets. Lengthening the body necessitated repeated repetition, first of the organs of movement with their muscles and nervous system, and then of the internal organs.
Extremely characteristic of ringlets is the segmented secondary body cavity, or coelom. This cavity is located between the intestines and the body wall. The body cavity is lined with a continuous layer of epithelial cells, or coelothelium.
These cells form a layer covering the intestines, muscles and all other internal organs. The body cavity is divided into segments by transverse partitions - dissepiments. A longitudinal septum, the mesenterium, runs along the midline of the body, dividing each compartment of the cavity into right and left parts.
The body cavity is filled with liquid, which in its chemical composition is very close to sea water. The fluid filling the body cavity is in continuous motion. The body cavity and abdominal fluid perform important functions. Cavity fluid (like any fluid in general) does not compress and therefore serves as a good “hydraulic skeleton”.
The movement of the cavity fluid can transport various nutritional products, secretions of the endocrine glands, as well as oxygen and carbon dioxide involved in the respiration process inside the body of the ringlets.
Internal partitions protect the body in case of severe injuries and ruptures of the body wall.
For example, an earthworm cut in half does not die. The septa prevent cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. Sea ships and submarines also have internal hermetic partitions. If the side is broken, then the water pouring into the hole fills only one damaged compartment. The remaining compartments, not flooded with water, maintain the buoyancy of the damaged ship.
Likewise, in ringworms, disruption of one segment of their body does not entail the death of the entire animal. But not all annelids have well-developed septa in the body cavity. For example, in echiurids the body cavity does not have partitions. A puncture in the body wall of an echiurid can lead to its death.
In addition to the respiratory and protective role, the secondary cavity acts as a container for reproductive products that mature there before being excreted.
Rings, with few exceptions, have a circulatory system. However, they have no heart. The walls of large vessels themselves contract and push blood through the thinnest capillaries.
In leeches, the functions of the circulatory system and the secondary cavity are so identical that these two systems are combined into a single network of lacunae through which blood flows. In some rings the blood is colorless, in others it is colored green by a pigment called chlorocruorin. Often ringlets have red blood, similar in composition to the blood of vertebrates.
Red blood contains iron, which is part of the hemoglobin pigment. Some ringlets, burrowing into the ground, experience an acute oxygen deficiency.
Therefore, their blood is adapted to bind oxygen especially intensively. For example, the polychaete Magelona papillicornis has a pigment called hemerythrin, which contains five times more iron than hemoglobin.
In ringlets, compared to lower invertebrates, metabolism and respiration are much more intense. Some polychaete ringlets develop special respiratory organs - gills. A network of blood vessels branches out in the gills, and through their wall oxygen penetrates into the blood and is then distributed throughout the body.
Gills can be located on the head, parapodia and tail.
The through intestine of ringlets consists of several sections. Each section of the intestine performs its own special function. The mouth leads into the throat. Some ringlets have strong horny jaws and teeth in their throats, which help them grasp live prey more firmly. In many predatory ringlets, the pharynx serves as a powerful weapon of attack and defense.
The pharynx is followed by the esophagus. This section is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push food into the next sections. In the wall of the esophagus there are glands, the enzyme of which serves for the primary processing of food.
Following the esophagus is the midgut. In some cases, goiter and stomach are developed. The wall of the midgut is formed by epithelium, very rich in glandular cells that produce digestive enzymes. Other cells in the midgut absorb digested food. Some ringlets have a midgut in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths on the sides of the intestine.
The hindgut ends at the anus.
Special organs - metanephridia - serve to secrete liquid metabolic products. Often they serve to bring out germ cells - sperm and eggs. Metanephridia begins as a funnel in the body cavity; from the funnel there is a convoluted channel, which opens outward in the next segment.
Each segment contains two metanephridia.
Rings reproduce asexually and sexually. Asexual reproduction is common in aquatic ringworms. At the same time, their long body breaks up into several parts. After some time, each part restores its head and tail.
Sometimes a head with eyes, tentacles and a brain forms in the middle of the worm's body before it splits into parts. In this case, the separated parts already have a head with all the necessary sensory organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These ringlets have lost their segmented body cavity. This is partly why, apparently, they lack the ability to reproduce asexually and restore lost parts.
Fertilization of eggs in ringed fish most often occurs outside the body of the mother's body. In this case, males and females simultaneously release reproductive cells into the water, where fertilization occurs.
In marine polychaetes and echiurids, the crushing of fertilized eggs leads to the development of a larva, which is not at all similar to adult animals and is called a trochophore.
The trochophore lives in the surface layers of water for a short time, and then settles to the bottom and gradually turns into an adult organism.
Freshwater and terrestrial ringworms are most often hermaphrodites and have direct development.
Freshwater and terrestrial ringworms do not have a free larva. This is due to the fact that fresh water has a salt composition of a completely different nature than sea water. Sea water is more favorable for the development of life. Fresh water even contains some toxic compounds (for example, magnesium) and is less suitable for the development of organisms.
Therefore, the development of freshwater animals almost always occurs under the cover of special low-permeable shells. Even more dense shells - shells - are formed in the eggs of ground rings.
Dense shells here protect the eggs from mechanical damage and from drying out under the scorching rays of the sun.
The practical importance of annelids is increasingly increasing due to the development of the intensity of biological research.
Here in the USSR, for the first time in the history of world science, the acclimatization of some invertebrates was carried out to strengthen the food supply of the sea. For example, the polychaete Nereis, acclimatized in the Caspian Sea, became the most important food item for sturgeon and other fish.
Earthworms not only serve as fishing bait and food for birds.
They bring great benefits to humans by loosening the soil, making it more porous. This facilitates the free penetration of air and water to the roots of plants and increases crop yields.
While burrowing in the ground, worms swallow pieces of soil, crush them and throw them to the surface well mixed with organic matter. The amount of soil brought to the surface by worms is amazingly large. If we were to distribute the soil plowed by earthworms every 10 years over the entire surface of the land, we would get a layer of fertile soil 5 cm thick.
Leeches are used in medical practice for hypertension and the threat of hemorrhage.
They release the substance hirudin into the blood, which prevents blood clotting and promotes the dilation of blood vessels.
Type of rings includes several classes. The most primitive are the marine primary rings - archiannelids.
Polychaetes and echiurids- inhabitants of the sea. Oligochaete ringlets and leeches- mainly inhabitants of fresh water and soil.
To the beginning of the encyclopedia
According to their body shape, worms are divided into three types: Flat, Round and Ringed. All worms are three-layered animals. Their tissues and organs develop from three germ layers - ectoderm, endoderm and mesoderm.
Type Flatworms and their characteristics
Type Flatworms unites about 12,500 species. In terms of their organization, they are higher than coelenterates, but among three-layered animals they are the most primitive. These animals can crawl slowly. The most characteristic feature of flatworms is their flattened body, shaped like a long ribbon.
The figure below shows the structure of a Flatworm using Planaria as an example.
|
Structure |
The body is flattened in the dorsal-abdominal direction, the space between the organs is filled with a special tissue - parenchyma (there is no body cavity) |
|
Coverings of the body |
Skin-muscle bag (skin fused with muscle fibers) |
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Nervous system |
Two nerve trunks connected by nerves (“scalenes”) |
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Sense organs |
Ocellus in the front of the body, tactile cells scattered throughout the body |
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The digestive system is blindly closed; there is a mouth --> pharynx --> branched intestines |
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Whole body surface |
|
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Selection |
A system of tubules that open outward on the sides of the body |
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Flatworm Reproduction |
Hermaphrodites; sperm mature in the testes, eggs mature in the ovaries; the female lays eggs from which young worms emerge |
Variety of Flatworms, their main classes
Type Roundworms and their characteristics
Type Roundworms- a large group of animals with a long, round body in cross-section, which is pointed at the anterior and posterior ends. Roundworms are characterized by the presence of free space inside the body - a primary cavity. It contains internal organs surrounded by abdominal fluid. By washing the cells of the body, it participates in gas exchange and the transfer of substances. The body of roundworms is covered with a durable shell - the cuticle. This group has about 20 thousand species.
The figure below shows the structure of a Roundworm using the Ascaris as an example.
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Structure |
An elongated cylindrical body, pointed at both ends, round in cross section, is a body cavity |
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Skin-muscle bag |
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Nervous system |
Ventral nerve cord |
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Mouth (3 hard lips) --> pharynx --> intestinal tube --> anus |
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Whole body surface |
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Selection |
Through the surface of the body |
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Reproduction |
Most are dioecious; the female lays eggs from which young worms emerge |
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Representatives |
Type annelids their characteristics
Type Annelids- a group of animals whose representatives have a body divided into segments resembling rings folded one after another. There are about 9 thousand species of annelids. Between the skin-muscular sac and the internal organs they have in general- secondary body cavity filled with fluid.
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Structure |
The body consists of segments, there is a body cavity |
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Leather; muscles - longitudinal and circular |
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Nervous system |
Suprapharyngeal and subpharyngeal ganglia and the ventral nerve cord, from which nerves arise in each segment |
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Mouth --> pharynx --> esophagus --> crop --> stomach --> intestines --> anus |
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The entire surface of the body; marine animals have special body extensions - gills |
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Selection |
In each segment there is a pair of tubules that open outwards with excretory pores |
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Reproduction of annelids |
Hermaphrodite; the female lays eggs in the cocoon, from which young worms emerge |
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Manifold |
1. Class Malochaetes - live mainly in soil and fresh water bodies, have small setae on each segment (representative - earthworm) 2. Class Polychaetes - live in the seas; have paired outgrowths with bristles on the sides of the body (representative - nereid, sandworm) |
_______________
A source of information: Biology in tables and diagrams./ Edition 2, - St. Petersburg: 2004.
Class Polychaetes, Class Oligochaetes, Class Leeches
Question 1. Describe the structural features of annelids.
Characteristic features of the annelid type:
The body is always segmented (segmentation in the internal structure is the repetition of many internal organs).
They have a secondary body cavity - the coelom.
The circulatory system is closed.
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. The suprapharyngeal node is the “brain”.
The sense organs are located on the head segments.
The organs that facilitate locomotion are setae (in polychaetes there are 8 on each segment) and parapodia with tufts of setae (in polychaetes).
Question 2. What are parapodia? What do you think is their evolutionary significance?
Parapodia are lateral outgrowths of the body in polychaete worms, arranged in pairs and serving as organs of movement. Evolutionarily, parapodia are the predecessors of limbs.
Question 3. Describe the structure of the circulatory system of annelids.
The circulatory system is closed, consists of vessels, some of which have contractile walls (“hearts”), which ensures blood circulation. Some groups do not have a circulatory system. The blood of a number of forms contains hemoglobin (a red blood protein that contains iron and carries oxygen from the respiratory organs to the tissues).
Question 4. Describe the ring secretion organs.
The excretory system is represented by segmentally located metanephridia. Their funnel faces the body cavity, and the other end opens outward.
Question 5. How does the reproduction process occur in an earthworm?
Earthworms are hermaphrodites, but they undergo cross-fertilization. The two worms approach and exchange sperm, which enter their spermatic receptacles. Then a mucous muff is formed on the body of each worm. By contracting the muscles, the worm moves it to the anterior end of the body. When the muff passes by the openings of the ovarian ducts and spermatic receptacles, eggs and sperm enter it. Then the muff slides off the worm and closes into a cocoon, where small worms develop from the fertilized eggs.
Question 6. What classes are united by the type of annelids?
The phylum Annelids unites several classes, of which three main ones are Polychaetes, Oligochaetes and Leeches.
Question 7. Why are some annelids called polychaetes, while others are called oligochaetes? How do oligochaetes differ from polychaete worms?
Oligochaetes are one of the subclasses of Annelids. The most striking and familiar representative of the taxon is the most trivial Earthworm.
Polychaetes are one of the subclasses of Annelids. The most famous representatives of the taxon are the sandworm and the nereid. Sometimes the animals are called polychaetes, which means “many hairs” in Greek.
Difference between oligochaete and polychaete worms
There are fewer species of oligochaete worms than polychaetes. There are only 3 thousand species of the first, about 10 thousand of the second.
The maximum size of polychaetes exceeds the maximum size of oligochaetes, reaching 3 meters.
Animals have different habitats. Oligochaete worms live mainly in the ground; most polychaete worms prefer warm and salty water bodies.
Oligochaetes perceive oxygen through the entire surface of the skin, while polychaetes breathe using pseudo-gills-setae.
Oligochaetes are hermaphrodites, polychaetes are dioecious animals.
Oligochaetes, emerging from eggs, are similar to their parents. Polychaetes go through a larval stage.
Oligochaetes devour dead foliage and corpses; most polychaetes are active predators.
Question 8. When and from whom did the first annelids originate? What major changes accompanied the emergence of the type? Discuss as a class what these transformations mean. Write down the results of the discussion in your notebook.
Annelids are descended from free-living flatworms. From the common ancestors of worms, under the influence of evolutionary factors, annelids also evolved. An important point in their evolution is the division of the body into segments (rings). Due to active movement, annelids have developed a circulatory system that supplies the body with nutrients and oxygen. Ancient annelids had a more complex structure compared to other worms.
Question 9. Make a table “Comparative characteristics of the structure of organs and systems in flat, round and annelid worms” (work in small groups).
Comparative characteristics of the structure of organs and systems in flat, round and annelid worms
Annelids are a fairly large group of invertebrate organisms. In addition, they are considered the most organized representatives of worms. They live mainly in fresh and salt water bodies, as well as in soil. Some species of tropical leeches have adapted to a terrestrial way of existence.
Type Annelid worms: general characteristics
The sizes of representatives of this group range from a few millimeters to six meters. A rather characteristic feature of such an organism is the presence of segmentation - their body consists of many rings, which explains the name of the type. External ringing corresponds to internal segmentation. That is why, when the body is wounded or damaged, the annelid loses only a few segments, which are soon regenerated.
From the outside, the body is covered with a cuticle that is not shed. Chitinous bristles grow from it - another characteristic feature of this species. Some representatives may have parapodia on their segments - rather primitive limbs, which in some cases are equipped with sensitive bristles or gills.
Ringed worms: structural features of internal organs
Representatives of this type are characterized by the presence of a secondary body cavity - the coelom. This cavity is filled with a specific liquid, thanks to which normal indicators are maintained.
There is a skin-muscular sac, which consists of epithelial balls, as well as muscles grouped into circular and longitudinal groups.
The digestive system is continuous, beginning with the mouth and ending with the anus. Annelids have three intestinal sections - anterior, middle and posterior. Some species have primitive salivary glands.
The body breathes through the skin. The only exceptions are some marine species of animals that have gills on their parapodia. As for the circulatory system, it is usually closed. It consists of the abdominal and dorsal aorta, which are connected to each other by annular vessels. These organisms do not have a heart - the movement of blood is ensured by contraction of the dorsal aorta. Blood can contain a wide variety of respiratory pigments.
Still quite simple. At the anterior end of the body there is a large nerve ganglion, which performs the functions of the brain. A nerve chain extends from it, which in each segment of the body forms a small ganglion - a collection of neurons. are represented by the eyes, organs of chemical sensitivity, as well as mechanoreceptors, which are distributed throughout the body of the worm.
Ringed worms: features of reproduction and development
Organisms of this group can be either heterosexual or hermaphrodite (they are much less common). For example, it has a hermaphroditic reproductive system, but two individuals are needed for fertilization. It can occur both in the external environment and through the introduction of sperm into the internal canals of the female.
Another interesting fact is that annelids with pronounced segmentation have a tendency to rapid and intense regeneration. Due to this, some species are characterized by direct development of organisms, without metamorphosis.
It is worth noting that the role of annelids is quite significant. For example, the well-known one is responsible for soil aeration. This group also includes leeches, which are often used in modern medicine. Hirudin, produced by the leech, is of particular value, as it thins the blood and is used in the fight against thrombosis and other dangerous diseases.