Annelids- This is the most highly organized type of worm. 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 carries nutrients, metabolic products and germ cells are excreted through it.
The body of annelids consists of repeating segments (rings, segments). In other words, their body is segmented. There can be several or hundreds of segments. The body cavity is not single, but is divided into segments by transverse partitions (septa) of the epithelial lining of the coelom. In addition, two coelomic sacs (right and left) are formed in each ring. Their walls touch above and below the intestine and support the intestines. Between the walls there are also blood vessels and a nerve cord. Each segment has its own nodes of the nervous system (on the paired abdominal nerve trunk), excretory organs, gonads, and external outgrowths.
The head lobe is called the prostomium. The back part of the worm's body is the anal lobe, or pygidium. The segmented body is called the torso.
The segmented body allows annelids to grow easily by forming new rings (this occurs posteriorly in front of the anal lobe).
The appearance of a segmented body is an evolutionary progress. However, annelids are characterized by homonomic segmentation, when all segments are approximately the same. In more highly organized animals, segmentation is heteronomous, when the segments and their functions are different. At the same time, in annelids, the formation of the head section of the body is observed by fusion of the anterior segments with a simultaneous increase in the cerebral ganglion. This is called cephalization.
The body walls, like those of lower worms, are formed by a skin-muscular sac. It consists of skin epithelium, a layer of circular and a layer of longitudinal muscles. Muscles achieve more powerful development.
Paired organs of movement emerged - parapodia. They are found only in polychaete annelids. They are outgrowths of a skin-muscular sac with tufts of bristles. In 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 a respiratory system and usually breathe over the entire surface of the body, gases are transported through the circulatory system rather than 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.
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. 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 large number 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 segments of the body. 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 in 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.
Annelids are a fairly large group of invertebrate organisms. In addition, they are considered the most organized representatives of worms. They live mainly in fresh and salt water bodies, as well as in soil. Some species of tropical leeches have adapted to a terrestrial way of existence.
Type Annelids: general characteristics
The sizes of representatives of this group range from a few millimeters to six meters. A rather characteristic feature of such an organism is the presence of segmentation - their body consists of many rings, which explains the name of the type. External ringing corresponds to internal segmentation. That is why, when the body is injured or damaged ringworm loses only a few segments, which are soon regenerated.
From the outside, the body is covered with a cuticle that is not shed. Chitinous bristles grow from it - another characteristic feature of this species. Some representatives may have parapodia on their segments - rather primitive limbs, which in some cases are equipped with sensitive bristles or gills.
Ringed worms: structural features of internal organs
For representatives of this type characterized by the presence of a secondary body cavity - the coelom. This cavity is filled with a specific liquid, thanks to which normal indicators are maintained.
There is a skin-muscular sac, which consists of epithelial balls, as well as muscles grouped into circular and longitudinal groups.
The digestive system is continuous, beginning with the mouth and ending with the anus. Annelids have three intestinal sections - anterior, middle and posterior. Some species have primitive salivary glands.
The body breathes through the skin. The only exceptions are some marine species of animals that have gills on their parapodia. As for the circulatory system, it is usually closed. It consists of the abdominal and dorsal aorta, which are connected to each other by annular vessels. These organisms do not have a heart - the movement of blood is ensured by contraction of the dorsal aorta. Blood can contain a wide variety of respiratory pigments.
Still quite simple. At the anterior end of the body there is a large nerve ganglion, which performs the functions of the brain. A nerve chain extends from it, which in each segment of the body forms a small ganglion - a collection of neurons. are represented by the eyes, organs of chemical sensitivity, as well as mechanoreceptors, which are distributed throughout the body of the worm.
Ringed worms: features of reproduction and development
Organisms of this group can be either heterosexual or hermaphrodite (they are much less common). For example, it has a hermaphroditic reproductive system, but two individuals are needed for fertilization. It can happen either in external environment, and by introducing sperm into the internal canals of the female.
Another interesting fact is that annelids with pronounced segmentation have a tendency to rapid and intense regeneration. Due to this, some species are characterized by direct development of organisms, without metamorphosis.
It is worth noting that the role of annelids is quite significant. For example, the well-known one is responsible for soil aeration. This group also includes leeches, which are often used in modern medicine. Hirudin, produced by the leech, is of particular value, as it thins the blood and is used in the fight against thrombosis and other dangerous diseases.
Annelids are a fairly large type of invertebrate (about 9,000 species). They are classified as free-living animals that have a very complex organization, unlike flat and roundworms. These include primary ringlets, multi- and oligochaete worms, echiurids and leeches. The most primitive species are considered to be archiannelids, which live in the marine environment.
Echiurids and polychaetes also live in the sea, but their organization is more complex. And leeches and oligochaete worms are inhabitants of soil and fresh water bodies.
Highly organized type
Annelids can reach 2.5 meters in length. Most species are free-living forms. Any annelid has a body consisting of 3 parts: the body (consists of rings), the head and the anal lobe. The head of the ringlets houses various sensory organs. Most annelids have well-developed eyes. Some of them have specifically acute vision, their lens is capable of accommodation. Eyes can be located in different areas: on the head, body, tentacles and even the tail. In addition, annelids have a highly developed sense of taste. Many of them have special olfactory cells on their heads or tails that perceive all kinds of odors or the effects of chemical irritants. Hearing organs are also present, and they work on the principle of locators. Not so long ago, hearing organs were discovered in many echiurids, very similar to the lateral line organ in fish. This allows them to always be ready: to hear all the rustles and splashes, because underwater audibility is much better than on land. Annelids are named for their body, which is made up of segments (rings). The number of these rings can reach a couple of hundred. However, most ringlets have only a few segments.
In principle, each ring represents an independent particle of the whole organism. Annelids move thanks to specific organs of movement - parapodia, which are located on the sides of each segment. They have the appearance of lobe-shaped outgrowths of the body, from which tufts of bristles protrude outward. However, not every ringlet has parapodia. They are found in polychaete worms and some primary ringworms. In oligochaete individuals, only the setae are the organs of locomotion. The body cavity of annelids is filled with a liquid similar in composition to chemical water. This liquid is constantly moving, due to which the process of transporting nutrients through the body of the rings, secretions of the endocrine glands, oxygen and carbon dioxide occurs, which are directly involved in the respiration process of any organism.
Reproduction of annelids
Annelids reproduce sexually and asexually. However, asexual reproduction dominates in aquatic ringworms. Fertilization of the egg occurs outside the female's body. The male and female synchronously release their reproductive cells into the water, where fertilization actually occurs.
The type Annelid worms has about 9 thousand species. The level of organization of animals included in this type is significantly higher than that of flat and roundworms. The time of origin of annelids is unknown. Their ancestors were probably primitive flatworms.
The main characteristics of the type Annelid worms:
- in the external structure there is a head lobe with sensory organs, a segmented body and a posterior lobe;
- the skin-muscle sac is well developed;
- coelom, or secondary body cavity; the digestive system usually consists of the oral cavity, pharynx, midgut and hindgut and ends at the anus;
- Most species have a closed circulatory system;
- The nervous system consists of a paired brain, a pair of peripharyngeal nerve trunks, and a ventral nerve cord. In most representatives of the type, paired nerve ganglia are located in each segment. The sense organs are represented by the eyes, olfactory pits, tentacle-like appendages, and balance organs - statocysts;
- primitive forms are dioecious; many have developed hermaphroditism.
The type is divided into two subtypes: belted and beltless. In the girdle subtype, the only class is polychaetes (polychaetes), in which two subclasses are distinguished - vagrant and sessile. The girdleless subtype includes the class of oligochaetes (oligochaetes) and the class of leeches.
Polychaetes- mostly marine annelids leading a bottom-dwelling lifestyle (nereis, aphrodite, peskozhil). They have well-developed sensory organs in the form of palps and eyes. On each segment of animals there are primitive legs - parapodia, which are used for movement. Their body is covered with a single-layer epithelium, secreting a thin cuticle. Worms are dioecious and have indirect development. The larva is called trochophore, it has cilia for movement.
Oligochaetes- inhabitants of soil and water bodies. Their parapodia are reduced, and their sensory organs are poorly developed. Animals do not have eyes, so light is perceived by light-sensitive cells. They breathe on the surface of the body.
Earthworms- hermaphrodites. The development of the worm occurs without a larval stage.
Features of the structure of the rings.
Skin-muscle bag consists of external circular and internal longitudinal muscles, which provides a variety of movements of the worm. Coelom - the secondary body cavity formed inside the mesoderm, is divided into sections, the number of which corresponds to the number of body segments. It performs support, distribution, excretory and sexual functions. The support function is explained by the incompressibility of the coelomic fluid. When muscles contract, the body becomes elastic. In general, nutrients come from the intestines, then they are distributed throughout the body. In the whole, reproductive products mature. The digestive system consists of the usual sections for annelids. In oligochaetes, the midgut forms a fold. In some predatory species of polychaetes, chitinous teeth are formed in the pharynx.
Respiratory organs- gills, individual sections of parapodia, the entire surface of the body.
Circulatory system closed. It consists of abdominal and dorsal longitudinal vessels communicating with annular vessels that act as the heart. Contractions of the dorsal and anterior annular vessels ensure blood flow through the body of the worm. Blood flows forward through the dorsal vessel, and backward through the abdominal vessel.
Excretory system formed by metanephridia - excretory canals, each of which passes through two segments of the body and goes out. The end of the canal that opens as a whole is surrounded by cilia.
Nervous system nodular, ganglion type.
In the process of evolution, arthropods evolved from annelids.
The meaning of annelids.
Polychaetes serve as food for fish, crabs and other animals.
Earthworms have a positive effect on soil fertility by loosening it. This facilitates the penetration of water and air into the soil. Worms mix and fertilize the soil with plant residues, promoting the formation of humus. They are also included in the diet of fish and other inhabitants of freshwater bodies.
Leeches are used in medicine to treat diseases such as vascular thrombosis, hypertension, etc.