The earthworm has sensory organs. Earthworm
The body of an earthworm is divided into segments by ring constrictions. Each segment has eight small bristles, which when the worm moves, rest against uneven soil.
The body wall is covered with a cuticle secreted by a single-layer epithelium. Below it is a layer of circular muscles, and below them are longitudinal muscles. Thanks to the alternating work of these muscles, the worm moves. The movement is facilitated by secreted mucus.
An earthworm is an annelid worm that has a secondary body cavity - as a whole. Its walls are lined with epithelium. The cavity is filled with fluid capable of transporting nutrients and oxygen, which is absorbed by the entire surface of the body. There is no respiratory system. (When it rains, the worms lack oxygen and crawl to the surface of the soil).
The mouth is located on the ventral side of the anterior segment, and the anus is located on the last segment. The worm feeds on fallen leaves and rotting plant debris, swallowing them along with the soil. Nutrients in the intestines are absorbed into the blood. Undigested residues are expelled through the anus.
The circulatory system is closed. The dorsal vessel carries blood from the posterior to the anterior end of the body. Several annular vessels in segments 7–11 play the role of the heart, pumping blood into the abdominal vessel. Blood moves through the abdominal vessel to the posterior end. From the main vessels, thinner ones depart, turning into capillaries. Blood contains hemoglobin, which carries oxygen. A closed circulatory system allows you to significantly increase your metabolic rate.
In each segment, except for the terminal ones, there is a pair of metanephridia - tubes that carry metabolic products out of the coelom (excretory system).
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. There are no sense organs. The worm is able to perceive light and touch due to tactile and light-sensitive cells scattered throughout the surface of the body.
Earthworms are hermaphrodites, but cross-fertilize. On segments 32–37 there is a girdle used for the construction of egg cocoons. The cocoon moves to the anterior end, spermatozoa obtained in advance during copulation with another individual enter it from the seminal receptacles, and fertilization occurs. The cocoon slides off through the head end of the worm. Development is direct, with young worms hatching from the eggs. Earthworms are characterized by the ability to regenerate - to restore a lost body fragment.
The importance of the earthworm in nature
- Earthworms make tunnels in the soil, allowing air and water to penetrate into the soil.
- They improve soil structure by gluing soil particles into small lumps.
- Soil fertility is promoted by worms dragging fallen leaves and other plant debris into burrows, digesting them and decomposing them to form humus.
- Earthworms serve as food for many animals: moles, shrews, hedgehogs, toads, ground beetles.
- They are intermediate hosts of helminths that cause diseases in young pigs, etc.
Charles Darwin wrote in 1881 that archaeologists should be grateful for the preservation of many ancient objects precisely to earthworms, under whose excrement coins, jewelry, and stone tools were safely stored for many centuries. In addition, the great naturalist established that in a few years the worms pass the entire arable layer of soil through their bodies, and their countless burrows form a kind of capillary network of the earth, providing its ventilation and drainage.
There are a huge number of earthworms on Earth: about 6,000 species. They live on all continents except Antarctica.
There are especially many of them in the tropics. An adult earthworm can reach a length of 15 cm; in the tropics, 3-meter individuals are found.
Lumbricus terrestis spends its entire life in the ground, tirelessly digging tunnels. They usually appear on the surface during rains due to lack of oxygen and at night.
The body of the worm consists of several tens, or even hundreds of segments (80 - 300). When moving, it relies on bristles, which are present on all segments except the first. Characterized by a closed circulatory system. The blood is red. There is one vein and one artery running through the entire body. Breathing is carried out over the entire surface of the body, which is covered with mucus. The nervous system is represented by two nerve ganglia (brain) and the abdominal chain. Capable of regeneration. Earthworms are hermaphrodites, that is, each sexually mature individual has a male and female reproductive system. Cross fertilization is common.
Photo: internal structure of the digestive system of earthworms.
Reproduction of earthworms.
Video: The principle of shedding the cocoon of an earthworm.
The structure of an earthworm: digestive, nervous and circulatory systems.
Video: Earthworm movement
An earthworm's burrow is a long channel that, on a hot summer day, descends to a depth of 1.5 meters. They feed on soil, fallen leaves and the remains of herbaceous plants. Penetrating the soil with their numerous passages, they loosen it, mix it, moisten it and fertilize it. During the day, the earthworm passes through itself an amount of organic substances equal to its body weight. If the soil is loose, then Lumbricus terrestis tears off a piece of soil with its lips and swallows it; if it is dry, it moistens it with saliva.
Annelids have the highest organization compared to other types of worms; For the first time, they have a secondary body cavity, a circulatory system, and a more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.
They live in soil, fresh and sea water.
External structure
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 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.
Internal structure
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.
Muscles
Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.
The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.
Digestive system
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-rotted plant remains, 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.
Circulatory system
The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is 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. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.
Breath
Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore the worms are very sensitive to moist soil and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when in the soil a lot of water, crawling to the surface.
Nervous system
In the anterior segment of the worm there is a peripharyngeal ring - the largest accumulation of nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.
This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.
Excretory organs
The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia - remove harmful substances into the external environment as they accumulate.
Reproduction and development
Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. During mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.
During copulation (mating) and oviposition, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous coupling (1).
The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).
Sense organs
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. Earthworms are capable of regeneration (easily restore the back part).
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.
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.
Type Annelids. Class Oligochaete worms - (answers)
Task 1. Do laboratory work.
Subject: “The external structure of an earthworm, movement, irritability.”
Goal of the work: study the external structure of an earthworm, observing its movement and reaction to irritations.
3. Draw an earthworm. Label the body parts.
4. Write down the results of your observations and draw a conclusion.
Annelids are descended from primitive (lower) worms with undifferentiated bodies, similar to flat ciliated worms. In the process of evolution, they developed a secondary body cavity, a circulatory system, and the body was divided into rings (segments).
Task 2. Fill out the diagram.
Task 3. Do laboratory work.
Subject: “The internal structure of an earthworm.”
Goal of the work: study the internal structure of an earthworm on a wet preparation.
1. Make sure that the workplace has everything necessary to perform laboratory work.
2. Using the instructions given in paragraph 19 of the textbook, complete the laboratory work.
3. Draw the internal structure of an earthworm and label its organs.
Record your observations and draw a conclusion.
Annelids are descended from lower worms with undivided bodies, similar to flat ciliated worms. In the process of evolution, they developed a secondary body cavity (coelom), a circulatory system.
Task 4. Fill out the table by entering the required numbers from the information provided in the columns.
Task 5. In the most favorable conditions (most often these are deciduous forests), the number of earthworms reaches 500-800 per 1 sq.m. Calculate and record how much soil earthworms process per day on an area of 20 hectares of land, if one earthworm can process about 0.5 g of soil during this time. Based on the data obtained, draw a conclusion about the role of earthworms in soil formation.
On one square meter on average (800+500)/2 = 650 worms1 Ha = 10000 m2 in 1 Ha - 650 * 10000 = 65 * 10^5 worms65 * 10^5 * 0.5 = 32.5 * 100 kg = 3250 kg of soil
Conclusion - worms process a huge amount of soil per day; without their participation, the soil would be less fertile.
General characteristics. Earthworms and many other soil and aquatic forms belong to this class. They are characterized by the absence of parapodia and a small number of setae, which usually sit in tufts on the sides of the segments (except for the anterior and posterior). The head section of the body is not separate. Most forms do not have tentacles. Hermaphrodites. Development occurs without metamorphosis.
About 3 thousand species of this class are known. They live mainly in the soil and at the bottom of fresh water bodies.
Soil earthworms play an important role in soil formation (Fig. 109).
Structure and vital functions. The body of oligochaete worms is highly elongated, cylindrical and consists of a different number of externally similar segments. The mouth is located at the anterior end of the body, and the anus is located at the posterior end.
Body sizes range from a few millimeters to 3 m (in some tropical forms). The integument contains a large number of skin glands that secrete mucus. Each segment is equipped with bristles collected in tufts.
In soil forms they play an important role in the movement of worms.
The body cavity is secondary, well developed, filled with coelomic fluid with cellular elements (lymphocytes, etc.). In most species it is divided segment by segment into separate chambers (Fig. PO, 111).
The nervous system is represented by the suprapharyngeal ganglion, the peripharyngeal ring, the subpharyngeal ganglion and the abdominal nerve cord.
Sense organs are poorly developed. Most forms lack eyes and tentacles. There are sensory setae, olfactory pits and statocytes.
Rice. 109. Earthworm:
/ - female genital opening;
2 -
male genital opening;
3
-belt
Rice. 110. Anatomy of a rainworm:
/ rotoinya deprive; 2
suprapharyngeal ganglion; AND- pharynx; / - esophagus; 5. 13 —
ring vessels; 6
dorsal vessel; 7 seed bags; N testes; 9 —
seed funnels; 10
vas deferens; // partitions between segments; 12
- metapefridine; 14
~ intestine; 15
- stomach; 16 - :у6\ 17 nycenode; /L’ - egg funnels; til ovary;
20
seminal receptacles. Roman numerals denote body segments
Digestive organs usually large and adapted to the passage of large masses of soil and bottom soil, on which most worms feed. The oral cavity is followed by the pharynx, esophagus, gizzard and intestines.
All these organs lie along the body without forming bends.
Blood system. The main vessels are dorsal and abdominal. The integument contains a dense network of capillaries, from which oxidized blood is collected in the eubpelvic vessel, which lies under the abdominal nerve cord.
Respiratory system, with rare exceptions, none.
The skin of oligochaete worms is penetrated by a network of capillaries that facilitate gas exchange through the integument (Fig. 112).
Sexual topic. All oligochaete worms are hermaphrodites and are characterized by cross-fertilization. The structure of the gonads differs among representatives of different groups.
Of the large number of oligochaete worms, various types of earthworms are of particular interest for agriculture. Their body is slightly flattened and consists of 50-248 externally similar segments. At the border of the anterior and middle thirds of the body, a belt of several thickened segments is clearly visible.
Earthworms prefer to live in moderately humid nights rich in humus. They avoid acidic and saline soils. They usually spend the winter in the ground at a depth of 2-3 m (in temperate zone conditions). They feed on organic matter contained in the soil, as well as dead parts of plants.
Rice. 111, Cross section of the body of an earthworm:
/ - metaephridia; 2
- metanephric funnel; 3
- ganglia of the ventral nerve cord; 4
- skin epithelium; 5
- non-river muscles; 6
- longitudinal muscles; 7
- bristles; 8
- dorsal fold of intestine; 9, 10
- dorsal and abdominal blood vessels; // - subneural vessel
They reproduce by cross-fertilization. The hermaphrodite genital organs of earthworms are distinguished by their structural complexity. One pair of ovaries. Near them, short oviducts begin with a funnel trimmed with cilia, the other end opening outward in the next segment. They are located inside large seminal sacs in which seminal fluid accumulates. Adjacent to the testes are funnels of the vas deferens, which open outward behind the openings of the oviducts. Usually in the 9-10th segments there are two pairs of small sacs - seminal receptacles, which open outwards with short ducts. When mating, two earthworms mate with their ventral sides. At the same time, their heads are turned in opposite directions, and the openings of the sperm ducts of one individual are adjacent to the sperm receptacle of the other. After this, a mutual exchange of seminal fluid occurs. Then the mated worms disperse.
When the eggs mature, a mucous ring is secreted on the worm's girdle. The worm sheds the ring through the anterior part of the body; when it passes by the openings of the oviducts and spermatic receptacles, in
Rice. 112. Cross-section of the wall of the skin-muscular sac of rain black;
/ - skin epithelium; 2 - layer of circular muscles; 3 - layer of longitudinal muscles; 4 - blood vessels and capillaries
113. Soil oligochaete annelids (enchytraeids) at the time of mating
Rice. 114. Freshwater annelid tubifex (Tubifex)
eggs and sperm enter it and fertilization of the eggs occurs. The discarded ring with the eggs enclosed in it hardens, turning into a cocoon. This is where the initial development of young worms occurs, which occurs without metamorphosis.
Earthworms play a huge role in soil-forming processes.
Worm burrows facilitate the penetration of water and air into the soil, thereby achieving uniform soil moisture and ventilation, which is important for successful plant growth. Worms loosen the soil and fertilize it, dragging plant debris into their burrows, which contributes to the formation of humus.
The number of earthworms in the soil is sometimes enormous, reaching 5 million individuals per 1 hectare, which is about 1 thousand kg by weight. If 50-100 earthworms live in the soil on an area of 1 m2, then they throw out onto the surface of 1 hectare from 10 to 30 tons of earth that passed through their intestines per year. Hence, the enormous importance of earthworms for creating soil fertility is obvious.
Small oligochaete worms from the family also take an active part in soil-forming processes. Enchy— treidae(Fig. 113). Their length usually does not exceed 1 cm, and their thickness is 1 mm. Often tens of thousands of different representatives of this family can be found in the soil layer over an area of 1 m2. There are especially many of them near the rotting remains of plants and animals. Many enchytraeids live at the bottom of fresh and brackish water bodies. Their life here passes in silt, in an almost oxygen-free environment. During starvation, they go into suspended animation.
Small oligochaete worms common in lakes and ponds pipe makers(family Tibificidae) (Fig. 114). Worms, having buried their back end in the silt, make oscillatory movements with their bodies. It has been noticed that the less oxygen in the water, the more they stretch their body and more often make oscillatory movements, increasing gas exchange through the integument.
A number of aquatic oligochaete worms exhibit asexual reproduction by budding. Sometimes entire chains of budding worms are formed. Aquatic oligochaete worms serve as important food for fish. They help accelerate the circulation of substances in the soil of reservoirs.
Type annelids (Annelida)
4.8.3. Class Oligochaeta (oligochaetes)
The earthworm (himbricus) is an animal with a long cylindrical body, reaching 12-18 cm. Its anterior end is cone-shaped, and its posterior end is flattened in the dorsoventral direction. And although he lives only on land, he was not able to fully adapt to the terrestrial way of life. The worm spends most of the day underground, burrowing into moist soil, thus saving itself from drying out. It leaves its burrow only at night, going in search of food or a sexual partner. The differences in the body structure of the Nereis and the earthworm can be explained by the adaptation of the latter to a terrestrial lifestyle.
The earthworm's body has a streamlined shape and is devoid of any outgrowths, since outgrowths can interfere with the free movement of the worm in the soil. The prostomium is small and round, there are no sensory appendages. All segments, with the exception of the first and last, bear four pairs of setae: two ventral and two ventrolateral. The setae emerge from setal sacs located in the body wall. Special muscles (retractors) can pull the bristles in or, conversely, push them out (protractors) (Fig. 4.29). Oligochaete setae are involved in locomotor activity. Long setae located on segments 10-15, 26 and 32-37 play an important role in the copulation process. Another structure used in the copulation process is belt, which is located on segments 32-37 (Fig. 4.30). The epithelium of the girdle contains many glandular cells, forming a noticeable thickening on the dorsal and lateral surfaces, reminiscent of a saddle. The girdle is involved in copulation and cocoon formation.
Rice. 4.29. Structure of the body wall of an earthworm (Lumbhcus terrestris). Cross section through the bristle area
Rice. 4.30. The anterior end of the body of an earthworm (Lumbricus terresths). Bottom view
The structure of the body wall of an earthworm (Fig. 4.29) is similar to the structure of the body wall of Nereis. The oral and anal openings are located at different ends of the body. Food is swallowed as a result of contraction of the muscles of the fixed pharynx. The intestine is straight. Typhlosol(a longitudinal fold on the dorsal side of the intestine that projects into the intestinal lumen) increases the surface area involved in food digestion and absorption. The worm feeds on detritus; it swallows the soil and digests the organic residues contained in it. Nutrients are absorbed and enter the capillaries surrounding the intestinal wall. The bulk of the soil passes through the intestines of the worm and is excreted in the form of coprolites.
The thin cuticle of oligochaetes is moistened by the constant release of coelomic fluid and mucus secreted by glandular epithelial cells through the dorsal pores. It is through the cuticle that gas exchange occurs by diffusion, and the branched network of capillaries located in the epithelium ensures this process.
All segments, with the exception of the first three and one last, carry a pair of nephridia that perform the functions of excretion and osmoregulation. Nephridia open on the surface of the body with special openings located somewhat anterior to the ventrolateral setae. Surrounding the intestine chloragogenic cells also participate in excretion processes.
Blood from individual segments is collected in a pulsating dorsal vessel. Five pairs of muscular lateral "hearts", located in segments 7-11, it is pushed into the median abdominal vessel. The “hearts” and spinal vessels have valves that prevent blood from flowing back. Lateral branches extend from the abdominal vessel, supplying blood to all segments of the body. In earthworms there is no noticeable concentration of sensitive structures at the anterior end of the body. They have individual photosensitive cells as well as cells that respond to chemicals and light; all these cells are scattered in the epithelium. The central nervous system of the earthworm is similar to the nervous system of the Nereis. In the abdominal nerve chain they have giant nerve fibers, which, in response to any strong irritation, cause a contraction of the entire musculature of the worm, although the general organization of the nervous system can ensure the coordinated work of the muscular layers associated with the digging or locomotor activity of the animal.
The earthworm's reproductive organ system is highly complex, and the behavioral reactions associated with reproduction are also complex. Perhaps this is due to the terrestrial lifestyle and the need to protect gametes and fertilized eggs from drying out. Hermaphrodite earthworms (Fig. 4.31). This can be seen as an adaptation to a relatively sedentary lifestyle. With this way of life, meetings of earthworms with each other occur very rarely, but if this happens, then any representative of the same species can arrange an earthworm as a sexual partner, since both of them are hermaphrodites. During the process of copulation, mutual fertilization occurs, i.e., the exchange of male gametes.
Rice. 4.31. The reproductive organ system of an earthworm. Side view
The genital organs of earthworms are concentrated at the anterior end of the body. The location of the reproductive organ system is shown in Fig. 4.31. A rather complex process of mating and the subsequent laying of fertilized eggs in cocoon happen as follows.
On warm, humid nights in spring and summer, the earthworm emerges from its burrow, rarely leaving it completely, and mates with its neighbor. They touch with their ventral surfaces so that the head of one of them is directed towards the caudal end of the other. In this case, the 9-11th body segments are located opposite the partner’s girdle. In the region of the girdle, as well as on the 10-15th and 26th segments, there are long bristles, which the worms stick into each other in order to ensure closer contact during copulation.
The epithelium of both worms secretes mucous coupling around segments 11-31. These couplings share the partners' sperm during copulation; a special closed groove appears in it for the passage of sperm.
In the area of the girdles around the partners, a common tube is secreted, which also holds the oligochaetes together.
In both partners, sperm from the seminal vesicle is brought out through the vas deferens, which opens outward on the 15th segment, and moves along the ventral spermatic groove towards the posterior end. The movement of sperm along the groove is ensured by contractions of the arcuate muscles located in the 15-32nd segments in the layer of longitudinal muscles. Having reached the 9th and 10th segments of the partner’s body, the sperm enters his seminal receptacles.
After the sperm exchange is completed, the partners separate (the process takes 3-4 hours). And after two days a cocoon begins to form.
Around each worm, a dense chitinous tube is secreted by epithelial glands; it becomes the shell of a cocoon. The cells of the girdle secrete into the cocoon albumen, which the embryos will subsequently feed on. As a result of the expansion of the segments located behind the cocoon, it is pushed forward. At this time, through the opening of the oviduct, located on the 14th segment, 10-12 eggs are laid into the cocoon. As the cocoon moves past the 9th and 10th segments, sperm enters it from the spermatic receptacles and fertilization occurs. Finally, the cocoon slides off the worm. The edges of the cocoon quickly close, which prevents its contents from drying out. The cocoon is initially yellow in color, but darkens as it dries.
Cocoons are formed every 3-4 days until all the sperm is used up. Cocoon formation can continue for a year without additional mating.
Annelids develop directly, i.e. they do not have free-swimming larval stages. Usually only one embryo develops in a cocoon. Young worms hatch 2-12 weeks after laying the cocoon, depending on environmental conditions.
Economic importance of earthworms
The burrowing activity of earthworms improves soil aeration and drainage properties. The dug passages facilitate the growth of roots in the soil. Earthworms bring earth particles containing inorganic components to the surface from deeper layers. Thus, they participate in mixing the soil.
Worms cannot swallow lumps larger than 2 mm in diameter, so the soil they bring to the surface does not contain pebbles and creates good conditions for seed germination.
Thanks to the activity of earthworms, seeds can be placed under a layer of soil, which contributes to their successful ripening.
Earthworms drag leaves into burrows, partially using them for food. The remains of leaves, as well as excrement, secretions and corpses of worms, enrich the soil with organic components.
Earthworm castings have a pH of about 7, so they prevent the soil from becoming alkalized or acidic.
Annelids have the highest organization compared to other types of worms; For the first time, they have a secondary body cavity, a circulatory system, and a more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.
They live in soil, fresh and sea water.
External structure
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 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.
Internal structure
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.
Muscles
Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.
The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.
Digestive system
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-rotted plant remains, 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.
Circulatory system
The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is 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. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.
Breath
Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore the worms are very sensitive to moist soil and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when in the soil a lot of water, crawling to the surface.
Nervous system
In the anterior segment of the worm there is a peripharyngeal ring - the largest accumulation of nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.
This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.
Excretory organs
The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia - remove harmful substances into the external environment as they accumulate.
Reproduction and development
Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. During mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.
During copulation (mating) and oviposition, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and a protein liquid to nourish the developing embryo. The secretions of the girdle form a kind of mucous coupling (1).
The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).
Sense organs
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. Earthworms are capable of regeneration (easily restore the back part).
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.
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.
Earthworm
Annelids have the following aromorphoses: 1. The body was divided into segments (metameres) with repeating sets of internal organs. 2. A secondary cavity has appeared - the coelom, which has its own mesodermal lining. 3. There was a further complication of the nervous system: the concentration of nerve cells on the abdominal side in each segment (the abdominal nerve chain was formed), a significant increase in the cerebral ganglia (nodes) (supraglottic, subpharyngeal nerve ganglia, peripharyngeal ring). 4. A closed circulatory system emerged, which ensured rapid transport of substances throughout the body. 5. Respiratory organs appeared, increasing the respiratory surface and the intensity of gas exchange. 6. The digestive system has become more complex: the midgut has differentiated into sections, which has led to a step-by-step process of digestion. 7. Parapodia were formed - limbs for movement. 8. Further complication of the excretory organs occurred: a metanephridial multicellular excretory system was formed.
Earthworm – Lumbricus terrestris(type Annelids, class Oligochaetes, family Lumbricidae) lives in moist, humus-rich soil. It feeds on organic matter, passing soil and plant debris through its intestines. Even Charles Darwin noted the beneficial effect of earthworms on soil fertility. By dragging the remains of plants into the burrows, they enrich it with humus. By making passages in the soil, they facilitate the penetration of air and water to the roots of plants.
Earthworms are active in the warm season. In winter they hibernate. Freezing kills the worms instantly, so they must burrow deeper into the ground where low temperatures cannot penetrate. In the spring, when the temperature reaches a suitable level and the ground is saturated with rainwater, their mating season begins. They reproduce very quickly, producing about a hundred young worms per year. In summer, worms are not as active. There is very little food - dying plant debris - at this time, and the soil is deprived of moisture, which can cause the death of worms. The autumn period is again characterized by worm activity. At this time, reproduction of offspring begins again, which lasts until the onset of winter.
Earthworms live relatively long. Some manage to live for about ten years if they do not become victims of birds and moles. Another threat to their life is the pesticides that are so widely used in gardening today.
So, the earthworm has an elongated, cylindrical body from 10 to 30 cm long. Dorsal side more rounded, it is darker, the dorsal blood vessel is visible through its skin. Abdominal side somewhat flattened and lighter colored. The anterior end of the body is thicker and darker in color. The body consists of rings - segments. In an adult worm, their number reaches 200. In the area of 32-37 body segments there is belt, rich in mucous glands. External segmentation corresponds to the division of the body cavity by partitions into separate chambers and the segmental (i.e. in each segment) arrangement of a number of internal organs. On each segment 8 bristles(they are easy to detect if you run your finger along the body of the worm in the direction from the rear end of the body to the front). The setae are arranged in four pairs on the lateral sides of the segments. Clinging to uneven soil, the worm moves forward with the help of the muscles of the skin-muscular sac.
Veils. The earthworm's body is covered skin-muscle bag. He is educated cuticle, single layer epithelium and two layers of muscles - external circular and internal longitudinal. The skin epithelium of the worm is rich mucous glands, which produce slime, covering the entire body of the worm and protecting it from drying out. Mucus also makes crawling in burrows easier by reducing friction with the soil.
Movement of an earthworm. When a worm crawls, waves of muscle contractions run through its body, and both the length and thickness of individual parts of its body are constantly changing. The movements produced by each part of the body consist in the fact that its constituent segments either stretch and become thinner, or contract and become thicker. As a result of such alternating stretching and contraction, the worm gradually moves forward: first, its head end is pulled forward, and then the posterior segments of the body are gradually pulled towards it; after this, the rear end of the body remains in place, and the head end is pushed even further forward, and thus the further advancement of the worm continues (it is convenient to observe it by letting the worm crawl along paper spread on the table).
Body cavity. Inside the skin-muscle sac of annelids there is secondary cavity body, or in general. This body cavity is not limited by muscles, like in roundworms, but has its own epithelial(coelomic) lining, i.e. the inner side of the longitudinal muscles is lined with epithelium of mesodermal origin, and there is also an epithelial lining on the side of the intestine lying in the body cavity. Due to the coelomic epithelium, internal two-layer transverse partitions are formed between the segments - dissepiments. The secondary cavity is divided into chambers, each segment containing a pair of coelomic sacs. The coelomic fluid is under pressure and plays a role hydroskeleton, so the worm feels elastic to the touch.
Digestive system comprises front, average And rear guts. Mouth located on the second segment on the ventral side of the body. Anal hole
type Annelids Earthworm
At the posterior end of the body, it looks like a small slit. Due to feeding on rotting plant remains and humus, the digestive system has a number of features. Its anterior section is differentiated into muscular throat, esophagus, goiter and muscular stomach. To increase the absorption surface, a fold has formed on the upper part of the intestine typhlosol(typhlozolis). Please note: differentiated sections of the foregut - pharynx, esophagus, crop, stomach - were absent in previous types of worms.
Breath. An earthworm breathes over the entire surface of its body due to the presence of a dense subcutaneous network of capillary blood vessels. Therefore, it is important that the worm’s body covers do not dry out, but excessive moisture (for example, very wet soil after rain) is just as destructive for them.
Circulatory system closed, that is, blood moves through the vessels without spilling into the body cavity. The movement of blood is determined by the pulsation of large vessels, mainly surrounding the esophagus. These are kind of hearts. Blood supplies all organs and tissues with nutrients, transporting them from the intestines, and oxygen entering the skin capillaries from the external environment. By spinal vessel blood moves from the back end of the body to the front, and along abdominal vessel- in the opposite direction. An earthworm's blood is red. An iron-containing protein, similar to vertebrate hemoglobin and transporting oxygen, is found in a dissolved state in the blood plasma, and red blood cells are absent.
Nervous system more complex than that of flatworms and roundworms. It consists of peripharyngeal nerve ring with ganglia and abdominal nervous chains. This is the so-called nervous system ladder type. Suprapharyngeal doubles ganglion performs the functions of the brain and is more developed than subpharyngeal. The nerve chain originates from the subpharyngeal node and consists of segmentally located pairs of ganglia, connected to each other by transverse and longitudinal commissures. Nerves extend from the ganglia to various organs. The earthworm's sense organs are poorly developed: there are no eyes or tentacles, but their skin contains numerous sensory cells and nerve endings.
Excretory organs presented segment by segment (i.e. in each segment) arranged in pairs metanephridia. They look like convoluted tubes and begin in the body cavity as a funnel with cilia. A channel departs from the funnel, which penetrates the transverse septum and passes into the cavity of the next segment. The terminal section of the metanephridium has an extension - uric bubble, which opens outward on the side of the worm’s body (i.e., in each segment there is a pair of very small excretory holes). In addition to metanephridia, the secretion involves chloragogenous cells, covering the surface of the intestine with a thin brown-yellow coating. Chlorogenic cells accumulate excretion products. Filled with metabolic products, these cells die, and their contents enter the body cavity, from where they are removed by metanephridia.
Reproduction. Earthworms hermaphrodites. The reproductive organs and girdle can be examined only during the breeding season - in the spring. To male
type Annelids Earthworm
the reproductive system includes two pairs of testes, located in segments 10 and 11, four vas deferens, which merge in pairs and open outwards doubles male sexual hole, located in the 15th segment. The female reproductive system includes pair ovaries located in segment 13, oviducts, which open outwards in the 14th segment a couple women's sexual holes. There are two pairs in segments 9 and 10 spermatheca, each of which opens outward with an independent hole.
Earthworms reproduce sexually. Cross fertilization, in a cocoon. Two worms meet, tightly wrap their bodies around each other, attach their ventral sides to each other and exchange sperm, which ends up in the spermatic receptacles. After this, the worms disperse. Next, the belt forms a mucous muff, in which eggs are laid. As the coupling moves through the segments containing the spermatheca, the eggs are fertilized by sperm belonging to another individual. The muff is shed through the anterior end of the body, becomes compacted and turns into an egg cocoon, where young worms develop.
Regeneration. Earthworms are characterized by a high ability to regenerate, i.e. From each piece of the torn body of an earthworm, a whole worm is restored.
Questions for self-control
Name the aromorphoses of the annelid type.
Name the classification of the type Annelids.
What is the systematic position of the earthworm?
Where do earthworms live?
What body shape do earthworms have?
What is the body of an earthworm covered with?
What body cavity is characteristic of an earthworm?
What is the structure of the worm's digestive system?
What is the structure of the worm's circulatory system?
How does an earthworm breathe?
What is the structure of the worm's excretory system?
What is the structure of the worm's nervous system?
What structure does the reproductive system of an earthworm have?
How does an earthworm reproduce?
What is the significance of an earthworm?
type Annelids Earthworm
Rice. Earthworm, its passages in the ground and movement.
Rice. Internal structure of an earthworm.
1, 16 - intestine; 2 - partitions; 3 - epithelial lining of the secondary body cavity; 4 - dorsal (back) blood vessel; 5 - ring blood vessel; 6 - skin-muscle bag; 7 - cuticle; 8 - skin epithelium; 9 - whole; 10 - metanephridium; 11 - eggs; 12 - ring muscles; 13 - longitudinal muscles; 14 - ventral (abdominal) blood vessel; 15 - abdominal nerve cord.
type Annelids Earthworm
Rice. The structure of the anterior end of the earthworm's body.
The prostomium is the protrusion of the upper part of the first segment, covering the mouth. Peristomium is the name of the first segment of the body.
type Annelids Earthworm
Rice. The structure of an earthworm.
A - head end; B - internal structure; B - nervous system.
1 - mouth opening; 2 - male genital opening; 3 - female genital opening; 4 - belt; 5 - pharynx; 6 - esophagus; 7 - goiter; 8 - stomach; 9 - intestines; 10 - dorsal blood vessel; 11 - ring blood vessels; 12 - abdominal blood vessel; 13 - metanephridia; 14 - ovaries; 15 - testes; 16 - seed sacs; 17 - seminal receptacles; 18 - peripharyngeal nerve node; 19 - peripharyngeal nerve ring; 20 - abdominal nerve chain; 21 - nerves.
type Annelids Earthworm
Rice. Longitudinal section of the body of an earthworm.
1 - mouth; 2 - pharynx; 3 - esophagus; 4 - goiter; 5 - stomach; 6 - intestine; 7 - peripharyngeal ring; 8 - abdominal nerve chain; 9 - “hearts”; 10 - dorsal blood vessel; 11 - abdominal blood vessel.
Rice. Reproduction of the Earthworm.
1 - mucous coupling; 2 - cocoon; 3 - emergence of young worms from the cocoon.
type annelids
Rice. The structure of the Nereid polychaete worm.
type annelids
Rice. Appearance of a Medical Leech.
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