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 posterior to 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 the 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 (left and 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, gas transport is carried out by the circulatory system, and not by diffusion through tissue fluid. In 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 (in the coelom). On the other side, the tubes open outward through the surface of the body. Each annelid segment contains two metanephridia (right and 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.
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 are playing important role in 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 worms can be of different colors various types: red, green or transparent. 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. Nervous system annelids includes a large nerve ganglion (the prototype of the brain) and an abdominal nerve chain 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 should be noted their important role in food chains in natural environment a 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 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.
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
|
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. |
|
Digestive system |
Digestive system - mouth, pharynx, esophagus, goiter, stomach, midgut, hindgut, anus, glands. |
|
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. |
|
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. |
|
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 physical properties habitats.
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.
Searched on this page:
- What features of annelids allowed them to populate most of the planet?
- formation of mucus that allows soil worms to tolerate unfavorable conditions
- What is common in adaptations to endure unfavorable conditions in protozoa and oligochaetes?
- which allows scientists to classify the polychaetes of oligochaetes and leeches as one type
- what features of annelids allowed them to colonize large
We invite you to visit the site
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 own way chemical composition 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.
Rummaging 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
Type Annelids is the most highly organized collection of worm species on Earth. This type unites about 12,000 species of different worms. Body of an annelid consists of a large number of segments, some of them segments have bristles, playing an important role for movement. Internal organs of annelids located in a body cavity called in general. Annelids have a circulatory system. Nervous system of annelids consists of a cluster of nerve cells that are located in the front part of the worm's body. These clusters form the suprapharyngeal and subpharyngeal nerve ganglion. Habitat of annelids- fresh and salt water bodies, soil.
The phylum Annelids are divided into three classes:
- Polychaetes.
Class Polychaetes.
The class Polychaetes includes a variety of sea worms. One of the typical representatives of this class is nereid. The body of this worm consists of many segments. Front segments They make up the head section, where there is a mouth, as well as organs of vision - eyes and organs of touch - tentacles.
On each of the segments there are located on the sides blades, on which the bundles are located bristles. With the help of bristles and blades, the nereid moves along the bottom of a reservoir or swims. Nereids feed small animals or algae. Nereid breathes the entire surface of the body, although some polychaetes have primitive gills on the blades.
Also belongs to this class sandstone, which lives in sand, burrows or a self-built limestone turtle, with the help of which it is attached to algae. Nereids, sandworms and others sea worms are food for fish and other larger aquatic inhabitants.
Circulatory system. The vessels of the circulatory system carry red blood. The spinal vessel has the ability to pulsate, i.e., contractile movements of the walls and usually drives blood from back to front.
Special vessels covering the intestinal tube and located metamerically in each segment (body rings) transfer blood to the abdominal vessel, which is not capable of independent pulsation. Blood moves in it from front to back. In addition to these directions of blood currents, the vessels that carry blood from the dorsal vessel to the parapodia are important. These are parapodial vessels. In parapodia, blood vessels acquire the character of capillaries, where oxidation of blood occurs, which is in close contact with the oxygen of the external environment. The dorsal vessel reaches the prostomymind, the abdominal vessel ends at the level of the pharynx, that is, it is somewhat shorter. The skin is also intensively supplied with capillary blood vessels. At the same time, the intestinal tube and all internal organs, as well as disseminations, are also abundantly irrigated with blood.
:
1—nerves to the palps. 2—nerves to the antennae (pyrrhus), 3—supraglottic ganglion, 4, 5—peripharyngeal nerve ring, 6—nerves of the ventral nerve cord, 7.—beginning of the ventral nerve cord
. I - the pharynx and buccal region are retracted; II - the buccal region is everted, the pharynx is pushed forward:
1 - buccal: section, 2 - pharynx
. The pharynx (1) with large jaws is pushed outward
Respiratory system. The capillaries of the circulatory system of parapodia and skin are very important in the respiratory processes of Nereis, while species of this genus do not have special gill projections.
Excretory system. In Nereis it consists of paired metanephridia. They are absent only in the five anterior metameres and three or four posterior ones. Nereis metanephridia are very typical. Each metanephridium consists of a glandular metanephridial body, shaped like a sac, penetrated by a convoluted nephridial canal. This canal begins outside the body of the metanephridium in the cavity of the corresponding coelomic sac with a funnel, or nephrostomy. The nephrostomy quickly narrows into a canal that penetrates the wall of the dissepiment lying in front of it and enters the cavity of the next coelomic sac (anterior to the previous one), where the body of the metanephridium itself lies. Inside the metanephridial body, the anterior part of the nephridial canal(closest to the nephrostomy) carries thin cilia that work in concert and drive the fluid located in the nephridial canal tube forward to the external outlet, i.e., into the external environment. This posterior part of the nephridial canal does not bear cilia. The external opening of the nephridial canal is called the nephridial pore. Thus, the liquid contents of the metanephridium, penetrating into the nephridial canal from the coelom, are discharged out through the nephropore. This is the anatomy of metanephridia. As for its function, it consists in removing liquid metabolic products through the nephridial canal, which accumulate partly in the form of grains in the coelomic cavity.
The type of annelids, or ringworms, covers about 9 thousand species of worms, which have a much more complex organization than representatives of other types of worms.
Certain structural features of the larvae, which are very reminiscent of the larval forms of free-living flatworms (the body is not divided into segments and is covered with ciliated epithelium), suggest that ringworms, like roundworms, originated from primitive flatworms, similar in structure to modern ciliated worms . This happened more than 600 million years ago.
The body of most forms consists of separate rings - segments. Many ringlets are characterized by the presence of lateral mobile outgrowths of the body of parapodia and tufts of setae, which are the prototype of the limbs. Some annelids have skin outgrowths called gills on the dorsal part of the parapodia.
External segmentation corresponds to the division of the internal body cavity by partitions into separate sections and the segmental arrangement of a number of internal organs. Nerve ganglia, annular blood vessels, excretory organs - metanephridia, midgut pouches and genitals are correctly repeated. The skin-muscular sac consists of the cuticle, epithelium, circular and longitudinal muscles, as well as the internal lining of the body cavity.
The nervous system is represented by a peripharyngeal nerve ring with a well-developed suprapharyngeal and less pronounced subpharyngeal nerve nodes, as well as an abdominal nerve cord that forms nodes in each segment of the body. Numerous nerves arise from them. Sense organs are better developed in polychaete annelids and are represented by one or two pairs of eyes located on the dorsal side of the first segment.
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.
Breathing is carried out in most cases over the entire surface of the body, some have special outgrowths - skin gills.
The digestive system is continuous, complex, divided into the pharynx, esophagus, stomach and intestine, sometimes having lateral outgrowths; ends with the anus.
The excretory system is represented by segmentally located metanephridia. Their funnel faces the body cavity, and the other end opens outward.
Reproduction of annelids occurs sexually and asexually - by budding. Among the ringlets there are dioecious species and hermaphrodites. Some ringlets have a rather complex reproductive system, while others do not have special reproductive organs - germ cells are formed from the internal lining of the body cavity and are brought out through metanephridia.
The phylum unites several classes, of which the three main ones are Polychaetes, Oligochaetes and Leeches.