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) internal organs, and accordingly, the organ systems of annelids are determined by 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. In the more ancient polychaete worms Each body segment has parapodia (paired primitive limbs with setae). 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 worms can be different colors at various types: red, green or transparent. It depends chemical structure 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 earthworm and leeches). 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 bait for fishing as bird food. 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.
Annelids are the most highly organized type of worms. Includes from 12 thousand (according to old sources) to 18 thousand (according to new) species. According to the traditional classification, annelids include three classes: polychaetes, oligochaetes, and leeches. However, according to another classification, polychaetes are considered in the rank of class, and oligochaetes and leeches are included in the rank of subclasses in the class Zyaskovye; In addition to these groups, other classes and subclasses are also distinguished.
The body length of annelids, depending on the species, varies from a few millimeters to more than 5-6 meters.
During embryonic development, ectoderm, mesoderm and endoderm are formed. Therefore, they are classified as three-layered animals.
In the process of evolution, annelids have a secondary body cavity, i.e. they are secondary cavities. The secondary cavity is called in general. It forms inside the primary cavity, which remains in the form of lumens of blood vessels.
The coelom develops from the mesoderm. Unlike the primary cavity, the secondary cavity is lined with its own epithelium. In annelids, the whole is filled with fluid, which, among other things, performs the function of a hydroskeleton (supporting shape and support during movement). Coelomic fluid also transports nutrients, and metabolic products and germ cells are excreted through it.
The body of annelids consists of repeating segments (rings, segments). In other words, their body is segmented. There can be several or hundreds of segments. The body cavity is not single, but is divided into segments by transverse partitions (septa) of the epithelial lining of the coelom. In addition, two coelomic sacs (right and left) are formed in each ring. Their walls touch above and below the intestine and support the intestines. Between the walls there are also blood vessels and a nerve cord. Each segment has its own nodes of the nervous system (on the paired abdominal nerve trunk), excretory organs, gonads, and external outgrowths.
The head lobe is called the prostomium. The back part of the worm's body is the anal lobe, or pygidium. The segmented body is called the torso.
The segmented body allows annelids to grow easily by forming new rings (this occurs posteriorly in front of the anal lobe).
The appearance of a segmented body is an evolutionary progress. However, annelids are characterized by homonomic segmentation, when all segments are approximately the same. In more highly organized animals, segmentation is heteronomous, when the segments and their functions are different. At the same time, in annelids, the formation of the head section of the body is observed by fusion of the anterior segments with a simultaneous increase in the cerebral ganglion. This is called cephalization.
The body walls, like those of lower worms, are formed by a skin-muscular sac. It consists of skin epithelium, a layer of circular and a layer of longitudinal muscles. Muscles achieve more powerful development.
Paired organs of movement emerged - parapodia. They are found only in polychaete annelids. They are outgrowths of a skin-muscular sac with tufts of bristles. In a more evolutionarily advanced group of oligochaetes, the parapodia disappear, leaving only the setae.
The digestive system consists of the foregut, midgut and hindgut. The walls of the intestine are formed by several layers of cells, they contain muscle cells, thanks to which food moves. The foregut is usually divided into the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The anus is located on the caudal blade. The process of absorption of nutrients into the blood occurs in the midgut, which has a fold on top to increase the absorption surface.
Characterized by a closed circulatory system. Previous types of worms (flat, round) did not have a circulatory system at all. As already mentioned, the lumen of blood vessels is the former primary cavity of the body, whose cavity fluid began to perform the functions of blood. The circulatory system of roundworms consists of a dorsal vessel (in which blood moves from the tail blade to the head), an abdominal vessel (blood moves from the head blade to the tail), half rings connecting the dorsal and abdominal vessels, small vessels extending to various organs and tissues . Each segment contains two half rings (on the left and on the right). The closed circulatory system means that blood flows only through the vessels.
Blood moves due to the pulsation of the walls of the spinal vessel. In some oligochaete worms, in addition to the dorsal one, some annular vessels contract.
Blood carries nutrients from their intestines and oxygen supplied through the integument of the body. The respiratory pigment, which reversibly binds oxygen, is found in the blood plasma and is not contained in special cells, as in vertebrates, for example, the hemoglobin pigment is found in red blood cells. Pigments in annelids can be different (hemoglobin, chlorocruarine, etc.), so the color of blood is not always red.
There are representatives of annelids that do not have a circulatory system (leeches), but in them it has been reduced, and a respiratory pigment is present in the tissue fluid.
Although annelids do not have respiratory system and usually breathe over the entire surface of the body, the transport of gases is carried out by the circulatory system, and not by diffusion through tissue fluid. Some marine species primitive gills are formed on the parapodia, in which there are many small blood vessels located close to the surface.
The excretory organs are represented by metanephridia. These are tubes that have a funnel with cilia at the end located inside the body (coelom). On the other side, the tubes open outward through the surface of the body. In every segment ringworm There are two metanephridia (on the right and on the left).
The nervous system is more developed compared to roundworms. In the head lobe, a pair of fused nodes (ganglia) form something like a brain. The ganglia are located on the peripharyngeal ring, from which the paired abdominal chain extends. It contains paired nerve ganglia in each body segment.
Sense organs of annelids: tactile cells or structures, a number of species have eyes, chemical sense organs (olfactory pits), and an organ of balance.
Most annelids are dioecious, but some are hermaphrodites. Development is direct (a small worm emerges from the egg) or with metamorphosis (a floating trochophore larva emerges; typical for polychaetes).
Annelids are thought to have evolved from worms with undifferentiated bodies, similar to ciliated worms (a type of flatworm). That is, in the process of evolution, two other groups of worms evolved from flatworms - round and annelid.
The body of which consists of repeating segments, or rings (hence their name - annelids).
General brief description annelids:
- there is a secondary body cavity (coelom);
- the body is covered on the outside with a cuticle secreted by the ectoderm.
- there is a circulatory system;
- the nervous system is represented by a paired suprapharyngeal node, connected by jumpers to the ventral nerve cord (usually double);
- excretory organs are located in each ring and are formed from ectoderm, they are equipped with cilia;
Structure
The elongated body of annelids seems to be composed of ring segments, the segments are separated by internal partitions; but they are not completely independent, since along the entire body there is a through intestine with oral and anal openings, an abdominal trunk of the nervous system and trunks of a closed circulatory system. These organ systems, piercing the septa one after another, stretch across the entire body of annelids. Each ring segment has a secondary body cavity (coelom). Most of the segments bear on the outside, on the right and on the left, two bunches of setae - organs of movement or attachment in tubes. In leeches, the bristles are lost for the second time.
Secondary body cavity (coelom)
The secondary body cavity (coelom) is of mesodermal origin. It is surrounded by a mesodermal membrane and filled with fluid. The cavity occupies the space between the walls of the body and the intestinal tube. The main part of the mesoderm lining the secondary cavity is the muscles that make up the body wall. They ensure the movement of the animal. In addition, the muscles of the intestinal wall, alternately contracting, push food through.
The secondary body cavity performs the following functions:
The secondary body cavity, to one degree or another, is a characteristic feature of all types of multicellular animals that follow in the evolutionary development, starting with annelids.
Classification
Annelids are a numerous type of worms that have a more complex body structure compared to flat and protocavitary worms. It is divided into three classes: Polychaetes, Girdles (including the subclasses Oligochaetes and Leeches), Mysostomidae.
Origin
According to a comparative study of the structure of worms, annelids evolved from primitive whole worms, similar to flat ciliated worms. Important evolutionary acquisitions of annelids are the secondary body cavity (coelom), the circulatory system, and the division of the body into separate rings (segments). Polychaete annelids are the ancestral group for other annelids. During the transition to freshwater and terrestrial lifestyles, oligochaete worms separated from them. Leeches evolved from oligochaete worms.
Questions about this material:
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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 and internal structure– repeatability 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 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
The phylum annelids include protostomes, the most highly organized of all worms. The phylum includes, according to various sources, 10–18 thousand species and is divided into three classes: polychaetes (the most numerous, more than 10 thousand species), oligochaetes and leeches. These worms live in bodies of water, including the ocean floor and icy waters Atlantic, and in the soil. The exception is several species of leeches that have adapted to living on land in tropical rainforests. Well, the most famous and common oligochaete earthworms in our latitudes are earthworms, so named for their custom of crawling out of the soil in dozens during rain to breathe oxygen. In addition, their presence is revealed by small excavation tubercles, especially noticeable in the spring - young worms loosen the soil, simultaneously saturating it with oxygen. We will talk about earthworms in more detail later, but now we will look at the important aromorphoses and structural features of annelids.
Aromorphoses of annelids
1. Overall - secondary body cavity, that is, a closed, fluid-filled space that separates the intestines from the walls of the body. It is important to emphasize that, unlike the primary cavity of roundworms, the coelom of annelids has an epithelial membrane, lining. In polychaete and polychaete worms the whole occupies a fairly large volume. Cells participating in excretory, gas exchange and other processes float freely in the liquid that fills it.
2. Closed circulatory system- its occurrence is directly related to the appearance of the coelom. As the worm larva develops, the secondary cavity displaces the primary cavity, the remains of which turn into blood vessels.
3. Metamerism- segmentation, duplication of internal organs, due to which the loss of a body part is not critical for the worm. Each ring has its own sex glands, excretory organs, nerve nodes, etc.
4. Parapodia- growths on the sides of the body in polychaetes that facilitate movement.
The structure of annelids
1. Body sizes from a quarter of a millimeter to three or even more meters.
2. B cross section the body is close to a circle or oval. Three sections are distinguished: the head (head lobe), the trunk and the anal lobe. Ringworms grow by forming new segments in the area of the anal lobe.
2. The body is segmented by constrictions into many homogeneous rings(segments). It is important that the whole also has partitions, in accordance with the external division. Top layer skin-muscle bag- cuticle, the next one is single-layer epithelium. There are two types of muscles: circular on the outside, longitudinal located deeper.
3. On the head, in addition to the mouth opening, different species may have eyes and organs of touch (various whiskers, palps, etc.).
4. They grow from the cuticle bristles, of which there can be a lot along the entire length of the body.
Organ systems
1. Digestive system not closed, it is divided into three sections, represented by the anterior, middle (here nutrients are absorbed) and hindgut. Some species of worms have acquired salivary glands.
2. Ringworms are heartless creatures, in their closed circulatory system there is only different types vessels through which blood moves. Interestingly, red color is not necessary for the blood of worms - it all depends on the pigments.
3. Breath can be carried out in two ways - either by the surface of the body (as in polychaete worms and leeches), or through primitive gills that are located on parapodia (in polychaete worms).
4. Nervous system begins in the head of the worm, where two nerve ganglia, suprapharyngeal and subpharyngeal, connected by cords, form the peripharyngeal nerve ring. A pair of nerve trunks with ganglia, connected by jumpers in each segment, emerge from the ganglion under the pharynx and stretch along the body. This is the so-called ventral nerve cord.
5. Sense organs Quite well developed in active worms: tactile cells, eyes (not all species have them), chemoreceptors, balance organ.
6. Excretory system presented in all rings in pairs metanephridia: tubes located in the coelom that open outward on the surface of the body.
Origin of annelids
1. The ancestors of annelids were free-living flatworms. How can you prove this? The larvae of polychaete worms are very similar to planarians. What does it mean? Trochophore, a polychaete larva, has cilia, ocelli, metanephridia in the form of tubes with stellate cells and a “flickering flame” formed by the beating of the cilia. In addition, the nervous system of the trochophore is very similar to nervous system planarians.
2. Oligochaetes evolved from ancient polychaetes as a result of simplification in their structure caused by life in the soil.
3. Leeches evolved from ancient oligochaete worms.
