The body of the ringlets is divided into the head section ( prostomium), the following rings (or segments, or metamers), the number of which is usually large (several dozen), and the posterior section (anal lobe, or pygidium). The head section of marine worms, called polychaetes, is well defined and bears various appendages: wide, narrow, etc. (Fig. 61). In freshwater and terrestrial ringlets, the head section is weakly expressed (Fig. 61). Several anterior rings may be fused with the prostomium. Body segments are usually similar in structure. This kind of division is called homonomic segmentation or homonomy metamerism. It is not only external, but deeply internal, since each segment is separated from neighboring segments by partitions and has a set of organs.
The skin consists of a single-layer epithelium and a thin cuticle secreted by it (Fig. 62). There are many glands in the skin that secrete mucus, which facilitates the movement of worms, and other secretions (for example, substances that help attract females to males in dioecious ringworms, poisonous to other animals, etc.).
Nervous system. This system is much better developed than that of other worms, and its structure very clearly reflects the division of the ringlet body into segments. Its central section consists, as a rule, of two head nodes lying on the dorsal side, peripharyngeal cords, which pass on the ventral side into a chain, usually very long and forming a node in each segment (Fig. 63, B), which explains its name. Thus, the abdominal chain was formed from two strands. In the lower forms of the type, the cords remain separated along their entire length and are connected by bridges, which resembles a ladder (Fig. 63, A). Such a system is less centralized, it is similar to the central nervous system of lower worms - flat and primitive (see Fig. 31, B, and 54).
The nodes and cords of typical annelids are much better developed and their structure is more complex than those of the latter. The entire central system of ringlets is separated from the epidermis, while in lower worms it is still connected to the epidermis. Each node of the abdominal chain innervates and affects the functioning of organs located in the ring where the node is located. The head nodes, better developed than the nodes of the chain, coordinate the work of the latter and, through them, the activity of the whole body. In addition, they innervate the eyes and other sensory organs located in the head of the body.
The senses are varied. Tactile cells are scattered in the skin, which are especially numerous on the appendages of the body. There are organs that perceive chemical irritations. All annelids have light-sensitive organs. The simplest of them are represented by special cells scattered throughout the skin. Therefore, almost all ringworms have skin sensitive to light stimulation. At the anterior end of the body, and in a number of leeches at the rear, the light-sensitive organs become more complex and turn into eyes. A number of forms have balance organs that are similar in structure to similar organs of jellyfish and other lower animals.
The progressive development of the nervous system of annelids provides more complex and energetic movements of their body, active work of all organ systems, better coordination of the functions of all parts of the body, more complex behavior and makes possible a more subtle adaptation of these animals in the environment.
Propulsion system. This system in annelids is more advanced than in previously studied worms. Ciliary movement is characteristic only of larvae; in adult forms, with rare exceptions, it is absent, and their movement is accomplished only through the work of muscles. The skin-muscle sac is developed much better than in flatworms and protocavitary worms (cf. Fig. 32, 53 and 62). Under the epidermis lies a well-developed layer of circular muscles (Fig. 62), consisting of long fibers with nuclei. When these muscles contract, the body of the worm becomes thinner and longer. Behind the circular muscles there is a much thicker layer of longitudinal muscles, the contraction of which shortens the body and makes it thicker. Unilateral contraction of the longitudinal and some other muscles leads to bending of the body and a change in the direction of movement. In addition, there are muscles running from the dorsal side to the abdominal side: muscles passing through the septa separating the rings; muscles of various appendages of the body, which play a supporting role in the movement of worms, etc. The strength of the muscles of the skin-muscular sac is great and allows worms to quickly penetrate deep into the ground. Many annelids can swim. The support for the muscles is mainly the hydroskeleton formed by the fluid of the body cavity, as well as border formations.
The movement of annelids is facilitated by auxiliary appendages (see Fig. 61, 62, 64): bristles(available in the vast majority of species) and parapodia(available in most sea worms). The bristles (see Fig. 62, 64, A, B) are solid formations of organic matter, a very complex carbohydrate - chitin, of different shapes, thickness and length. The bristles are formed and driven by special muscle bundles. The setae are arranged (singly or in tufts) in regular longitudinal rows on almost all rings of the worms. Parapodia (Fig. 64, B) are powerful lateral outgrowths of the body with well-developed muscles. The parapodia are movably connected to the body, and these appendages act like a simple lever. Each parapodia usually consists of two lobes: dorsal and ventral, which, in turn, can be divided into second-order lobes. Inside each of the main blades there is a supporting bristle. The parapodia bear tufts of bristles that extend far beyond the body. The parapodium has two palps - dorsal and ventral, in the epidermis of which there are various sensory organs that perceive mechanical and other irritations. The movement of annelids is greatly facilitated by their division into rings, as a result of which the flexibility of the body increases.
The body of the rings contains compacted plates called border entities, which underlie the epidermis, separate the muscles, are highly developed in the partitions between the rings. They give strength to the entire body, serve as a support for the musculoskeletal system, are important for the functioning of the circulatory and digestive systems, and play a protective role.
Circulatory system. In annelids, due to the significant complication of the structure of their body and the sharply increased activity of their vital functions, a more advanced system of transporting substances has developed - the circulatory system. It consists of two main vessels - dorsal and ventral(Fig. 62 and 65). The first passes over the intestine, coming close to its walls, the second - under the intestine. In each segment both vessels are connected circular vessels. In addition, there are smaller vessels - there are especially many of them in the walls of the intestine, in the muscles, in the skin (through which gases are exchanged), in the partitions separating the segments of the body, etc. Blood moves due to the contraction of the vessels themselves, mainly the spinal and anterior annular ones, in the walls of which muscle elements are well developed.
Blood consists of a liquid part - plasma in which blood cells float - blood cells. Plasma contains respiratory pigments, i.e. special complex organic compounds. They absorb oxygen in the respiratory organs and release it to the tissues of the body. Some ringlets in the plasma contain one of the most advanced respiratory pigments - hemoglobin; these rings have a reddish blood color. For the most part, the blood of annelids contains other pigments and its color can be greenish, yellowish, etc. Blood cells are quite diverse. Among them there are phagocytes, which, like amoebas, release pseudopods that capture bacteria, all sorts of foreign bodies, dying body cells and digest them. As noted earlier, all animals have phagocytes. Thus, the circulatory system not only ensures the transport of various substances, but also performs other functions.
Body cavity. The body cavity of the ringlets differs in structure from the primary cavity. The latter does not have its own walls: on the outside it is limited by the muscles of the skin-muscular sac, on the inside by the intestinal wall (see Fig. 53). The body cavity of annelids, called secondary or coelom, is surrounded by a single-layer epithelium, which, on the one hand, is adjacent to the skin-muscular sac, and on the other, to the intestine (see Fig. 62). Consequently, the intestinal wall becomes double. The whole is filled with a watery fluid, constantly in motion, in which cells similar to blood cells (phagocytes, cells with respiratory pigments, etc.) float. Thus, the secondary body cavity, in addition to the role of the hydroskeleton, performs functions similar to those of the blood (transfer of substances, protection from pathogens, etc.). However, it should be emphasized that the coelomic fluid moves slower than blood and it cannot come into such close contact with all parts of the body as a branched network of capillaries.
Respiratory system. In annelids, the exchange of gases mainly occurs through the skin, but the respiration processes in connection with the appearance of the circulatory system and coelom are more advanced in them than in the previously considered worms. Many ringlets, mainly marine ones, have branched appendages that play the role of gills (see Fig. 61, B). The respiratory surface also increases due to the presence of various outgrowths of the body. Improving respiratory processes is of great importance for annelids due to the activation of their lifestyle.
Excretory system. The main excretory organs are metanephridia(Fig. 66, B). A typical metanephridia consists of a funnel and a long convoluted tube, in the walls of which blood vessels branch. In each segment, with the exception of some, there are two of these organs, to the left and to the right of the intestine (see Fig. 65). The funnel faces the cavity of one segment, and the tube pierces the septum, passes into the other segment and opens outward on the ventral side of the body. Dissimilation products are extracted by metanephridia from the coelomic fluid and from the blood vessels entwining them.
In a number of annelids, metanephridia are associated with tubes of the protonephridial type, closed at the ends facing the body cavity by flame cells. It is possible that metanephridia arose from protonephridia, which connected with funnels that developed on the partitions between the rings (Fig. 66, A). It is believed that these funnels, called coelomoducts, originally served for the exit of reproductive products from the body cavity.
On the walls of the coelom there are numerous cells that absorb decay products from the cavity fluid. There are especially many of these cells called chloragogenous, is present on the walls of the middle part of the intestine. Decay products removed from the coelomic fluid and contained in these cells can no longer have a harmful effect on the body. Cells loaded with such products can escape through metanephridia or through pores in the walls of the body.
Digestive system. The digestive system of ringlets (see Fig. 65), due to a more active lifestyle than in the previously considered groups of animals and the progress of the entire organization, is also more perfect. In ringlets: 1) the division of the digestive system into various sections is more pronounced, each of which performs its own function; 2) the structure of the walls of the digestive tube is more complex (digestive glands, muscles, etc. are more developed), as a result of which food is processed better; 3) the intestine is connected to the circulatory system, due to which the digestion of nutrients and their absorption is more intense and the supply of substances necessary for the work it performs is improved.
The digestive tube is usually straight and divided into the following sections: oral cavity, pharynx, esophagus, which can expand into a crop, muscular stomach (present in a number of species, such as earthworms), midgut (usually very long), hindgut (relatively short), opening outward through the anus. Gland ducts flow into the pharynx and esophagus, the secretion of which is important in processing food. In many predatory polychaete ringlets, the pharynx is armed with jaws; the front part of the digestive tube can turn out in the form of a trunk, which helps to take possession of the prey and penetrate its body. The midgut in a number of species has a deep invagination ( typhlosol), stretching along the entire dorsal side of this intestine (see Fig. 62). Typhlosol increases the surface of the intestines, which speeds up the digestion and absorption of food.
Reproduction. Some ringlets reproduce asexually and sexually, while others exhibit only sexual reproduction. Asexual reproduction occurs by division. Often, as a result of division, a chain of worms can result that have not yet had time to disperse.
The structure of the reproductive apparatus is different. Polychaete ringlets (they live in the seas) are dioecious and have a simply constructed reproductive apparatus. Their gonads develop on the walls of the coelom, the germ cells enter the water through breaks in the body walls or through metanephridia, and fertilization of the eggs occurs in the water. Ringlets living in fresh water and damp soil (oligochaetes), as well as all leeches are hermaphrodites, their reproductive apparatus has a complex structure, fertilization is internal.
Development. The crushing of the fertilized egg, as a result of which the resulting blastomeres are arranged in a spiral (Fig. 67), resembles the same processes in ciliated worms. Polychaete ringlets develop with transformation: larvae are formed from their eggs trochophores(Fig. 68), completely different from adult worms and turning into the latter only after complex transformations. Trochophore is a planktonic organism. It is very small, transparent, and there are usually two belts of cilia along the equator of its body: one, upper, above the mouth, the other, lower, under the mouth. Consequently, the trochophore consists of two parts: the upper, or anterior, and the lower, or posterior, ending in the anal lobe. Trochophores of some species may have several belts of cilia. At the upper end there is a tuft of cilia attached to the parietal plate (the larval sensory organ). Under the plate is the nerve center, from which the nerves extend. The muscular system consists of fibers running in different directions. There is no circulatory system. The space between the body walls and the intestines is the primary body cavity. Excretory organs are protonephridia. The digestive apparatus consists of three sections: anterior, middle and posterior, ending with the anus. Thanks to the work of the cilia, the larva moves and food, consisting of microscopic organisms and organic pieces, enters the mouth. Some trochophores actively capture small animals with their mouths. In its structure, the trochophore resembles protocavitary worms, but in some respects it is also similar to the larvae of marine ciliated worms. The walls of the body, the nervous system, protonephridia, the beginning and end of the digestive apparatus, trochophores, were formed from the ectoderm, most of the intestine - from the endoderm, muscle fibers - from cells called mesenchymal and originating from both layers.
When a trochophore transforms into an adult worm, it undergoes a number of significant changes. In these changes, the most important role is played by the rudiments of the third germ layer - mesoderm. Some rudiments of mesoderm are still present in the larva before the onset of metamorphosis; they lie on each side between the walls of the body and the posterior part of the intestine (Fig. 68, B, 12). Other rudiments of mesoderm are formed later from the anterior edge of the anal lobe, which turns into growth zone worm (Fig. 68, B, 13). Metamorphosis of the larva begins with the fact that its rear part lengthens and the constrictions of the body walls are divided into 3, 7, and rarely more segments. After this, the rudiments of the mesoderm, lying between the walls of the body and the posterior part of the intestine, also lengthen and are divided into as many sections as the number of segments formed as a result of external constrictions. There are two of them in each ring (Fig. 68, D, 14). The segments formed from the back of the trochophore are called larval or larval, they are characteristic of the later stages of trochophore development, when it already begins to look a little like an adult worm, but still has few segments. In the process of further development, segments are formed by the growth zone mentioned above. These segments are called postlarval, or postlarval(Fig. 68, D). There are as many of them formed as the number of segments an adult worm of a given species has. In the postlarval segments, the mesodermal rudiments are first divided into sections (two in each ring), and then the outer integument.
The main organ systems of an adult worm are formed as follows (Fig. 69, A). From the ectoderm the epidermis, the nervous system, and the anterior and posterior ends of the digestive tube develop. Mesodermal primordia in each ring grow and displace the primary cavity. Eventually the right and left rudiments converge above and below the intestine, so that along it, above and below, dorsal and abdominal blood vessels are formed. Consequently, the walls of the vessels are formed from the mesoderm, and their cavity represents the remains of the primary body cavity. In the middle of the rudiments, the cells move apart, a coelomic body cavity appears and grows, which is surrounded on all sides by cells of mesodermal origin. This method of coelom formation is called teloblastic. Each mesodermal rudiment, growing, converges in front and behind with neighboring rudiments (Fig. 69, B) and septa appear between them, and the mesodermal cells surrounding the remains of the primary cavity between the septa form ring blood vessels. The outer layer of mesodermal primordia, adjacent to the ectoderm, gives rise to muscles, the inner layer surrounds the digestive tube. Consequently, the intestinal walls now become double: the inner layer (with the exception of the anterior and posterior ends, originating from the ectoderm) developed from the endoderm, the outer layer from the mesoderm. The metanephridia funnels are formed from the cells of the mesodermal layer, and their tubes (representing the remains of protonephridia) are from the ectoderm.
Gradually, all parts of the body of an adult worm develop; layers of muscles are differentiated, the number of blood vessels increases, the intestine is divided into sections, glandular cells, muscle fibers, blood vessels, etc. develop in its walls. The head lobe (prostomium) of an adult worm is formed from the upper part of the trochophore, the body ring from larval and postlarval segments, and the pygidium is from the anal lobe of the larva.
Origin. Various hypotheses have been put forward about the origin of annelids. Proponents of one hypothesis believe that annelids evolved from turbellarians. Indeed, there are similarities in the embryonic development of both groups of animals. The central nervous system of the ringlets (i.e., the cephalic nodes and the abdominal chain) could have formed from the same system of more complex turbellarians, in which the nodes moved to the anterior end of the body and two main ones remained from the longitudinal cords, and thus a central nervous system of the scalene type arose, preserved in lower annelids. The dermal-muscular sac of flatworms could develop into a similar ring system, and metanephridia could arise from protonephridia. However, from an evolutionary point of view, it is impossible to assume that the most highly organized worms descended directly from the lowest worms, in which the nervous and muscular systems were still poorly developed, there was no body cavity, the intestine was not differentiated into three more sections and digestion mainly remained intracellular, etc. d. Obviously, the ancestors of higher worms were worms with a more complex structure than turbellarians.
According to another hypothesis, ringlets began with nemerteans, i.e. worms, undoubtedly descended from turbellarians, but having a much more complex structure than the latter (significant development of the nervous and muscular systems, the appearance of a circulatory system, a through intestine, etc.). The author of this hypothesis, the outstanding Soviet zoologist N.A. Livanov, suggested that in the most progressive group of nemerteans, metamerically located cavities arose in the skin-muscle sac, which served as a support for the muscles and later turned into coelomic cavities, as a result of which the movement of animals sharply improved. Opponents of this hypothesis believe that nemerteans, in which one of the main features is a trunk, which is absent in ringlets, could not be the ancestors of the latter. However, it must be assumed that the trunk developed in nemerteans after a long evolution, when they had stronger rivals than before in hunting animals. Annelids could have evolved from unspecialized nemerteans, whose organization was already complex, but the trunk was not developed. Another objection to the hypothesis under consideration is more serious. From this hypothesis it follows that the circulatory system arose before the coelom, and the latter developed from the very beginning in the form of metameric formations. Meanwhile, worms are known, undoubtedly related to annelids, in which metamerism is not yet expressed, the whole is continuous and there is no circulatory system. It was previously believed that the worms mentioned were simplified due to adaptation to a sedentary lifestyle, but new research confirms the original primitiveness of the coelomic worms in question.
The authors of the third hypothesis believe that the ancestors of ringworms were protocavitary worms, but not as specialized as rotifers and roundworms, but closer to the ancestors of this type. This hypothesis is based mainly on the structure of the trochophore, which, as shown above, has important similarities (primary body cavity, protonephridia, through intestine) with protocavitary worms, but still lacks the features of annelids. Having accepted this hypothesis, it should be assumed that the coelom arose as a result of the development of epithelium on the walls of the primary body cavity, and body metamerism and the circulatory system appeared later. From the same hypothesis it follows that nemerteans, despite the progressive features of their organization, were not related to the emergence of more highly organized types of animals. On the contrary, the nonmertean hypothesis of the origin of annelids rejects the importance of protocavitary worms for the formation of new types of animals.
It is impossible to consider here in sufficient detail the various objections to each of the mentioned hypotheses, since this requires more detailed information about the structure and development of all types of worms, but there is no doubt that coelomic worms could not have arisen directly from the lowest worms.
Type annelids- This is a very large group of invertebrates, the type belongs to the subkingdom Eumtazoa and the kingdom Animalia. The number of subspecies today is, according to inaccurate estimates, 12,000 - 18,000.
The rich diversity of subspecies is determined by the large number of subtypes: various species are combined into large groups - leeches (numbering about 400 species), polychaetes (approximately 7000 species), oligochaetes, mysostomids.
The origin of the type traces its history back to the evolution of mollusks and arthropods; annelids can truly be called ancient creatures. Today there are ringed, round and.
Worms, both common and annelid, are the oldest inhabitants of the planet; for thousands of years they have practically not changed their appearance.
A distinctive feature of their body structure is the segments (or segments) that make up the entire body. The minimum length of the worm is 0.25 mm, the maximum is 3 m.
The length directly depends on the number of segments; their number can be 2-400 pieces. Each of the segments forms a complete unit and has a strict set of the same structural elements. The entire body is enclosed in a skin-muscular sac that covers the entire body of the worm.
The general structure of annelids includes:
- head lobe (scientifically “prostomium”)
- body consisting of a large number of segments
- anal opening at the end of the body
The skin-muscle bag as a part of the body has several sections. Annelids and their structure are unusual in their constant layering of fragments. In general, there are two sacs in the body of a worm: an external one, enveloping the entire body, like skin, and an internal one, lining the surface under the organs.
Movement in the body is produced due to the contraction of blood and nerve vessels: this explains the reason for the pulsating nature of movements. There are special muscles in the intestines of the worm; they are responsible for the digestion of food and its subsequent elimination.
The higher development of the circulatory system indicates the evolutionary superiority of annelids over their historical ancestors, mollusks and arthropods (it is from these creatures that annelids originate).
The innovation is that their circulatory system is closed. The above-mentioned blood vessels in the abdominal and dorsal cavities transfer blood from one segment to another.
It is through the flow of blood that movement occurs. Thus, the activity of the body and its ability to move and navigate the terrain completely depend on the functioning of the circulatory system.
If we talk about external organs of movement, then parapodia will be responsible for them. This scientific term refers to the bicuspid flippers that grow on the outer sides of the worm.
When adhering to the surface (most often the soil), the parapodia ensure repulsion of the annelids and movement forward or to the side. The method of movement does not affect the differences between worms that reproduce sexually or non-sexually.
Learn more about the vital systems of the annular body.
The food system is very diverse, because... has a very segmented structure. The foregut is divided into 3 sections and includes the mouth, pharynx, esophagus, crop and stomach. The hindgut ends at the anus.
The respiratory system is very developed and formed in the form of gills that are quite invisible on the surface of the cover. These gills have a completely different appearance: their structure can be feather-like, leaf-like, or completely bushy.
It is important to note that the interlacing of the gills includes blood vessels.
The excretory system of worms has a structure adapted to the structure of their body. This means that metanephridia, paired tubular organs with a special excretory canaliculus, are duplicated in each of the body segments.
The removal of cavity fluid is carried out through the opening of all identical tubules and subsequent adhesion.
The anus is not located directly on the body. When the cavity fluid is alienated, a special channel opens to the outside, and the supply occurs precisely through it. Then the hole closes and the integument regains its integrity.
Most species of annelids are dioecious, but this is not necessarily the case. In species whose origin occurred historically less recently, hermaphroditism is observed, which has developed secondarily. This means that individuals can also be bisexual.
How do ringed animals sense the external environment?
Type of nervous system- ganglian. This means that in the animal’s body the nervous system is designed in such a way that all nerve vessels belong to one sensitive nerve node. It coordinates incoming information, and the system of nerve ganglia represents the central nervous system.
The elements of the nervous system of the ring are well cohesive and interconnected, the sensory organs, as ways of analyzing the external environment, are located on the head, the ganglia, as part of the abdominal chain, line the abdominal cavity and are connected in pairs.
There are two important centers in the head lobe: suprapharyngeal and subpharyngeal ganglia, in turn they are formed into a common node. The organs of vision, touch, and balance are carried through special paths to the suprapharyngeal node.
The supraglottic and subpharyngeal nodes are connected by columns, so messages are transmitted between the organs, and a nerve ring appears that communicates with the abdominal region.
Annelids do not have a brain, as such. The entire nervous system in their body should be considered the brain.
The sense organs are located on the head of the body; this area is the most sensitive. Ringlings exhibit surprisingly good development of organs for perceiving environments and conditions of the outside world.
They can see and feel the pressure on the surface of their covers, and also analyze the chemical composition of the soil and the environment in which they live.
When moving, they maintain balance; this sense is especially sensitive so that ringworms can sense the position of their body in the conditions of the soil as a closed solid system.
Balance also helps them stay on the surface of the earth, this is especially true when any aggressors in the form of animals or people take the worms to the surface.
How do ringworms reproduce?
Taking into account the sexual characteristics of various species (worms are dioecious or bisexual), in general, the reproduction of annelids can take place in two ways:
- sexual
- asexual
If we are talking about an asexual method of reproduction, then most often it is budding or division into parts. The worm simply breaks into pieces; any tail end that falls off is able to grow its own head lobe with its organ system.
This way, the worms reproduce and increase their own chances of survival. Even if the maternal individual is divided into two parts or even more, then none of them will die, each will grow back the missing part.
The division of one body into several, as a method of reproduction, occurs quite often, especially in species that live in the soil. Budding is observed much less frequently, except perhaps only in sillids (budding can occur on the entire surface of the integument of this species).
The asexual method of reproduction in earthen annelids should be regarded as a special mechanism of adaptation to living conditions in their environment. A worm living in the outer layers of the soil can always be attacked by a bird or a person.
The protective mechanism assumes the impossibility of destroying the organism by crushing. For the worm to actually die, it must be crushed, not cut.
The sexual method of annelids during reproduction is traditional for species living in water. Females and males release the products of their reproductive systems into the water, thus external fertilization occurs (annelids always reproduce in the external environment, not inside their body).
The fry gradually mature. Their appearance can sometimes copy that of an adult, but this condition is not necessary: the appearance of an immature and an adult worm can be radically different and not even resemble each other’s shapes.
As for hermaphrodites, internal cross-fertilization occurs in them. Male reproductive organs are presented in the form of testes located in seed capsules, which in turn are placed in special bags. The female reproductive organs include a pair of ovaries, a pair of oviducts, and egg sacs.
The development of new individuals occurs outside the cell; the larval stage is passed. Fertilized female cells continue their division and development, being suspended on a girdle near the egg cocoon. In leeches, this cocoon is of fundamental importance when growing immature worms: it is from it that nutritional resources are drawn.
Features that characterize all ringlets, regardless of their types
All annelids have similar properties; their common characteristics are an extremely important system of knowledge that allows us to evaluate the evolutionary development of other species.
Annelids represent a special type of organization of biological life; their body structure is characterized by rings, a ring type of segmental body structure.
It is for this reason that the following properties, inherent only to their type, will become distinctive; other species, types and kingdoms may have with them only some common elements, but not an identical paradigm of patterns.
So, annelids are characterized by the following:
- Three layers. In embryos, ectoderm, endoderm and mesoderm develop simultaneously.
- The presence of a special coelomic body cavity lining the organs and viscera. The coelom is filled with a special coelomic fluid.
- The presence of a skin-muscular sac, due to which motor function is performed and the functioning of the nervous, circulatory and digestive systems is ensured.
- Bilateral symmetry. Formally, you can draw an axis along the center of the body and see mirror symmetry with a repetition of the structure and various vital systems.
- The appearance of simple limbs that facilitate movement.
- Development of all major vital systems within one individual organism: digestive, excretory, nervous, respiratory, reproductive.
- Dioecy
What kind of lifestyle do ringworms follow?
Ringlings hardly sleep and can function both during the day and at night. Their lifestyle is irregular, they are especially active during rain or when increased amounts of moisture are concentrated in the soil (this tendency is noticeable in species called earthworms).
Annelids live in all possible environments: in salty seas, fresh water bodies, on land. Among the worms there are both those who obtain their food on their own, and those who are scavengers (here it is worth highlighting the usual scavengers, blood-suckers, etc., who belong to them).
You can often find real predators (the best example: leeches, they are classified as the most dangerous species in this type, because they pose a potential threat to humans). However, for the most part, worms are very peaceful and feed on the soil, or rather, process it. Worms can reproduce both year-round and only in a certain season.
The importance of worms in maintaining healthy soil conditions has always been key, because... Thanks to intensive movement in the strata, the necessary oxygen and water are carried into the ground.
The enrichment of the soil composition occurs due to the fact that the worm absorbs the soil, passes it through its systems and processes it with enzymes, and then brings the soil out and captures a new portion.
Thus, there is a constant renewal of earth resources; the existence of the rest of the biological world directly depends on the existence of worms.
Type annelids unites about 9,000 species that have the most perfect organization among other worms. Their body consists of a large number of segments; many have bristles on the sides of each segment, which play an important role in movement. Internal organs are located in a body cavity called coelom. There is a circulatory system. In the anterior part there is a cluster of nerve cells that form the subpharyngeal and suprapharyngeal nerve nodes. Annelids live in fresh water bodies, seas and soil.
Most of the representatives of annelids belong to the classes: oligochaetes, polychaetes and leeches.
Class oligochaetes
Representative of the class oligochaetes - earthworm lives in burrows in moist humus soil. The worm crawls to the surface in damp weather, at dusk and at night. In an earthworm, the anterior and abdominal parts of the body can be easily distinguished. In the anterior part there is a thickening of the girdle; on the ventral and lateral sides of the body, elastic and short bristles are developed.
The body of the worm is covered with skin made of integumentary tissue, in which the cells adhere tightly to each other. The skin contains glandular cells that secrete mucus. Under the skin there are circular and deeper - longitudinal muscles, thanks to the contraction of which the body of the worm can lengthen or shorten, thereby moving through the soil.
The skin and muscle layers form skin-muscle sac, inside which there is a body cavity where the internal organs are located. Earthworms feed on rotting plant debris. Through the mouth and pharynx, food enters the crop and muscular stomach, where it is ground and enters the intestine and is digested there. Digested substances are absorbed into the blood, and undigested substances are excreted along with the soil through the anus.
Circulatory system of an earthworm closed and consists of dorsal and abdominal blood vessels, connected to each other by ring vessels from each segment. Around the esophagus there are larger annular vessels that act as the “hearts” of the large vessels; side branches branch off, forming a network of capillaries. The blood does not mix anywhere with the fluid of the body cavity, which is why the system is called closed.
The excretory organs are represented by convoluted tubes through which liquid and harmful substances are removed from the body.
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. The earthworm does not have specialized sensory organs. There are only different kinds of sensitive cells that perceive external stimuli (light, smell, etc.).
Earthworms are hermaphrodites. However, they have cross-insemination; two individuals participate in this process. When eggs are laid on the worm's belt, copious mucus is formed, into which the eggs fall, after which the mucus darkens and hardens, forming a cocoon. Then the cocoon is thrown off the worm through the head end of the body. Inside the cocoon, young worms develop from fertilized eggs.
Among the oligochaetes there are dwarfs, whose body length does not exceed a few millimeters, but there are also giants: Australian earthworm 2.5-3 m long.
Characteristic of earthworms regenerative ability. Earthworms are called soil formers, since they, by making passages in the soil, loosen it and promote aeration, that is, the entry of air into the soil.
Class polychaetes
This includes a variety of marine worms. Among them nereid. Her body consists of a large number of segments. The anterior segments form the head section, on which the mouth and sensory organs are located: touch - tentacles, vision - eyes. On the sides of the body, each segment has lobes on which numerous bristles sit in tufts. With the help of blades and bristles, Nereids swim or move along the bottom of the sea. They feed on algae and small animals. Breathe with the entire surface of the body. Some polychaetes have gills- primitive respiratory organs.
Refers to polychaetes sandstone, living in burrows, in the sand, or building itself a gypsum turtle, which is attached to algae. Many marine fish feed on nereids and other annelids.
Leech class
The most famous representative of this class is medical leech, which was already used in ancient times to treat people. Leeches are characterized by the presence of two suckers: the front one, at the bottom of which the mouth is located, and the back one.
The posterior sucker is large, its diameter exceeding half the greatest width of the body. Leeches bite through the skin with three jaws lined with sharp teeth along the edges (up to 100 on each jaw). Strong bloodsucker. In medicine, it is used for diseases of blood vessels (formation of blood clots), hypertension, and pre-stroke conditions. Leeches are applied to a certain part of a sick person to suck out blood; as a result, blood clots dissolve, blood pressure decreases, and the person’s condition improves. In addition, the salivary glands of the medicinal leech produce a valuable substance - hirudin, - preventing blood clotting. Therefore, after leech injections, the wound bleeds for a long time. While in the leech's stomach, the blood, under the influence of hirudin, is preserved for months without being subject to coagulation and rotting.
The leech's digestive system is designed in such a way that it can accumulate large reserves of blood, preserved with the help of hirudin. The size of a leech that has sucked blood increases significantly. Thanks to this feature, leeches can starve for a long time (from several months to 1 year). A leech lives up to 5 years. Leeches are hermaphrodites. In nature I achieve! They reach sexual maturity only in the third year of life and lay cocoons once a year in the summer.
Leeches are characterized by a straight, developed structure. Leeches include the non-blood-sucking predatory leech - large pseudokonskaya. It eats worms (including leeches), soft-bodied animals, larvae of aquatic insects, small vertebrates (tadpoles), which it can overcome.
Annelida are the most highly organized worms with a coelom. Their sizes range from a few millimeters to 3 m. The elongated body is divided into segments by internal annular partitions; sometimes there are several hundred such segments. Each segment may have lateral outgrowths with primitive limbs - parapodia, armed with setae. The musculature consists of several layers of longitudinal and circular muscles. Breathing is carried out through the skin; excretory organs - paired nephridia, located segment by segment. The nervous system consists of a “brain” formed by paired ganglia and a ventral nerve cord.
The closed circulatory system consists of abdominal and dorsal vessels connected in each segment by small annular vessels. Several of the thickest vessels in the anterior part of the body have thick muscular walls and act as “hearts.” In each segment, blood vessels branch, forming a dense capillary network.
Some annelids are hermaphrodites, while others have different males and females. Development is direct or with metamorphosis. Asexual reproduction (by budding) also occurs.
Annelids are divided into 3 classes: polychaetes, oligochaetes and leeches.
Polychaetes(Polychaeta) have primitive limbs (parapodia) with numerous setae on each segment. Bilobed parapodia are often associated with branched appendages - gills, with the help of which gas exchange is carried out. On the clearly distinct head there are eyes (in some species even capable of accommodation), tactile antennae and balance organs (statocysts). Some species are capable of luminescence.
During the breeding season, males release sperm into the water, and females release a large number of eggs. In some species, mating games and competition for territory have been observed. Fertilization is external; the parents then die. Development occurs with metamorphosis (free-swimming larva). Asexual reproduction is rare.
Oligochaeta are predominantly soil worms. Among them there are both giant earthworms up to 2.5 m long and dwarf forms. All segments, except the oral one, have bristles arranged in tufts. Parapodia are not pronounced, the head is poorly separated. The thin cuticle is constantly moistened by secreted mucus; Gas exchange occurs through the cuticle by diffusion.
Oligochaete worms are predominantly hermaphrodites with cross-fertilization; the genitals are distributed over several body segments. The complex structure of these organs is an adaptation to a terrestrial lifestyle. Parthenogenesis is known in some species. There is no metamorphosis; A dozen young worms emerge from the cocoons formed during the copulation process after a few weeks.
Leeches (Hirudinea) have a flattened body, usually colored brown or green. There are suckers on the anterior and posterior ends of the body. The body length is from 0.2 to 15 cm. Tentacles, parapodia and, as a rule, setae are absent. The muscles are well developed. The secondary body cavity is reduced. Breathing is cutaneous, some have gills. Most leeches have 1–5 pairs of eyes.
The lifespan of leeches is several years. They are all hermaphrodites. Eggs are laid in cocoons; there is no larval stage. Most leeches suck blood from various animals, including humans. Leeches pierce the skin with their proboscis or teeth on their jaws, and a special substance - hirudin - prevents blood clotting. Sucking blood from one victim can continue for months. Blood in the intestines does not deteriorate for a very long time: leeches can live without food for even two years. Some leeches are predators, swallowing their prey whole.
Leeches live in fresh water bodies and are also found in seas and soil. Leeches serve as food for fish. Medical leech used by humans for medicinal purposes. 400–500 species.
Annelids evolved from primitive flatworms in the Cambrian. The first annelids were polychaetes, which gave rise to oligochaetes, and through them, leeches.
The type of annelids, uniting about 12,000 species, represents, as it were, a node in the family tree of the animal world. According to existing theories, annelids originate from ancient ciliated worms (turbellar theory) or from forms close to ctenophores (trochophore theory). In turn, arthropods arose from annelids in the process of progressive evolution. Finally, in their origin, annelids are related by a common ancestor to mollusks. All this shows the great importance that the type under consideration has for understanding the phylogeny of the animal world. From a medical point of view, annelids are of limited importance. Only leeches are of particular interest.
General characteristics of the type
The body of annelids consists of a head lobe, a segmented body and a posterior lobe. Segments of the body throughout almost the entire body have external appendages similar to each other and a similar internal structure. Thus, the organization of annelids is characterized by repeatability of structure, or metamerism.
On the sides of the body, each segment usually has external appendages in the form of muscular outgrowths equipped with bristles - parapodia - or in the form of bristles. These appendages are important in the movement of the worm. Parapodia in the process of phylogenesis gave rise to the limbs of arthropods. At the head end of the body there are special appendages - tentacles and sticks.
A developed skin-muscular sac consists of a cuticle, an underlying layer of skin cells and several layers of muscles (see Table 1) and a secondary body cavity, or whole, in which the internal organs are located. The coelom is lined with peritoneal epithelium and divided by septa into separate chambers. Moreover, in each body segment there is a pair of coelomic sacs (only the head and posterior lobes are devoid of coelom).
The coelomic sacs in each segment are placed between the intestine and the body wall, they are filled with a watery fluid in which amoeboid cells float.
Overall it performs a supporting function. In addition, nutrients enter the coelomic fluid from the intestines, which are then distributed throughout the body. In the whole, harmful metabolic products accumulate, which are removed by the excretory organs. Male and female gonads develop in the walls of the coelom.
The central nervous system is represented by the suprapharyngeal ganglion and the ventral nerve cord. Nerves from the sensory organs pass to the suprapharyngeal node: eyes, balance organs, tentacles and palps. The abdominal nerve cord consists of nodes (one pair in each body segment) and trunks connecting the nodes to each other. Each node innervates all organs of a given segment.
The digestive system consists of the foregut, middle and hindgut. The foregut is usually divided into a number of sections: the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The hindgut opens with the anus on the posterior lobe. The intestinal wall contains muscles that move food along.
The excretory organs - metanephridia - are paired tubular organs, metamerically repeated in body segments. Unlike protonephridia, they have a through excretory canaliculus. The latter begins with a funnel that opens into the body cavity. Cavity fluid enters the nephridium through the funnel. A tubule of nephridium extends from the funnel, sometimes opening outward. Passing through the tubule, the liquid changes its composition; the final products of dissimilation are concentrated in it, which are released from the body through the external pore of nephridium.
For the first time in the phylogenesis of the animal world, annelids have a circulatory system. The main blood vessels run along the dorsal and ventral sides. In the anterior segments they are connected by transverse vessels. The dorsal and anterior annular vessels are capable of contracting rhythmically and perform the function of the heart. In most species, the circulatory system is closed: blood circulates through a system of vessels, nowhere interrupted by cavities, lacunae or sinuses. In some species the blood is colorless, in others it is red due to the presence of hemoglobin.
Most species of annelids breathe through skin rich in blood capillaries. A number of marine forms have specialized respiratory organs - gills. They usually develop on the parapodia or palps. Vessels carrying venous blood approach the gills; it is saturated with oxygen and enters the body of the worm in the form of arterial blood. Among annelids there are dioecious and hermaphroditic species. The gonads are located in the body cavity.
Annelids have the highest organization compared to other types of worms (see Table 1); For the first time, they have a secondary body cavity, a circulatory system, respiratory organs, and a more highly organized nervous system.
| Table 1. Characteristics of different types of worms | ||||||
| Type | Skin-muscle bag | Digestive system | Circulatory system | Reproductive system | Nervous system | Body cavity |
| Flatworms | Includes layers of longitudinal and circular muscles, as well as bundles of dorso-abdominal and diagonal muscles | From the ectodermal foregut and endodermal midgut | Not developed | Hermaphrodite | Paired brain ganglion and several pairs of nerve trunks | Absent, filled with parenchyma |
| Roundworms | Only longitudinal muscles | From the ectodermal anterior and posterior gut and the endodermal midgut | Same | Dioecious | Peripharyngeal nerve ring and 6 longitudinal trunks | Primary |
| From the external circular and internal longitudinal muscles | From the ectodermal foregut and hindgut and the endodermal midgut | Well developed, closed | Dioecious or hermaphrodite | Paired medullary ganglion, peripharyngeal nerve ring, ventral nerve cord | Secondary | |
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Animals belonging to the type of annelids, or ringworms, are characterized by:
Annelids live in fresh and marine waters, as well as in soil. Several species live in the air. The main classes of the annelid phylum are:
Class polychaete ringletsFrom the point of view of phylogeny of the animal world, polychaetes are the most important group of annelids, since their progressive development is associated with the emergence of higher groups of invertebrates. The body of polychaetes is segmented. There are parapodia consisting of dorsal and ventral branches, each of which carries an antennae. The muscular wall of the parapodia contains thick supporting setae, and tufts of thin setae protrude from the apex of both branches. The function of parapodia is different. Typically these are locomotor organs involved in the movement of the worm. Sometimes the dorsal barbel grows and turns into a gill. The circulatory system of polychaetes is well developed and always closed. There are species with cutaneous and gill respiration. Polychaetes are dioecious worms. They live in the seas, mainly in the coastal zone. A typical representative of the class is the Nereid (Nereis pelagica). It is found in abundance in the seas of our country; leads a bottom lifestyle, being a predator, it captures prey with its jaws. Another representative, the sandbill (Arenicola marina), lives in the seas and digs holes. It feeds by passing sea mud through its digestive tract. Breathes through gills. Class oligochaete ringletsOligochaetes originate from polychaetes. The external appendages of the body are setae, which sit directly in the body wall; no parapodia. The circulatory system is closed; skin breathing. Oligochaete ringlets are hermaphrodites. The vast majority of species are inhabitants of fresh water and soil. A typical representative of the class is the earthworm (Lumbricus terrestris). Earthworms live in soil; During the day they sit in holes, and in the evening they often crawl out. Rummaging in the soil, they pass it through their intestines and feed on the plant debris contained in it. Earthworms play a large role in soil-forming processes; they loosen the soil and promote its aeration; they drag leaves into holes, enriching the soil with organic matter; deep layers of soil are removed to the surface, and superficial layers are carried deeper. The structure and reproduction of an earthworm The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments or segments. In the anterior third of the earthworm's body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side. A characteristic feature of the internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells. Under the skin there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. When the circular muscles contract, the animal’s body elongates and becomes thinner; when the longitudinal muscles contract, it thickens and pushes the soil particles apart.
The digestive system begins at the front end of the body with the mouth opening, from which food enters sequentially into the pharynx and esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop, and a small muscular stomach (the muscles in its walls help grind the food). The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotten remains of plants, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil. The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it - the abdominal vessel. In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one. The red color of blood is due to the presence of hemoglobin in the plasma. Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore earthworms are very sensitive to soil moisture and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when there is a lot of water in the soil, they crawl to the surface. The excretory system is represented by metanephridia. Metanephridia begins in the body cavity with a funnel (nephrostom) from which a duct emerges - a thin loop-shaped curved tube that opens outward with an excretory pore in the side wall of the body. In each segment of the worm there is a pair of metanephridia - right and left. The funnel and duct are equipped with cilia, causing the movement of excretory fluid. The nervous system has a structure typical of annelids (see Table 1), two abdominal nerve trunks, their nodes are interconnected and form the abdominal nerve chain. The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Like hydra, earthworms are capable of regeneration. Reproduction occurs only sexually. Earthworms are hermaphrodites. At the front of their body are the testes and ovaries. Earthworms undergo cross fertilization. During copulation and oviposition, girdle cells on the 32-37th segment secrete mucus, which serves to form an egg cocoon, and protein fluid to nourish the developing embryo. The secretions of the girdle form a kind of mucous muff. The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the muff stick together and a cocoon is formed, which remains in the earthen burrow. Embryonic development of eggs occurs in a cocoon, and young worms emerge from it. Earthworm tunnels are located mainly in the surface layer of soil to a depth of 1 m; in winter they descend to a depth of 2 m. Through the burrows and tunnels of earthworms, atmospheric air and water penetrate into the soil, necessary for plant roots and the vital activity of soil microorganisms. During the day, the worm passes through its intestines as much soil as its body weighs (on average 4-5 g). On each hectare of land, earthworms process an average of 0.25 tons of soil every day, and over the course of a year they throw out 10 to 30 tons of soil they processed to the surface in the form of excrement. In Japan, specially bred breeds of fast-reproducing earthworms are bred and their excrement is used for biological soil cultivation. The sugar content of vegetables and fruits grown in such soil increases. Charles Darwin was the first to point out the important role of earthworms in soil formation processes. Annelids play a significant role in the nutrition of bottom fish, since in some places worms make up up to 50-60% of the biomass of the bottom layers of reservoirs. In 1939-1940 The Nereis worm was transplanted from the Azov Sea to the Caspian Sea, which now forms the basis of the diet of sturgeon fish in the Caspian Sea.
Leech classThe body is segmented. In addition to true metamerism, there is false ringing - several rings in one segment. There are no parapodia or setae. The secondary body cavity was reduced; instead there are sinuses and gaps between organs. The circulatory system is not closed; the blood passes only part of its way through the vessels and pours out of them into the sinuses and lacunae. There are no respiratory organs. The reproductive system is hermaphroditic. Medical leeches are specially bred and then sent to hospitals. They are used, for example, in the treatment of eye diseases associated with increased intraocular pressure (glaucoma), cerebral hemorrhage and hypertension. For thrombosis and thrombophlebitis, hirudin reduces blood clotting and promotes the dissolution of blood clots. |
