The body of the earthworm is round in shape; the length of most representatives of this genus is no more than 15 centimeters, occasionally more than twenty, and the length of the largest is slightly more than thirty centimeters.
Consists of 100-180 segments. The segments have small, fairly elastic bristles that are practically invisible, but if you run your finger from the rear tip to the front, you can immediately feel it. The worm needs bristles so that it can cling to uneven ground surfaces while moving. On the front of the worm's body there is a small thickening that serves as the place where the genital organs are located. The cells located in this thickening are activated during reproduction to lay eggs. If you look closely, you will notice that the belly of the earthworm is somewhat lighter than the rest of the parts. The worm has not only a circulatory system, but also a nervous, tactile and digestive system.
General characteristics of annelids
Type annelids. Structure of an earthworm: external structure (ventral view), cross section, circulatory and digestive systems (front end of the worm)
Representatives of the phylum Annelida are the most highly organized worms. They mostly live freely in the seas, fresh water bodies, and soil. About 9 thousand species are known.
The body is elongated, divided into relatively uniform (homonomous) segments ( metameres ), separated by internal partitions, and has a head and sometimes a posterior lobe. Almost every segment has paired organs (nerve ganglia, excretory organs, etc.). In cross section they have a rounded shape. The segments have bristles.
The integument is represented by a skin-muscle sac. Covered with cuticle, which is secreted by the epidermis. There are many mucous glands in the skin. Two layers of muscles: the outer layer is circular and the inner layer is longitudinal. Most polychaete worms develop organs of movement - parapodia . These are mobile muscular outgrowths of the body (primitive limbs), which consist of dorsal and abdominal branches. They can cling to nearby objects.
They have a fluid-filled secondary abdominal cavity ( coelom ), with each segment containing a pair of coelomic sacs (except for the head and posterior lobes), which are located between the intestine and the body wall. The whole differs from the primary cavity in that it is lined with a special epithelium, which, on the one hand, is adjacent to the body wall, and on the other, to the walls of the digestive tube and separates the cavity fluid from the tissues and organs. The lining leaves grow together above and below the intestine and form a mesentery, which divides the coelom into right and left parts. The liquid is in constant motion, thanks to which it transports nutrients, oxygen, carbon dioxide, and gland secretions. Forms a hydrostatic skeleton. The secondary cavity develops in the middle of the mesoderm and is lined with epithelium of mesodermal origin.
Appearance (a) and structural diagram (b) of an earthworm
Germ layers
The germ layers are the basis of all organs. In annelids, the ectoderm (outer layer of cells), endoderm (inner layer of cells) and mesoderm (intermediate layer of cells) appear early in development as three germ layers. They give rise to all major organ systems, including the secondary cavity and the circulatory system.
| Germ layers | What is being formed |
| ectoderm |
|
| endoderm |
|
| mesoderm |
|
The respiratory organs of different groups of higher multicellular organisms can be built with the participation of all three germ layers.
These same organ systems are subsequently preserved in all higher animals, and they are formed from the same three germ layers. Thus, higher animals in their development repeat the evolutionary development of their ancestors.
Earthworms , or earthworms , are a group of oligochaete annelids.
Their characteristic features:
- the gonads are located in several segments;
- copulation of two hermaphroditic individuals and cross-fertilization occurs;
- eggs are laid in a cocoon secreted by a belt, development is direct;
- live in moist soil.
Circulatory system of annelids
Circulatory system of annelids
The circulatory system is closed, consists of interconnected abdominal and dorsal vessels, which are connected in each segment by circular ones. Blood flow is ensured by contractions of the spinal and several anterior annular vessels. Blood moves through the abdominal vessel to the back of the body, and through the spinal vessel to the front. The blood of most annelids is red, sometimes green, depending on the type of respiratory pigments. May be colorless, transparent or blue. Leeches have a reduced circulatory system. The function of blood is performed by coelomic fluid.
Habitat
During the day, earthworms stay in the soil, making tunnels in it. If the soil is soft, the worm penetrates it with the front end of its body. At the same time, he first compresses the front end of the body so that it becomes thin, and pushes it forward between the lumps of soil. Then the front end thickens, pushing the soil apart, and the worm pulls up the rear part of the body. In dense soil, the worm can eat its way through the soil through its intestines. Lumps of soil can be seen on the surface of the soil - they are left here by worms. After heavy rain has flooded their passages, the worms are forced to crawl out to the surface of the soil (hence the name - earthworm). In summer, worms stay in the surface layers of the soil, and in winter they dig burrows up to 2 m deep.
| Rice. 51. Earthworm and its movement in the soil. The front end of the earthworm's body from below |
Reproductive system of annelids
Sexual, sometimes asexual reproduction (budding, fragmentation). Mostly the genitals appear only in certain segments of the body, sometimes in all. Oligochaete worms are hermaphrodites, while polychaete worms are predominantly dioecious. Sexual dimorphism is not expressed. Fertilization is both internal and external. In marine forms, indirect metamorphosis is developed (with a larva, incomplete metamorphosis), while in freshwater and terrestrial forms, direct metamorphosis is developed.
Some annelids are capable of regeneration.
Position in taxonomy (classification)
Earthworms belong to the phylum Annelids, the class Beltworms, and the subclass Oligochaetes.
Since ancient times, humanity has paid special attention to such unsightly creatures as the earthworm. And scientific minds, in the person of Charles Darwin, decades later, studied their structure and significance in agriculture for many years. And not without reason. Indeed, with the onset of spring warmth, earthworms begin painstaking work and work, without knowing it, for the benefit of people.
Variety of annelids
Type annelids: class Polychaetes or Polychaetes, class Oligochaetes or Oligochaetes and class Leeches
Class Polychaete worms or Polychaeta
Class Polychaetes or Polychaeta: Amphitrite, Sea Mouse, Green Nereis, Peskozhil and Serpula
The most common class. There are about 6 thousand species. Dimensions from 1 mm to 3 m. Mainly marine animals, free-living. Only some representatives are parasites. Body shape – elongated, slightly flattened. The head is well developed, the rear part is pronounced. Segments of the body can be either identical in structure and function, or different. The body is covered with single-layer epithelium and various outgrowths (especially the head lobe). It has projections (with two branches: ventral and dorsal) with tufts of setae on the sides of the body for active locomotion ( parapodia ) on each segment. The epithelium may have glandular cells, and in primitive species - cilia. In sessile species, epithelial secretions may harden and form a tube.
Some species have respiratory organs - gills (body outgrowths on parapodia). Others breathe throughout the entire body.
The circulatory system is closed. They have organs of vision and a pair of tentacles (organs of touch). Regeneration is pronounced.
Dioecious animals. Sexual dimorphism is not expressed. Development in polychaetes is indirect. trochophore ) covered with cilia emerges from the egg Some species can reproduce asexually.
Representatives: peskozhil, nereis, Pacific palolo , etc.
Class Oligochaetes or Oligochaetes
Class Oligochaetes or Oligochaetes: Ripistes, Stilaria, Elosoma and Chaetogaster
About 5 thousand species are known. Distributed mainly in fresh water bodies and soil. Sizes vary greatly. There are no parapodia. The head lobe is poorly expressed. Hermaphrodites. Fertilization is internal. Development is direct. The eggs are laid in a cocoon. Asexual reproduction occurs by fragmentation. Representatives: tubifex, earthworm , etc.
Earthworm (Lumbricus terrestris)
Earthworm: appearance, internal structure, cross section, pattern of movement of the worm in the soil, reproduction, mating of two individuals, exchange of sexual products. Earthworms in summer and worms overwintering in deep burrows
Dimensions reach 15 – 30 cm. Lives in the soil. Forms soil and increases its fertility. Feeds on plant remains. The body is elongated, consists of segments, the number of which is 140 - 180. Near the 31st segment there is a special formation - the girdle. This is an organ that secretes a substance from which a cocoon is built, two individuals stick together during mating. Each segment has four pairs of setae (remnants of parapodia). The skin is covered with mucus secreted by the cutaneous glands. The integument is represented by a skin-muscular sac, which consists of four layers: mucous cuticular lining, epithelial cells, muscle layer, and thin film. The muscles are represented by two layers of muscle fibers: annular septa. The body cavity is lined with epithelium and separated by septa. Filled with liquid, which acts as a hydrostatic skeleton.
Digestive system of an earthworm
The oral opening at the anterior end of the body (head lobe) leads into the thick-walled pharynx, which is located in the first 6 segments. The pharynx passes into a narrow esophagus, into which special limestone glands open, which neutralize humic acids (from the soil). Behind the esophagus there is a crop, which passes into the muscular stomach (the ectodermal section ends). Then the food passes into the mid and hind intestines, which end in the anus - on the posterior segment.
Excretory system of an earthworm
The excretory system is represented by metanephridia. The openings of the metanephridia are located on the dorsal side. They open and close. Not noticeable to the naked eye.
Earthworm circulatory system
The circulatory system is closed and consists of dorsal, abdominal and annular vessels. The six pairs of annular vessels that surround the esophagus and connect the dorsal and abdominal vessels are called the hearts. Their walls, like the walls of the spinal vessel, pulsate. The blood is red and contains hemoglobin in the plasma. It flows in the dorsal vessel to the head end, in the abdominal - in the opposite direction.
Respiratory system of an earthworm
There is no respiratory system. They breathe through the entire surface of the body, so a large number of capillaries branch in it.
Nervous system of an earthworm
The nervous system is represented by the peripharyngeal nerve ring and the ventral nerve cord.
Sense organs of an earthworm
Sense organs: photosensitive, tactile, chemical sensation cells. Light-sensitive cells are located on the head blade.
Reproductive system of an earthworm
Reproduction is only sexual. On the ventral side, on the 14th segment, there is a pair of small round female genital openings. On the 15th segment there is a pair of male genital openings in the form of transverse slits. Hermaphrodites. The gonads are located at the level of 9 – 15 segments. Fertilization is predominantly cross-fertilization. During reproduction, two individuals exchange seminal fluid, which is stored in the seminal receptacles. The secretions of the girdle form a kind of mucous muff. The worm crawls out of the coupling with its back end first. The edges of the coupling stick together and form a cocoon. The eggs are laid in a cocoon (the egg and sperm are released from the seminal receptacle). The cocoon lies in the ground. Young individuals develop from eggs. Development is direct. Such complex reproduction is due to the habitat and the low opportunity to meet a sexual partner.
Earthworms are capable of regeneration.
Class Leeches (Hirudinea)
Leech class (Hirudinea): False-cone leech and Medicinal leech
About 500 species are known. Dimensions - from 1 to 20 cm. They live in the seas and fresh water bodies, sometimes found on land. They come from oligochaete worms. The body is flattened. Setae and parapodia are absent. Predators and bloodsuckers, free-living and parasites (mainly ectoparasites). The outside of the body is covered with a dense cuticle. The head lobe is not expressed. They have two suckers (the front one with the mouth opening and the back one). May have chitinous teeth in the throat for cutting through the skin of the victim. Some species have a long proboscis, which is used to attack prey. The body cavity is reduced and turned into a system of lacunae (sinusoids). The space between the internal organs is filled with parenchyma.
The digestive system is well developed. In bloodsuckers, the salivary glands produce a substance that prevents blood clotting ( hirudin ). The midgut forms lateral protrusions in which blood is stored. Metanephridia are located in only a few segments.
The circulatory system is reduced due to the parasitic lifestyle. It is represented by a system of individual lateral sinusoids (remnants of the body cavity).
The function of blood is performed by coelomic fluid. The blood is colorless or red (has hemoglobin).
Hermaphrodites. Lose the ability to regenerate. Fertilization is internal. Direct development is characteristic.
Representatives: medical leech, horse leech , etc.
Reproduction and development
Earthworms are hermaphrodites. During the process of copulation of two individuals, mutual fertilization occurs, that is, the exchange of male gametes, after which the partners separate.
The ovaries and testes are located in different segments at the anterior end of the body. The location of the reproductive organ system is shown in Figure 51. After copulation, a belt is formed around each worm - a dense tube that secretes the cocoon shell. The cocoon receives nutrients that the embryos will subsequently feed on. As a result of the expansion of the rings located behind the cocoon, it is pushed forward towards the head end. At this time, 10-12 eggs are laid through the opening of the oviduct into the cocoon. Further, when the cocoon moves, sperm from the sperm receptacles received from another individual during copulation enters it, and fertilization occurs. After this, the cocoon slides off the worm and its holes quickly close. This prevents the eggs it contains from drying out.
The development of earthworms is direct, that is, they do not have larvae; a young worm hatches from the egg.
The meaning of annelids
Annelids are an important link in the food chain. Many species form the food supply for fish (for example, Nereis). From the Sea of Azov, Nereis was moved to the Caspian Sea in order to maintain the food supply of valuable industrial fish species.
Earthworms are soil formers; they improve the properties of the soil, enrich it with oxygen and organic matter, make it loose, facilitate the access of moisture, and mix the soil layers. Through the digestive system, the earthworm passes as much soil as its body weighs (approximately 4 - 5 g). Abroad (in Japan, America, etc.) earthworm cultures are bred to obtain soils, and various plants (vegetables, herbs, etc.) are grown on them. The importance of earthworms as soil formers was noted by Charles Darwin.
Some types of annelids are eaten by humans (palolo) and used as live bait in fishing (sandfish, etc.). The annelid tubifex is used as food for aquarium fish.
Medical leeches are used for hypertension (high blood pressure), to reduce blood clotting and resolve blood clots. To do this, leeches are caught or specially bred. The substance hirudin is obtained, which is used in medicine and the perfume industry.
What leaf does Lupine have?
The leaves are alternate, palmately compound, on long petioles, articulated with the stem by a fleshy leaf pad with elongated stipules. The inflorescence is a multi-flowered apical raceme. The flowers are arranged alternately, semi-whorled or whorled. The flower is zygomorphic.
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Intestinal blood plexus
- The intestines are rich in blood capillaries, which form a close network.
- Consists of a dense network of capillaries in the intestinal wall.
- There are 2 networks of capillaries in the intestine: 1) external plexus; 2) internal plexus.
- The external plexus lies on the surface of the intestine and receives blood from the ventral vessel through the ventro-intestinal and septal tract and transmits it to the internal plexus.
- The internal plexus is located between the layer of circular muscles of the intestines and the inner epithelial lining.
- The internal plexus conveys blood along with absorbent nutrients to the dorsal vessel through the dorsointestinal tract.
