Earthworm in brief. Earthworm color, body shape and size
Earthworms, they are earthworms, this is far from being one species, but an entire suborder of the class Oligochaete worms, which belongs to the phylum Annelids. The earthworm is characterized by most of the structural features of its type and class.
Earthworms are ubiquitous. Our area is home to more than a dozen species similar to each other (European earthworms), whose body length is 10-20 cm, the number of segments is 100-180. At the same time, the Australian earthworm can reach a length of 3 meters.
During the day, earthworms crawl in the soil. At night and after rain they can come to the surface. With the onset of cold weather, they go underground, to a depth of 2 m. The back of the body is slightly flattened. When crawling out of the soil, the worm holds the edge of the hole with its rear end.
The body of an earthworm, as a representative of annelids, is divided into segments by ring constrictions. As in all oligochaetes, the parapodia are reduced; only tufts of setae are preserved from them, which allow the worm to cling, rest against the ground and facilitate pushing the body forward. In other words, the bristles provide adhesion to the substrate.
The surface of the body is moist and covered with mucus, which facilitates movement in the soil and also facilitates the penetration of oxygen into the body.
The epithelium secretes a layer of transparent cuticle and also contains many mucous cells. Under the epithelium there are circular and longitudinal muscles. The body of an earthworm can contract and elongate. Circular muscles make the body of the worm thin and long, longitudinal muscles shorten and thicken. The longitudinal layer of muscles is more powerful. Alternate contraction of these muscles ensures movement. Each segment can change its shape separately.
The coelomic sacs of adjacent segments communicate with each other, thus the liquid in them mixes.
An earthworm often swallows soil, eating its way through. Nutrient particles are absorbed from the soil in the intestines. If the soil is soft, then it drills it with the front end. First, the front end is stretched and thinned, inserted between lumps of soil. Then the front end thickens, causing the soil to move apart. Next, the worm pulls up the back of the body.
They feed on rotting plant debris. In addition, they can drag fallen leaves from the surface. By dragging plant debris into the soil, worms contribute to their decomposition and the formation of fertile soil.
The digestive system consists of the mouth, pharynx, esophagus, crop, gizzard, midgut, hindgut, and anus. Swallowing food is done through the muscular pharynx. The stomach grinds food; in addition to the muscles of the walls, swallowed grains of sand are involved in this. On the dorsal side, the wall of the midgut forms an invagination, increasing the absorption surface. The midgut is lined with ciliated epithelium, in which there are many unicellular glands. Complex organic substances are broken down in it, and simpler substances are absorbed into the blood. In the walls of the earthworm's midgut there is a dense network of blood vessels. The hindgut is small and ends at the anus.
A special feature of earthworms are calcareous glands, whose ducts flow into the esophagus. The substances they release neutralize the acids contained in the soil.
Breathing occurs over the entire surface of the skin. In the superficial layers of the body wall there is a dense network of blood vessels. When it rains, earthworms crawl to the surface due to lack of air in the soil.
The circulatory, nervous and excretory systems are similar to polychaetes. However, in the circulatory system there are so-called “hearts” - annular vessels capable of muscular contraction. Located in segments 7-13. In a number of species, ring vessels are present only in the anterior part of the body.
There are no metanephridia (excretory organs of annelids) in the anterior three segments.
Sense organs are poorly developed. The skin contains sensitive cells - organs of touch. There are also cells in the skin that perceive the degree of illumination.
Earthworms are hermaphrodites. The reproductive system is located in several segments of the anterior part of the body. The testes are located in front of the ovaries.
Mutual cross fertilization. Each of the mating worms transfers sperm to the partner's seminal receptacle.
In the first third of the body of earthworms there is a special belt; its glandular cells secrete mucus, which, when dried, forms a muff. Unfertilized eggs are laid in it. After mating, spermatozoa enter from the seminal receptacles. Fertilization occurs. After this, the sleeve slides off the worm's body and turns into a cocoon. Small worms develop from the eggs.
Capable of regeneration. If a predator tears off part of the worm's body, the other half completes the missing part. If the worm is divided into two parts, it will produce two individuals, which can be considered asexual reproduction. However, the earthworm itself does not reproduce this way.
Behind the mouth opening there is a strong muscular pharynx, which passes into a thin esophagus, and then into an extensive goiter. In the crop, food accumulates and is moistened. After this, it enters the muscular chewing stomach, which looks like a bag with thick hard walls. Here the food is ground, after which, by contraction of the muscular walls of the stomach, it moves into a thin tube - the intestine. Here, under the influence of digestive juices, food is digested, nutrients are absorbed through the intestinal wall into the body cavity and enter the blood. With the blood, nutrients are carried throughout the worm's body. Undigested food remains are thrown out through the anus.
Excretory organs
The excretory organs of the worm consist of the finest whitish convoluted tubes. They lie in pairs in almost every segment of the worm's body. Each tube opens at one end with a funnel-shaped expansion into the body cavity. The other end opens outward on the animal's ventral side with a very small opening. Through these tubes, unnecessary substances that accumulate there are released from the body cavity.
Nervous system
The nervous system of an earthworm is more complex than that of a hydra. It is located on the ventral side of the body and looks like a long chain - this is the so-called ventral nerve cord. Each segment of the body has one double nerve ganglion. All nodes are connected to each other by jumpers. At the anterior end of the body in the pharynx area, two jumpers extend from the nerve chain. They cover the pharynx on the right and left, forming a peripharyngeal nerve ring. There is a thickening in the peripharyngeal ring above. This is the suprapharyngeal ganglion. Many thinnest nerves extend from it to the front part of the worm’s body. This explains the great sensitivity of this part of the body. This structural feature of the earthworm has a protective value. Branching throughout the tissues and organs of the body, the nervous system of the earthworm and other animals regulates and unites the activities of all organs, connecting them into one whole - the animal’s body.
Body symmetry
Unlike Hydra and many other coelenterates, the body of the earthworm exhibits clearly defined bilateral symmetry of the body. In animals with this structure, the body is divided into two identical halves, right and left - the only plane of symmetry that can be drawn along the main axis of the body from the mouth to the anus. Bilateral symmetry is characteristic of worms and many other animals.
The transition of worms from radial symmetry of the body, characteristic of their ancestors - coelenterates, to bilateral symmetry is explained by their transition from a swimming or sessile lifestyle to crawling, to a terrestrial lifestyle. Consequently, the development of different forms of symmetry in multicellular animals is associated with changes in the conditions of their existence.
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.
1. GENERAL REMARKS. EXTERNAL SIGNS
Let's start by getting acquainted with the body structure of earthworms. The structure of the body is the basis of knowledge about animals. Do we want to understand the variety of forms of a group of animals that interests us for some reason, or get acquainted with their way of life, their connection with their environment, or approach the solution of certain practical issues related to these animals, etc. - the question of structure body is the basic prerequisite for solving any others. In particular, with regard to earthworms, in order to determine the genus and species of any of their representatives (and, as we will see later, there are a considerable number of them), it is not enough to know its external signs, but it is necessary to establish a number of structural features by dissection internal organs.
At the same time, we will become familiar with the work of the described organs and their significance in the life of worms.
In the body of an earthworm (Fig. 1), one can distinguish the anterior (or head) end of the body, which is thicker, with stronger muscles and usually darker in color, and the posterior (or tail), thinner and paler. The back end of the worm is often flat. The mouth is located at the head end of the body, and the anus is located at the tail end. The dorsal side is also well distinguished from each other, more convex and usually darker, and the ventral side is lighter and flatter; in worms preserved in alcohol or formaldehyde, the ventral side may be concave in places or along the entire length.
The entire body of an earthworm is divided by transverse constrictions into separate sections called segments or segments. This ringing, or segmentation, is the leading feature of their organization: each of the segments, in principle, has the same structure and contains basically the entire complex of organs characteristic of these animals. In the anterior part of the body the segments are larger; towards the rear their size gradually decreases. The number of segments in common species varies from 90 to 300; it is subject to significant fluctuations in different specimens of the same species, but, unlike many of their aquatic relatives, it does not change with age. Only in some tropical species the number of segments reaches 600. Looking closely at the surface of the body, you can see that each segment is divided into three parts by two shallow grooves. This is the so-called secondary ringing, which also reflects some features of the internal organization of each segment. The body segments are numbered, with the head segment being considered the first.
The head segment, in addition to the mouth opening, has one more characteristic feature: on its front part there is a head lobe - a movable, shape-changing appendage hanging over the mouth. In earthworms, the head segment can be of two types: either the head lobe, protruding on the dorsal side into the area of the first segment, is separated from it by a transverse groove, or it reaches the groove between the 1st and 2nd segments. In the first case, the head segment is called epilobic, in the second - tanilobic. These differences in the shape of the head lobe are important in identifying worm species (Fig. 2).
The head lobe is an organ of touch and smell; With it, the worm examines objects encountered on its way.
In the anterior part of the body in adult individuals there is a so-called girdle, i.e. a thickening covering from 5 to 12 segments, usually differently colored compared to the rest of the body (Fig. 3). The skin in the girdle area contains a large number of glands that secrete nutrients for eggs when egg cocoons are laid. Therefore, during the breeding season, the girdle looks very swollen, and when there are no cocoons being laid, the region of the girdle differs from neighboring areas only in color and a different character of the body surface. The shape of the girdle can be ring-shaped, if it is developed equally strongly on all sides, or saddle-shaped, if it is poorly developed on the ventral side. On the sides of the ventral side of the girdle there are elongated thickenings, which we will call maturity ridges (Fig. 35). In some species these ridges are replaced by several pairs of mature tubercles. The shape, length, color and location of the girdle, ridges and tubercles serve as significant species characteristics of earthworms.
Along the entire length of the worm's body you can see small bristles, which are clearly visible through a magnifying glass. They are found on all body segments except the 1st. In earthworms of the fauna of the USSR, the setae are located 8 on each segment, in pairs or singly. The bristles form 4 longitudinal rows on each side of the worm’s body, which are usually designated by the letters of the Latin alphabet - a, b, c, d (Fig. 4). Their location is of great importance in identifying worms. The rows of setae a and b, c and d are usually close together in pairs. The degree of their convergence varies among different species. When identifying worms, the ratio of the distances between the rows of bristles must also be taken into account. These distances are denoted by the letters aa, ab, bc, cd and dd (as is customary to denote line segments in geometry). The ratio of the distances between the bristles to the size of the outer contour of the cross section through the worm is also important.
The bristles are important organs of movement: the worm can cling to soil particles with them or be repelled by them when moving in soil burrows and on the surface of the earth. You can also verify their presence by running your finger along the ventral side of the body from the tail end to the head. If a live worm is placed on a sheet of paper, a characteristic rustling sound will be clearly audible as it moves, caused by the friction of the hard bristles. On some segments, the setae are modified into special sex setae, which are important during the mating of worms.
The genital openings are located on the ventral side of the body, in front of the girdle. This includes a pair of male genital pores, usually located on elevations - the so-called glandular cushions (Fig. 34) and a pair of female genital pores, poorly distinguishable from the outside.
In addition, most species have 2-3 pairs of seminal receptacle pores. The meaning of all these holes will be discussed below.
On the dorsal side of preserved worms, dorsal pores are clearly visible in the intersegmental grooves, the anterior border of which is important in determining the types of worms.
The body color of earthworms depends, on the one hand, on the color of their blood, on the other, on skin pigments. It is necessary to strictly distinguish between the body color of worms, which can only be discussed in relation to living individuals and which depends on the combination of skin pigment and blood color, from skin pigmentation, which is determined only by the presence of pigments. Worms lacking pigment have a pink or red body color during life, and when preserved they become white or greyish; pigmented species can be red, brown, brown, yellow and blue.
The body length of USSR earthworms ranges from 2 to 30 cm with a thickness from 2 to 12 mm. In tropical countries there are species that reach a length of 3 m. The bulk of worms inhabiting soils around the world are represented by species that are 5-20 cm in length.
All further presentation refers to earthworms of the Lumbricidae family. Worms of other families (except for botanical gardens, where worms are sometimes brought along with tropical plants) can only be found in the Ussuri region, Central Asia and the southern part of the Black Sea coast of the Caucasus.
2. COVERS OF THE BODY
The body of earthworms is covered with single-layer epithelium. It contains supporting, glandular and cambial cells (Fig. 5).
Supporting cells perform a protective function. The outer part of these cells secretes the substance of the cuticle - a thin transparent film covering the epithelium. The cuticle consists of two systems of parallel fibers that intersect each other at right angles. There may be holes in the cuticle at the intersections. The direction of the fibers is diagonal with respect to the longitudinal axis of the body (Fig. 6), which best ensures the strength of the cuticle when stretched from the inside (it is curious that the connective tissue fibers in the skin of mammals also have a diagonal arrangement with respect to the longitudinal axis of the body). Throughout life, the cuticle wears out all the time and is renewed by the activity of the epithelium. In canned specimens, the cuticle may lag behind, and sometimes it can be removed entirely, like a stocking.
The cuticle is responsible for the smoothness of the skin's surface, which makes it easier for the body to glide when moving on hard surfaces. It also determines the characteristic glossiness of the surface of the body.
The activity of glandular cells is of great importance in the life of worms. Most of them secrete a mucous substance, which always lubricates the surface of the cuticle; it comes out to the surface of the body through holes in it (Fig. 5 and 6). This increases the ease of sliding on the substrate and protects the body from drying out. With any strong irritation, mucous secretions appear on the surface of the body in huge quantities: the worm is instantly enveloped in a thick layer of thick sticky mucus. The formation of a mucous sheath on the body plays an important role during mating and the formation of egg cocoons. In addition, mucous secretions cover the walls of worm tunnels inside the soil, which gives them considerable strength.
In addition to ordinary mucous cells, the skin epithelium of earthworms contains so-called protein glandular cells throughout the entire surface of the body (Fig. 5). In the area of the girdle (Fig. 25), near the bristles of the genital openings and in other places of the body there are skin glands, the significance of which will be discussed below.
An important component of the skin epithelium are small cells located in its deep part, at the border with the underlying muscles, and not in contact with the outer parts of the supporting and glandular cells (Fig. 5). These are cambial cells, which are a reserve; due to them, worn-out functioning cells are renewed and tissue growth occurs in young animals. These cells are also mobilized during wound healing after injuries and other injuries.
Bristles are also formed from special cells of the skin epithelium. Only the outer part of the bristles protrudes on the surface of the body. With its inner end it is deeply immersed in the body wall and can pierce it through, almost reaching the body cavity. The bristles are placed in bristle sacs, which are ingrowths into the body of the skin epithelium (Fig. 7). They consist of a substance similar to the substance of the cuticle, are fragile and wear out quickly. Therefore, throughout life, new bristles are formed in the depths of the bristle sacs. Each bristle is formed from one cell that is part of the bottom of the bristle sac.
The bristles of earthworms are not the same in shape: they are sticks, sometimes almost completely straight, sometimes with clearly curved ends. At some distance from the outer end of the bristles there is a small thickening - a nodule, i.e., a place to which muscles are attached that retract the bristles deep into the body (retractor muscles; Fig. 7). In addition to them, the setal sacs contain protractor muscles, which are attached at one end to the end of the seta and at the other to the body wall; by their contraction, the bristles are pushed outward, and also (with their non-simultaneous contraction) can make quite a variety of movements.
For genital bristles, see below (page 54).
Speaking about the integument of the body, let us mention the interesting phenomenon of the glow of earthworms, which has long attracted the attention of many prominent naturalists. In particular, the famous researcher of insect life, Fabre, wrote about luminous earthworms. In different countries, special types of “phosphorus” worms have been described. It turned out, however, that glow in the dark can be observed in the most common species. The famous Czech explorer Veidovsky reported that while digging through a dung heap one night in search of earthworms, he saw spots of flickering bluish-white light that appeared and disappeared at different points. It turned out that the light came from ordinary dung striped worms, which he collected in large quantities. He noticed that his fingers began to glow in the dark after he picked up the worms. Thus, the mucous secretions of the worms glow, and only under special conditions, since the glow is not always observed. There are indications of glowing fluid protruding from the oral and anal openings.
There can be no doubt that in all these cases the glow is caused by bacteria contained in the secretions of the worms. During the life of many bacteria, light energy is released, which is released during chemical reactions. It must be said that almost always the glow of animals owes its origin to bacteria, one way or another associated with it.
Some researchers believe that glow is beneficial for worms: some think that flashes of light help individuals find each other on the surface of the earth when mating (although worms do not have eyes, they are still able to perceive light on the surface of the front part of the body); others attribute the glow to the role of a factor that scares away enemies; still others think that the glowing mucus left by the worms along their path attracts the attention of enemies and makes them less noticeable. However, all this is nothing more than speculation, not supported by accurate observations.
3. MUSCULARITY AND MOVEMENT. BODY CAVITY
The main part of the locomotor system of earthworms is the powerfully developed muscles of their body wall (Fig. 8). It is constructed like a so-called skin-muscle bag. Under the skin epithelium there is a layer of circular muscles, the contraction of which reduces the diameter of the worm. The layer of circular muscles is underlain by a layer of longitudinal muscles (Fig. 18), the contraction of which reduces the length of the worm. At the border between these two layers there is a very thin layer of diagonal muscle fibers.
Over most of the body, the longitudinal muscle layer has significantly greater thickness than the annular layer, but in the anterior 8-12 segments of the body, the annular layer can reach the thickness of the longitudinal layer. These segments play a particularly important role when the worm drills into the ground (Fig. 9).
Previously, it was thought that the passages of worms in the ground are formed by their absorption of the earth, that is, that the worm, as it were, eats into the ground. However, as Darwin already showed, these moves are mainly the result of active muscular work, thanks to which particles of even very hard soils can be moved apart. Ingestion of soil during digging can certainly occur, but it is of secondary importance. For large species of earthworms, 30-40 minutes is enough to burrow into dense soil for the entire length of the body. This ability to make passages in the soil, allowing earthworms to penetrate deep into the ground, sometimes to a depth of 2 m or more, largely determines the cosmic role of earthworms as soil formers. This requires great muscle power, which they possess. The musculature of the body wall makes up 38-44% of the body volume, and in the strongest species this figure rises to 50%. In this regard, invertebrate worms are second only to leeches, in which the body muscles can account for up to 65% of the body volume.
On the surface of the earth and inside ready-made underground passages, the worm, as well as when burrowing, moves through regularly alternating contractions of the longitudinal and annular muscles, combined with the movement of the bristles (peristaltic movements). In a calm state, worms move rather slowly, but under strong stimulation they can contract very quickly, even making something like jumps, especially when they have to escape persecution. In these movements, the longitudinal muscles play a special role, contributing to the speed of forward movement. Worms can move upward quite quickly in vertical passages they make in the ground. Experiments in glass tubes with species of the genera Lumbricus and Allolobophora showed that the worms rest their dorsal surface on the hard surface of the tube. In addition, the movement of the worm is aided by the mouth, which acts like a suction cup (Japp, 1956).
This justifies not only the incomparably greater thickness of the longitudinal muscles compared to the annular layer, but also the peculiarities of its structure. In many species, a peculiar orderliness in the arrangement of muscle fibers is observed in the longitudinal muscles. The latter are strengthened on strands of connective tissue in parallel rows, so that on a cross section they appear to be arranged in a herringbone pattern. This arrangement of muscle fibers is called pinnate. It is not observed in all species; Many species are characterized by the usual fascicular arrangement of longitudinal muscle fibers.
For the efficiency of muscle work, the fact that under the wall of the body there is a cavity filled with fluid is important. This cavity is similar in origin and character to the abdominal cavity of higher vertebrates and humans, i.e., like theirs, it contains the insides and is lined with squamous epithelium, called “peritoneal.” In worms, the body cavity is divided according to the body segments by intersegmental partitions. In addition, the body cavity is divided into right and left sides by the mesentery, which connects the abdominal side of the body with the intestine. In general, the body of the worm is like two tubes nested one inside the other: the wall of the outer tube is the body wall, the inner wall is the intestine. The space between them is occupied by a body cavity filled with fluid. All liquids, as is known, are very elastic and practically incompressible at arbitrarily high pressures. Therefore, the cavity fluid is an antagonist of the action of the muscles and, to a certain extent, replaces the worm’s missing skeleton. When the muscles of the body wall contract, the pressure on it from the inside from the cavity fluid (turgor) increases, and, due to its incompressibility, the surface of the worm acquires the properties of an elastic solid. This helps him when moving, and especially when digging underground passages; With the front end of the body, the worm is drilled into the ground like a solid wedge.
Let us mention once again that during the movements of earthworms the combined action of the muscles of the body wall and the bristles is very important. The work of the bristles (except for drilling into the ground) becomes especially important during steep climbing. It is known that many species of worms can climb trees, that they are found in large barrels placed to collect rainwater, or in mature cabbage heads under the outer leaves, or in the middle of the head, etc.
4. INTESTINES AND NUTRITION
The mouth, located at the anterior end of the body, leads into a small oral cavity with folded walls, followed by a muscular pharynx (Fig. 10). Due to the fact that the pharynx is connected by a complex interweaving of muscle fibers to the body wall, it not only makes swallowing movements and compresses ingested substances, but can also turn out through a wide open mouth. These movements allow the grasping of objects such as leaves, pebbles, etc., used for food or for other purposes. In the thickness of the pharyngeal wall and beyond there are numerous pharyngeal glands, the ducts of which open directly into the pharynx or into a special pocket in the dorsal thickened part of its wall. The pharyngeal glands secrete a mucous fluid that envelops ingested food particles. In this respect, their activity is similar to the activity of the salivary glands of other animals. But, in addition, the pharyngeal glands produce a substance that digests proteins; it is active in an alkaline environment and is similar in its action to the enzyme that enters the intestines from the pancreas in vertebrates. Thus, the chemical processing of proteins begins in earthworms already in the oral cavity, which is probably due to the need for the most complete extraction of protein substances from food, which, as a rule, is extremely poor in these substances.
The pharynx passes into the esophagus (Fig. 10). This is a rather narrow cylindrical tube, the walls of which have well-developed muscles. On the sides of the esophagus there are 1-3 pairs of lateral pockets (Fig. 10) - the so-called calcareous glands. In some species they are located deep in the wall of the esophagus and are therefore invisible from the outside. These glands are called calcareous due to the fact that crystals of lime carbonate are found in them under a microscope. That these glands produce lime is proven by the fact that food masses are significantly enriched with it as they pass through the intestines (the amount of lime carbonate in the intestinal contents can increase from 0.8 to 1.3-1.8%). It was assumed that the role of these glands is to neutralize the acids contained in the ingested soil. This assumption is in good agreement with the above-mentioned need for an alkaline environment for the activity of digestive enzymes. However, this hardly exhausts the role of calcareous glands. There are many other assumptions regarding their function, and the most varied ones; This already shows that the function of the calcareous glands must be considered unclear.
Behind the esophagus there is a voluminous expansion of the intestinal tube - the so-called goiter (Fig. 10), occupying 2-3 segments. It accumulates swallowed food, which from there enters in portions into the following sections of the intestine. In the absence of such a device, the body would not have time to cope with the processing of incoming material. The goiter has fairly thin elastic walls, due to which it stretches well.
Directly behind the goiter is another extension of the intestinal tube - the muscular stomach. Inside, it is lined with epithelium with a thick cuticle, and its wall consists of annular and longitudinal layers of muscle, with the inner, annular layer having a “feathery” structure, similar to the longitudinal layer of muscle of the body wall, being especially well developed. The task of the stomach is to grind food; In this process, the main role is played, just as in chickens and other granivorous birds, by the friction of mineral soil particles against each other, between which there are organic food substances. Darwin observed that grains of sand and pieces of brick that passed through the intestines of earthworms took on a rounded shape instead of an angular one. There are new observations and experiments proving the importance of mineral soil particles for grinding food in the intestines of worms; in their absence (for example, if worms are placed in peat), they starve, despite abundant food in the form of leaves (Zrazhevsky, 1953).
The gizzard is followed by the midgut, which extends to the posterior end of the body.
A deep dorsal fold, or typhlozol, stretches along the entire length of the midgut, thanks to which in transverse sections the contour of the intestinal cavity takes on a horseshoe-shaped outline (Fig. 11). The physiological significance of this peculiar feature of the organization of the intestine is clear: in this way, an increase in the absorption surface of the intestine is achieved. The intestinal wall contains a large number of glandular cells that produce mucous secretions and digestive enzymes. Among the latter, as in the pharynx, there are enzymes that digest proteins, and, in addition, enzymes that convert starch into sugars (maltose and glucose); Fats are also converted into a soluble state in the intestines. Thus, in worms, as in vertebrates, nutrients in the form of solutions are absorbed by the intestinal wall. The movement of food is accomplished by the action of the intestinal muscles, which consists of an inner circular and outer longitudinal layer of muscle (note that the arrangement of the layers here is the opposite of that in the body wall). Some species have several layers of muscle in the intestinal wall.
In the last 10-15 segments of the body, the intestine lacks a dorsal fold, and its epithelium acquires cilia. This part is called the hindgut. Absorption no longer occurs in it, but only the process of formation of lumps of feces, i.e., those coprolites that are so important for the soil structure, takes place. On the last segment of the body, the intestine opens outwards with an anal opening, which looks like a vertical slit.
An interesting debate is between two famous naturalists of the last century on the issue of earthworm food - Etienne Claparède (France), an excellent expert on invertebrates (in particular, annelids), and Charles Darwin (England). Claparède found in different parts of the intestines of earthworms the remains of crushed leaves mixed with soil, and on this basis he believed that the worms swallow the soil only for the purpose of making the plant remains they swallowed better grindable. Darwin, without denying that worms feed on fallen leaves and other plant debris, at the same time argued that they also use the ingested soil for nutrition. He observed that places where they could only feed on soil rich in organic matter (for example, neatly swept yards) were also abundantly populated by worms. All further studies confirmed the correctness of Darwin's observations.
We will touch on the question of the ability of worms to choose their food later, when we talk about the functions of their nervous system and sensory organs.
The amount of earth absorbed and processed in the intestines of earthworms is of great importance. It turned out to be huge: by weighing coprolites, it was established that worms inhabiting cultivated soils pass through their intestines in 24 hours an amount of soil equal to their body weight.
In order to complete our consideration of the intestine, let us mention the characteristic tissue that envelops the entire midgut and dorsal blood vessel from the outside and fills the dorsal fold of the intestine. When opening a live or just killed earthworm, the yellow color and loose velvety surface of the midgut, on which red blood vessels stand out in contrast, attract attention. This tissue is called chloragogenous, or yellow. Its connection with the intestine is purely topographical: it is a modified part of the lining of the body cavity (peritoneal epithelium) adjacent to the intestine. Yellow tissue consists of large cells, the plasma of which is filled with droplets of substances that have a yellowish color. The origin and nature of this substance, and at the same time the function of the tissue itself, are not entirely clear. Some researchers consider this tissue to be a place for storing reserve nutritional materials, similar to the adipose tissue of vertebrates. Indeed, inclusions of yellow tissue cells contain fat, protein and a substance similar to glycogen (animal starch). At the same time, it is known that this tissue contains large amounts of uric acid, that foreign substances introduced in the form of solutions into the body cavity (paint) accumulate in the cells of chloragogenous tissue, and that the final nitrogenous metabolic products to be excreted from the body usually have yellow or brown color. All this makes us think about the excretory function of this tissue. It is very likely that, along with the accumulation of reserve nutrients, the cells of the yellow tissue have the ability to extract waste products formed during the metabolic process from the blood circulating in it and the fluid filling the body cavity. Once inside the cells of the yellow tissue, these substances are switched off from the blood flow and become harmless. Gradually accumulating in the cells of this tissue, they can remain there for a long time, but they can also be released from the body, since the cells of the yellow tissue often break off and enter the body cavity, and from there they are brought out along with the splashing of the cavity fluid through the dorsal pores.
5. CIRCULATORY SYSTEM. NUTRIENT AND OXYGEN DISTRIBUTION FUNCTIONS
The distribution of nutrients absorbed by the surface of the intestine is carried out in earthworms using a highly developed circulatory system. The layout of its main vessels is as follows (Fig. 8, 10 and 12). The dorsal (above the intestine) and ventral (beneath the intestine) vessels run along the entire body. The dorsal vessel is equipped with muscles, which, through wave-like contractions, drive blood from the rear end of the body to the front. In several anterior segments (from the 7th to the 11th or, in other species, from the 7th to the 13th), the dorsal vessel communicates with the ventral vessel by 5-7 pairs of transverse vessels. These vessels are equipped with particularly strong muscles and are called hearts. They fully justify this name, since they serve as the main apparatus that ensures blood circulation. Blood flowing from the hearts into the abdominal vessel moves towards the posterior end of the body. Along the way, it enters the vessels that supply the body wall,” as well as the vessels leading to the intestines, to the excretory organs (Fig. 13), and in the corresponding segments to the genitals. In all these parts of the body, the vessels break up into a network of microscopic capillaries. From the capillaries, blood flows into the transverse vessels, which ultimately collect blood from the entire body into the dorsal vessel.
There are other longitudinal and transverse vessels, which can be seen in Fig. 8, 10, 12 and 13; We will not dwell on them. Of particular importance is the dense plexus of small vessels around the intestine (Fig. 13). Nutrients absorbed by the intestines come here, and from here they are distributed throughout the body. Note that almost all vessels have muscles, although not as highly developed as in the spinal vessels and hearts, which prevents the possibility of blood stagnation in the peripheral parts of the circulatory system.
The blood of earthworms, as already noted, is red. This color is due to the presence of a substance very close to hemoglobin in the blood of vertebrates. However, in worms it is not contained in the blood cells, but is dissolved in the liquid part of the blood (blood plasma). Earthworms have in their blood only colorless cells of several types, generally the same as the mud of colorless blood cells in vertebrates.
As is known, hemoglobin in vertebrates ensures the transport of oxygen from the respiratory organs to all living cells of the body. A substance similar to hemoglobin plays the same role in earthworms. However, they do not have special respiratory organs: they breathe over the entire surface of the body. The thin cuticle and tenderness of the skin of earthworms, as well as the rich network of skin blood vessels, provide the ability to absorb oxygen from the environment. But we note that the cuticle of earthworms is well wetted by water and oxygen, apparently, must first dissolve in the water that wets the skin. This entails the need to keep the skin moist. This alone makes it clear how important the humidity conditions of the external environment are for the life of worms. As the skin dries out, breathing becomes impossible for them. However, if there is a lack of moisture in the soil, the worm can fight this for a long time, using the water reserves available inside the body. In these cases, the skin glands come to his aid (see page 15), and in case of an acute lack of moisture, he utilizes the cavity fluid for this purpose, spraying it out of the dorsal pores.
The absorption of oxygen by the surface of the body is facilitated by the presence of a very rich network of blood capillaries, penetrating even into the skin epithelium (Fig. 14). From here, blood through the vessels of the body wall and transverse vessels enters the main trunks of the bloodstream, thereby achieving a supply of oxygen to the entire body. The red top coloration of most species of earthworms (not pigmentation, see page 15) is determined precisely by the presence of a rich network of cutaneous blood vessels.
All this creates the possibility of worms living in conditions of very low oxygen levels. In this respect, they come close to some of their distant freshwater relatives - the tubifex worms (Tubifex tubifex, Limnodrilus hoffmeisteri, etc.), which, living in deep silt, can endure an almost complete absence of oxygen. Regarding earthworms, there are observations that they can live with an oxygen content in the air space surrounding them equal to 2.5% (as is known, the air usually contains 21%). Even with 0.4% oxygen in the air, worms can absorb half the amount of oxygen they need to maintain life, and can survive in these conditions for quite a long time. In addition, once in an oxygen-free environment, worms can switch to a special type of metabolism, in which the source of energy for life processes is not oxidation reactions (which require oxygen), but the breakdown of a starch-like substance - glycogen, which also occurs in an oxygen-free environment. However, the glycogen reserves of worms are not particularly large, and, in addition, with this method of metabolism, acids are released that have a harmful effect on the body of the worms.
While underwater, worms can absorb oxygen just as well as in the air. It is known that they can live in water for months in the presence of the minimum oxygen they need and other conditions necessary for them. This fact is of great importance for understanding many phenomena in the life of earthworms.
6. ORGANS OF EXCRETION. ABSORPTION AND RELEASE OF WATER
The excretory function is performed in earthworms (as in all ringworms) by tubular organs located in pairs in each segment, except the anterior ones. These organs are called nephridia, which means “kidney-like organ” in Greek. Nephridia are located in the body cavity on the sides of the intestine (Fig. 8 and 12). Each of them is a convoluted tube starting inside the body with an opening into the body cavity located on the capitate extension, the cells of which are equipped with cilia. This expansion is called a funnel by analogy with similar formations in more primitive rings (Fig. 15). Almost immediately behind the vortex, the nephridium pierces the intersegmental septum and penetrates into the next segment of the body. There it first forms a highly convoluted thin tube, which passes into a wider middle part of the nephridium, equipped with cilia. Then the nephridium, making several loops, passes into the excretory part, which ends on the ventral side of the body with an external opening (Fig. 15), or nephridial pore. It is very difficult to find it from the outside, since its edges are always tightly closed. Not far from the nephridial pore there is an extension of the nephridial tube, which is something like a bladder. Nephridia are equipped with a very rich network of blood vessels. The blood leaving the nephridium enters the transverse vessel, and from it into the dorsal vessel (Fig. 16).
It should be noted that in one of the earthworms (Allolobophora antipae) the nephridial tubes do not open with pores independent from each other, and their outer parts flow into longitudinal excretory canals, which run from the right and left along the entire body and at its posterior end flow into the intestine not far away from the anus. Thus, here the connection of the excretory apparatus into one anatomical whole is outlined and a connection is established with the hind intestine.
The cells of the thin part of the nephridial tube capture nitrogen metabolism products from the blood circulating outside the network of nephridial capillaries to be excreted from the body. These substances enter the cavity of the nephridial tube and here they mix with the cavity fluid entering through the funnel at the inner end of the nephridium. The cavity fluid also contains excretory products, dead cells, worn bristles, etc., enter there. The fluid of the nephridial tube is driven by the beating of cilia towards the excretory end, from where it is periodically released through the external pore by contraction of the muscles of the body wall (Roots, 1955).
There is evidence that the terminal nephridium vesicle is emptied once every three days. Other observations indicate that a worm weighing 1. -1.8 g secretes 0.82 cm3 of excrement per day. Such quantities must be excreted from the body several times a day. The excreta contains, in general, the same substances as in mammals, namely: urea, ammonia, creatinine, salts, etc., but in much lower concentrations. However, normal excrement of worms contains 0.3% protein, while in higher animals there is no protein in the excretory products.
The cells of the middle part of the nephridial tube have the ability to phagocytose, i.e., to actively ingest water-insoluble substances from the body cavity (dead cells, coagulated protein, bacteria, etc.). These substances accumulate there for an indefinite period of time. Sanitary services of this kind are also performed by other cells within the body: amoeboid blood cells, cells of the body cavity and the aforementioned cells of chloragogenous or yellow tissue (see p. 26). Especially many amoeboid cells, swallowed foreign bodies, are found in the body cavity. They get here by actively crawling out of the vessels, squeezing between the cells of the vascular wall. These cells are removed from the body cavity in different ways. Firstly, they crawl through the intestinal wall and, entering its cavity, are excreted along with feces (this has been observed many times); secondly, as already mentioned, they can exit with the cavity fluid through the nephridia and, thirdly, they can exit along with the cavity fluid sprayed through the dorsal pores. In general, one might think that the cavity fluid is replaced quite quickly. That is why it becomes so important in worms during the process of excretion. Its role in the life of worms will become clearer after becoming familiar with the water regime of their body.
We have already pointed out the importance of water in the body of worms when we talked about the role of the cavity fluid (98.8% of its composition is water) during muscle work and the need to moisturize the skin for breathing (p. 30). Water continuously enters the body of the worms and is released back into the external environment in the ways indicated above. Thus, the body of the worm and, in particular, the body cavity is constantly rinsed with water. Therefore, for the normal performance of these physiological functions, worms need environmental conditions that would ensure that water enters their body in significantly greater quantities than most terrestrial animals.
How does water enter the body of worms?
Let us note first of all that worms never drink. They absorb water over the entire surface of their body; water passes through the integument and muscles, accumulating in the body cavity. In this case, worms can only use water in a liquid state. A worm in an environment containing water vapor may die from drying out if it has no other source of moisture.
Under normal conditions, the body of worms contains about 84% water. Despite such a significant supply of water, it turns out to be far from the limit. If the worm is given the opportunity to further increase the supply of water in its body, it will immediately do so. You can easily verify this if you put earthworms in a hearth. After a few hours, their weight will increase by 10-12% due to water absorbed by the surface of the body. After being removed from the water, the worm returns to its original weight, and this happens in a very short time (1-2 hours). The removal of excess water from the body occurs in a very unique way: it is absorbed by the intestinal cells, from them enters the cavity of the latter and is removed mainly through the anus, and partly through the mouth.
Under normal living conditions in the soil, the function of removing excess water lies with the nephridia. The presence of a constant current of water through the body by absorption by the surface of the body and excretion of it in excess by the kidneys is a phenomenon very common among aquatic animals. It is undoubtedly inherited by earthworms from their aquatic ancestors.
The metabolism of aquatic animals occurs with increased circulation of water through their body; they cannot be threatened by a lack of water, whereas in soil conditions with this type of water exchange, a sufficient amount of moisture becomes the main factor ensuring the possibility of existence. Therefore, soil moisture conditions are of primary importance in the question of colonizing them with certain types of earthworms.
About the ability of earthworms to lose water during periods of drought and wintering, which is associated with their transition to a state of hidden life, see below (p. 105).
7. NERVOUS SYSTEM AND SENSORY ORGANS. REFLEXES
Along the midline of the ventral side of the body in earthworms, under the muscles, there is a nerve trunk called the ventral nerve cord. In each segment of the body there is a nerve node, or ganglion, which is a collection of nerve cells and gives off 3 pairs of nerves. Ganglia are connected to each other by bridges, connectives, which, in addition to nerve fibers, also contain nerve cells. At the anterior end of the body, in the 3rd segment, the abdominal nerve cord is divided into right and left pharyngeal connectives, forming a peripharyngeal nerve ring connecting to the suprapharyngeal or cephalic ganglion (Fig. 17). This ganglion is paired and consists of the right and left halves, tightly connected to each other. But unlike all the other nerve ganglia lying on the ventral side of the body under the intestines, this ganglion is located on the dorsal side of the body and lies above the intestines. This ganglion can be contrasted with all others due to the fact that morphologically it is comparable to the brain of higher forms (arthropods). Numerous nerve trunks extend forward from it, profusely branching and forming dense nerve plexuses in the first three segments. Under the pharynx, at the point where the pharyngeal connectives diverge, lies the subpharyngeal ganglion, which is the result of the fusion of several ganglia of the abdominal nerve chain.
As can be seen in cross sections, nerve cells lie along the periphery of the ganglion, and its middle part is occupied by a plexus of processes of nerve cells (Fig. 18). In the abdominal nerve chain, attention is drawn to three very thick fibers that run the entire length of the worm's body under the connective tissue capsule of the nerve chain on its dorsal side. These are the so-called neurochords, which until recently were mistaken for giant nerve fibers. However, it is now finally clear that they are a kind of supporting formations (Nevmyvaka, 19476). These formations are similar in structure, function, and position between the nervous system and the intestines to the notochord of vertebrates.
The nerves extending from the ganglia of the abdominal chain contain motor fibers that end in the muscles and sensory fibers, through which irritations enter the nervous system from the periphery. The bodies of sensory nerve cells are located in the periphery, including in the outer epithelium (Fig. 18). Nerve cells here stand in a row of epithelial cells. This extremely ancient type of relationship between the elements of the nervous system was preserved in earthworms from their distant ancestors, primitive multicellular animals. It is very interesting that not only the cells of the outer epithelium become sensitive nerve cells here, but, as was recently established, also intestinal cells originating from the inner germ layer (Nevmyvaka, 1947a).
Sensory nerve cells and their endings are also found in other places in the body. They are also richly supplied with nephridia, bristle sacs and other organs. Thus, in earthworms, as in higher animals, the work of internal organs occurs under the control of the regulatory and centralizing role of the nervous system.
Of the reflexes of earthworms, the best known are those observed in the act of crawling. As the worm moves along the entire length of the body, from the anterior end to the posterior, peristaltic waves of combined muscle contractions run through. They follow each other, and each subsequent wave can occur long before the first reaches the rear end of the body. It would seem obvious, by analogy with higher animals, that the cause of these waves of contractions is the sequential transmission of stimuli along the abdominal nerve chain. However, to the surprise of the researchers, it turned out that cutting the abdominal nerve trunk and even cutting out several nerve nodes from it does not stop the running waves of muscle contractions: the contraction wave passes through the site of damage in the same way as it did in a normal worm. With the same result, in addition to breaking the abdominal nerve chain, you can remove the musculature of several segments or damage it with acid.
Analysis of these and similar experiments showed that the forward movement of the worm represents a long chain of reflex acts, in which each segment is a largely autonomous physiological unit. Irritations from the periphery lead to contraction of the muscles of this segment. As a result of this contraction, the peripheral apparatuses in the adjacent segment are irritated, which cause contractions in it, etc. Thus, combined muscle contractions in each segment can represent an independent reflex, starting with the excitation of sensory cells in the periphery and ending with the effect of contraction of the muscles of this segment . This is the most primitive type of reaction to external influences. Some of its complication is the transmission of the resulting irritation along the nerve chain to the adjacent posterior segment of the body, in response to which the muscles of this segment contract. In Fig. Figure 19 shows a diagram of the reflex during arcuate flexion of the worm, when a wave of muscle contractions goes along one side of the body. This reflex is the main one in the forward movement of the worm. This method of transmitting irritations throughout the body, as stated above, indicates weak centralization of its nervous system.
Experiments with the removal of the suprapharyngeal ganglion indicate the same thing. It was noted above that morphologically the suprapharyngeal ganglion can be compared with the brain of higher forms (arthropods). In many ringed fish, the suprapharyngeal ganglion has a rather complex structure. However, in land worms the suprapharyngeal ganglion has undergone simplification and its physiological role is very small. After removal of the suprapharyngeal ganglion, it is possible to note only some general relaxation of the muscles of the anterior part of the body, changes in the perception of light; it may also play a role in reproduction. But it is not possible to notice significant changes in the movements of the worm after the wound heals: the worm also buries itself in the ground, also avoids danger and carries out all those rather complex reflex reactions that we will get acquainted with later. It is especially surprising that the ability to “learn”, i.e., in modern terminology, to conditioned reflexes, does not disappear in worms lacking the suprapharyngeal ganglion.
The subpharyngeal ganglion is of somewhat greater importance, since after its removal the worm is deprived of many of its inherent abilities: its taste abilities are greatly affected (p. 45).
It would be a mistake to think that the weak centralization of the nervous system and the relative autonomy of individual segments, revealed during the forward movement of the worm, mean the absence of reactions of the organism as a whole. We can say in advance that such reactions cannot but exist, and, indeed, they are very easy to detect. With weak irritation at the rear end of the body (with a light touch), the worm crawls forward; with irritation at the front, it quickly contracts and crawls in the other direction; with strong irritation anywhere, the worm begins to strongly contract in an arched manner, in different directions (so-called gymnastic movements); worms exhibit rapid reactions to light, smells, etc. Thus, the above-mentioned imperfections of the nervous system and its weak centralization are revealed only with careful observation and in specially designed experiments.
So, we know that the worm has a fairly rich arsenal of possibilities for carrying out certain reactions to changes occurring in their environment.
Let us now consider how he can recognize these changes. The means for this are the senses.
As already mentioned, the entire surface of the worm’s body is covered with a huge number of sensitive nerve cells. These cells serve as organs of touch, which are very developed in worms. It is known that it is enough, after carefully approaching, to blow weakly on the worm so that it responds with a sharp contraction of the longitudinal muscles; with the help of such a movement he hides in a hole. In addition to the sensory nerve cells, the outer epithelium contains a very large number of free nerve endings between the cells, which most likely also provide the function of touch.
As has been known for more than a hundred years, earthworms, despite the lack of eyes, perceive light well. The perception of light is produced by special photosensitive cells, which for the most part are located singly between the cells of the outer epithelium (Fig. 20). Inside these cells, in addition to the nucleus and a dense network of the finest fibers - neurofibrils, there is a transparent light-refracting body of a bean-shaped or elongated shape; it is called a lens by analogy with the lens of the eye of more highly organized animals. A nerve process extends from the cell body, entering the subcutaneous nerve plexus and connecting it with the central nervous system. Such a cell undoubtedly represents the simplest eye, like an isolated and autonomous cell of the retina of higher animals. Light-sensitive cells are concentrated mainly in the anterior segments of the body; Most of them are in the head lobe, where there can be over 50 of them (Fig. 21). In subsequent segments, their number quickly decreases, they are not found in the middle of the body, and in the last three segments they become more numerous again. In some species of earthworms, in addition to isolated light-sensitive cells in the outer epithelium, there are large groups of light-sensitive cells located under the skin along the nerves, especially in the head lobe (Fig. 22).
Darwin carefully studied the perception of light by earthworms. He found that if he carefully illuminated them with a silent lantern, having only a narrow beam of light, the intensity of which was reduced by red or blue glass (the color of the glass is indifferent), then only very few worms reacted, namely, they went into their holes. Darwin made observations of those species that emerge from their burrows at night in search of food or to mate; these are the large red worm (Lumbricus terrestris), the long worm (Allolobophora longa) and some others. Their rear end usually remains in the burrow. Under stronger illumination (particularly accurate results were obtained by concentrating light rays using a magnifying glass), the worms, quickly contracting their longitudinal muscles, hide in their burrows, “like rabbits,” notes Darwin, citing the expression of one of his friends who observed his experiments. At the same time, Darwin proved that worms react precisely to light, and not to radiant heat emanating from the light source. Experiments with heated pieces of iron approaching worms showed that they are little sensitive to radiant heat. However, when the worms are “busy” with something, that is, when they drag leaves into their holes, eat, etc., they do not notice the light, even when the light was concentrated on them using a large burning glass. They do not react to light during mating either. It was later proven that very weak light can attract worms as they move in the direction of its source.
The ability to sense light plays a very important role in the life of worms, since sunlight has a detrimental effect on them (worms are very sensitive to the ultraviolet part of the solar spectrum). The reaction of going into darkness saves their life (Smith, 1902).
Worms do not have special hearing organs. Worms do not react to very strong sounds transmitted through the air if the solid substrate with which they are in contact does not vibrate. But they perceive the vibration of solid bodies with which they are connected, caused by sounds, very subtly. For example, according to Darwin’s observations, “when a pot containing a pair of worms, which turned out to be completely insensitive to the sounds of the piano, was placed on the instrument itself, then when the note C was played in the bass clef, both instantly hid in holes. After a while they again appeared on its surface, but when the note G was struck in the treble clef, they disappeared again.” These vibrations of the piano lid were apparently perceived by the worms' sense organs.
The method of collecting worms practiced in Florida is based on a highly developed sense of touch: a board or stick is stuck into the ground, abundantly populated with worms, and another stick is moved along its upper edge, like a bow on a violin (this method is called “violin” there). They write that the worms leave their burrows and come to the surface in large numbers.
From time to time, reports have appeared in the scientific literature about the sounds made by earthworms. Indeed, when the body and bristles rub against the ground, during piston movements in wet burrows, when rubbing food in the throat, when dragging leaves and pebbles, etc., sounds may occur. The more worms there are and the larger they are, the more perceptible they are. But it is very doubtful that these sounds have any biological significance.
In addition to sensitive nerve cells, nerve endings and light-sensitive cells, a large number of organs represented by cell complexes are scattered in the outer epithelium. They are sometimes called sensitive kidneys. Several dozen sensitive cells form a cylindrical or ovoid complex (Fig. 23). These are sensory nerve cells and long nerve processes that go to the ventral nerve cord. The surface of the cuticle in the area of the sensitive bud is slightly raised, and each cell is equipped with a sensitive hair. These microscopic organs are distributed in large numbers throughout the body, but they are especially numerous in the 1st segment and in its head lobe, where in large species there are about 1800 of them. Their function has not been precisely established. Some researchers believe that some of them may have a tactile function. But one can hardly doubt that they also carry out the functions of smell and taste. This conclusion is supported by the fact that these organs are present in large numbers in the oral cavity.
The sense of smell, i.e. the ability to recognize various substances in a gaseous state (which is the ability to perceive odors), is relatively weakly developed in worms. In Darwin's experiments, the worms did not react to the smell of tobacco juice, perfume, or acetic acid, but they found pieces of onions (which they love very much) and cabbage leaves by smell. The worms reacted negatively to ether brought close to the front end of the body and immediately moved away from it.
The sense of taste, i.e. the ability to recognize chemical differences in substances upon contact with them, is very finely developed in worms and, along with the sense of touch, serves as their main source of perception of events in the outside world. Darwin's experiments, developed recently by a number of researchers, have proven absolutely indisputably the ability of worms to choose their own food, and the objections expressed by some authors on this matter (for example, Tarnani, 1928) are undoubtedly based on errors.
A very accurate experimental setup for determining the taste abilities of worms, developed by Mangold (1924, 1951), is as follows. Cherry leaves rolled into a tube or a bunch of several pine needles are tied in several places with thread and boiled. All flavoring substances are thus removed from them. Then one half of such a “taste tester” is dipped in pure 20% gelatin, the other half is dipped in the same gelatin to which the test substance was added - crushed leaves of various trees and herbs, acids, quinine, etc. Such taste testers are placed overnight on the surface of the soil of flower pots in which worms are cultivated. In the morning, they count how many testers the worms dragged into the holes and at the same time note which end of the tester the worm grabbed. It must be said that the worms, collecting food that they came across on the surface of the earth, never brought it deep into the burrows, but left it not far from the outer hole or only moved it towards it. Therefore, the above calculation is not difficult to do. If the worm does not distinguish between the ends of the taste tester, then with a sufficiently large number of repetitions of the experiment it should turn out that the worm takes hold of both ends equally often. If he prefers the test substance to pure gelatin, then the end soaked in it should often be in front when dragging. On the contrary, if the substance tastes worse than pure gelatin, then the worm should grab onto it less often. This experiment is modified by introducing taste testers soaked in various substances to the worms, followed by determining the number of those drawn into the burrows. The results were processed statistically. Experiments have shown that worms prefer rotting leaves to those that have just fallen in autumn; They like fresh green leaves even less and, to an even lesser extent, dried green leaves. They are more attracted to pure gelatin than dried leaves. Decaying leaves of different plants can be arranged in this row in order of decreasing susceptibility of worms to them: willow, sweet lupine, walnut, black acacia, poplar, oak, bitter lupine, linden, beech, glues, horse chestnut. Fresh leaves are arranged in a completely different sequential row. The worms refuse gelatin mixed with quinine, and they sense this substance already at a concentration of 0.07%. They refuse mineral acids at any concentration, but they like the addition of 1-2% citric and phosphoric acids to gelatin. They are indifferent to sugars, but they completely refuse very strong solutions of sugar. A negative reaction to saccharin is detected even starting from insignificant concentrations.
The ability to determine the shape of bodies in worms appears to be absent. Their preference for dragging leaves into burrows by the front end and pine needles by the base (a fact established by Darwin) has been confirmed by further research. However, Mangold's experiments established that the worms are guided only by their sense of taste, which allows them to distinguish the tip of the leaf from the petiole.
Speaking about the reflex activity of earthworms, it should be noted that they have long been proven to have the ability to learn and change behavior in connection with previously experienced sensations, i.e. conditioned reflexes. Without going into the details of the rather complex experiments that established this fact, we mention that worms can “remember” a road on which they are not threatened with electric shock, and if the electric shock is accompanied by the touch of sandpaper, then the worms begin to avoid sandpaper without electric shock , although it itself does not cause changes in the direction of movement of the worms. In experiments to determine the taste abilities of worms, it also turned out that the reaction to the proposed substance changes in connection with previous tests. Worms usually at first refuse food that is unfamiliar to them, but then they often get used to it and take it in the presence of other food that is familiar to them.
As already noted (p. 39), the apparatus that ensures the presence of conditioned reflexes can also be localized in parts of the nervous system that do not correspond to the brain of higher-level forms. Determining exactly where this function is localized in earthworms is a matter for future research.
To finish our consideration of reflex reactions in earthworms, let us also touch upon the issue of pain in them.
Can these animals experience pain?
The remarkable Russian zoologist V. Fausek considered pain sensations as useful devices, the role of which is to signal the presence of damage to the body. He tried to trace the emergence of this feature in the evolution of the animal world and cites the earthworm as an example of an animal for which the feeling of pain is not yet available. If we, while pricking an earthworm, notice its rapid whip-like movements, then an analogy with a creature writhing in pain suggests itself. How unreasonable, however, this analogy is shown by the following simple experiment: if a worm calmly crawling forward is cut in half with a razor, then the back half will contract like a whip, imitating the pain sensations of higher animals, and the front will calmly continue to crawl forward, “not noticing” the damage caused . Attributing the feeling of pain to the back half of the worm and denying it to the front is clearly absurd. But this means that we have no right to attribute the feeling of pain to a contracting whole earthworm.
8. INTERNAL SECRETION ORGANS
Let us mention the presence in earthworms of substances that are produced in certain places of the body and serve as chemical causative agents of various manifestations of the body’s vital activity. Such substances are called hormones (a Greek word meaning “stimulating”), and the process of their formation is called internal secretion. In vertebrates, hormone production occurs partly in special endocrine glands (for example, adrenal glands, thyroid gland, pituitary gland), as well as in organs that simultaneously perform another function (for example, gonads, pancreas, brain cells).
Earthworms do not have special endocrine glands, but hormones are produced in different parts of the nervous system. It has long been known that in the ganglia of the abdominal nerve cord of worms there are so-called chromaffin cells that secrete adrenaline, i.e., a substance produced by the central part of the adrenal glands of higher vertebrates. This substance is known to be a specific stimulant of the nervous apparatus that moves the muscles of the walls of blood vessels and serves as an important means for regulating the width of the lumens of the vessels of the circulatory system, and thereby blood pressure. In earthworms, this substance plays the same role.
Recently, it was discovered that a significant part of the nerve cells of the suprapharyngeal ganglion also have an intrasecretory function (Herlant-Meewis, 1956). There are two types of such nerve secretory cells: some of them have homogeneous protoplasm, others have granular protoplasm. The former are believed to serve as regulators of the activity of the gonads, and the substance produced by them apparently inhibits the activity of the gonads: they begin to function in the months when the reproduction of worms ends, and disappear during the reproduction periods. Granular cells are important in the healing of wounds and restoration of lost parts of the body (regeneration): during these processes, secretion in them is especially enhanced.
The activity of the girdle of earthworms, which consists in the production of shells and nutritional contents of egg cocoons, is also undoubtedly regulated by hormones. At one time it was believed that hormones that stimulate the activity of the glandular cells of the girdle are produced by male reproductive cells that mature in the seminal sacs. However, this turned out to be incorrect. But the activity of the girdle is undoubtedly regulated by some kind of hormones: if you transplant a piece of the girdle from a worm with an inactive girdle to one that is in the midst of sexual activity, then the transplanted piece quickly acquires the properties of the latter’s girdle. The place of production of hormones that regulate the activity of the glands of the girdle is still unknown.
9. REPRODUCTIVE ORGANS
Earthworms reproduce only by laying eggs enclosed in special egg cocoons.
Let's look at how their organs are structured to ensure the formation of eggs, their fertilization and laying. The combination of these organs forms the reproductive apparatus. Male and female reproductive organs are found in earthworms in the same individual; thus, among them there are no male or female individuals, but they are all hermaphrodites, or, as they are commonly called, hermaphrodites.
Eggs are formed in the pars of very small female gonads - the ovaries, which are attached to the septum between the 12th and 13th segments on the ventral side (Fig. 24). The ovaries are very simple. They are complexes of developing eggs; the earliest stages of development are in the part adjacent to the intersegmental septum, where the ovary consists of small cells. The largest cells are located at the free posterior end of the ovary, facing the body cavity. Here the egg cells reach their final size (about 0.1 mm in diameter) and fall into the body cavity. Earthworm eggs are spherical or slightly elongated. They are almost transparent, since their protoplasm contains only a very small amount of grains of nutritious material - the yolk. The lack of sufficient nutritional material for the developing embryo inside the egg makes it necessary to supply it with nutrition from the outside using the protein of the egg cocoon.
Eggs finish maturing in the so-called egg sacs. These are blind sac-like projections of the intersegmental septa into which eggs that have been torn off from the posterior part of the ovary fall.
The eggs are brought out through short oviducts, which begin as oviducts in the 13th segment, then pierce the septum between the 13th and 14th segments and open on the ventral side of the 14th segment (Fig. 24). The egg funnels are equipped with cilia, the work of which catches the eggs from the egg sacs and at the right moment (during the formation of the egg cocoon) they are brought out through the oviduct.
The male gonads - the testes - are also very small. Among two pairs, they are placed on the partitions between the 9th and 10th segments and between the 10th and 11th (Fig. 24). Male reproductive cells - sperm - are just beginning to develop in these tiny little bodies. Complexes of future spermatozoa in the form of microscopic lumps of rounded cells fall into the body cavity and from there enter the seminal sacs, which are voluminous growths of intersegmental partitions. The number, shape, location and relative sizes of seed sacs vary and serve as an important feature in identifying worms.
In some species of earthworms (in the genera Octolasium and Lumbricus), the abdominal part of the body cavity near the testes is separated by a special wall from the main cavity of the segment; the so-called testicular capsules are obtained. Thanks to their presence, the developing lumps of sperm cannot spread throughout the entire cavity of the segment and a more direct path is created for them into the seminal sacs (Fig. 24).
The seminal funnels and vas deferens are used to remove spermatozoa to the outside (RPS. 24). The funnels are usually large; they are clearly visible when opening the worms. The seminal ducts, which receive sperm from the seminiferous funnels, are very thin cylindrical tubes that run posteriorly along the abdominal wall of the body. The vas deferens from the funnels of the 10th and 11th segments in the 12th segment merge with each other, and the common tube of the vas deferens on each side of the body usually stretches to the 15th segment, where it passes through the thickness of the body wall and ends with the male genital opening (sometimes ), usually having the appearance of a vertical slit.
The male genital openings sit on more or less strongly developed glandular pads. These cushions, in addition to glandular cells, contain a large number of vessels, which fill with blood during mating.
An original feature of the reproductive apparatus of oligochaete ringlets, to which earthworms belong, are the seminal receptacles (Fig. 24) - small spherical hollow sacs tightly pressed to the wall of the body cavity. The ducts of the seminal receptacles pass through the thickness of the body wall and open with external pores located in the intersegmental grooves. The walls of the seminal receptacles contain muscles, through the action of which seminal fluid can be absorbed into the seminal receptacle and, conversely, splashed out of it. This muscle acts like the rubber cap of a pipette. There are 2 or 3 pairs of seminal receptacles; they may be located laterally, on the ventral side, or they may (as in the genus Eisenia) be shifted to the dorsal side, up to the midline. But you need to keep in mind that some species of earthworms do not have seminal receptacles.
The organs that ensure reproduction include the belt of earthworms. In worms that have reached sexual maturity, the girdle is always noticeable, but its appearance depends on the season and state of nutrition. During breeding periods, the girdle swells greatly. Its function is the formation of egg cocoons.
The girdle is a modification of the outer epithelium. In the girdle area, the outer epithelium is very thickened. All cells acquire a glandular character; Among them, three types can be identified: 1) relatively small cells that do not contain grains - mucous cells; 2) medium-sized cells containing large grains that form the shell of the egg cocoon; 3) huge fine-grained cells that produce a protein substance that makes up the contents of the egg cocoon and serves as food for developing embryos (Fig. 25). In addition to glandular cells, a large number of blood vessels and nerve endings can be seen in the girdle.
A number of other glands on the ventral side of the body between the girdle and the anterior end of the body are also involved in the reproductive function. Particularly noticeable are the glands on the 10th and 11th segments, which give this part of the body surface a whitish tint in mature worms. In addition, near the abdominal setae in the indicated part of the body, on some segments, sometimes only on one side of the body, glands are developed, visible in the form of small swellings. Often the setae themselves are changed, turned into so-called genital setae, which function during mating to hold the partner and to move apart the pores of the seminal receptacles. Sometimes the genital setae differ from ordinary ones only in their larger sizes, but in some species they are very different in shape (Fig. 26). On the one hand, sharp stylets are formed, which are apparently injected into the partner’s skin during mating, and on the other, bristles are inserted into the pores of the seminal receptacles.
Many people underestimate the importance of the work of earthworms. These representatives of the invertebrate kingdom are best known for crawling out of the ground in large numbers after heavy rain. They are often used as bait by numerous fishing enthusiasts. Darwin also noted the fact that worms perform an important function in nature, acting as a kind of agricultural technicians. In the process of creating an extensive system of tunnels, which the earthworm digs through, excellent aeration is formed by supplying air to the inner layers of the soil.
Thanks to excellent aeration, the respiratory activity of many plants is facilitated. Feeding on organic matter and waste, worms ensure the grinding of soil components, enriching them with their secretions. The amazing ability of representatives of this species is the ability to disinfect large areas of soil, sterilizing it from harmful bacteria. Thanks to countless holes, forming a kind of capillary system, ideal drainage and ventilation of the soil is ensured.
The body of an earthworm can reach three meters in length. However, on the territory of Russia there are mainly individuals whose body length does not exceed 30 centimeters. In order to move, the worm uses small bristles, which are located on different parts of the body. Depending on the variety, there can be from 100 to 300 segments. The circulatory system is closed and very well developed. It consists of one artery and one central vein.
The structure of an earthworm is very unusual. Breathing is realized with the help of special hypersensitive cells. The skin produces protective mucus with a sufficient amount of natural antiseptics. The structure of the brain is quite primitive and includes only two nerve nodes. Based on the results of laboratory experiments, earthworms have confirmed their outstanding regeneration abilities. The severed tail grows back after a short period of time.
The genital organs of the earthworm are also very unusual. Each individual is a hermaphrodite. She also has male organs. Based on biological factors, all such worms can be divided into several subgroups. Representatives of one of them search for food on the surface of the soil layer. Others use the soil itself for food and emerge from the ground extremely rarely.
The earthworm is a type of annelid. Under the skin layer there is a developed muscle system consisting of muscles of various shapes. The mouth opening, from which food enters the esophagus through the pharynx, is located on the front of the body. From there it is transported to the area of the enlarged crop and the small size of the muscular stomach.
Burrowing and bedding earthworms live in places with loose and moist soil. Preference is given to moist soils of the subtropics, marshy lands and the banks of various reservoirs. In steppe areas, soil varieties of worms are usually found. Litter species live in the taiga and forest-tundra. The coniferous broad-leaved strip can boast the highest concentration of individuals.
What kind of soil do worms like?
Why do earthworms love sandy loam and loam soils? Such soil is characterized by low acidity, which is best suited for their life. Acidity levels above pH 5.5 are detrimental to the organisms of these representatives of the ringed type. Moist soils are one of the prerequisites for population expansion. During dry and hot weather, worms go deep underground and lose the opportunity to reproduce.
Character and lifestyle of the earthworm
The active and productive life of an earthworm occurs at night. As soon as night falls, many individuals crawl to the surface of the ground in search of food. However, the tail usually remains in the ground. By morning, they return to their holes with prey, dragging pieces of food into them and masking the entrance to their shelter with blades of grass and leaves.
The role of earthworms in nature is difficult to overestimate. The worm literally passes an incredible amount of soil mixture through itself, enriching it with beneficial enzymes and killing harmful substances and bacteria. The worm moves by crawling. Retracting one end of the body and clinging to the roughness of the ground with its bristles, it pulls up the back part, making its many passages in a similar way.
How do earthworms survive winter?
During the winter, the vast majority of individuals hibernate. A sharp drop in temperature can instantly destroy worms, so they try to burrow into the soil to a depth in advance, often exceeding one meter. Earthworms in the soil perform the most important function of its natural renewal and enrichment with various substances and microelements.
Benefit
In the process of digesting semi-fermented leaves, the worms’ body produces specific enzymes that contribute to the active generation of humic acid. Soil that has been loosened by earthworms is optimal for a wide variety of representatives of the plant kingdom. Thanks to a system of intricate tunnels, excellent aeration and ventilation of the roots is ensured. Thus, the movement of the earthworm is an important factor in the task of restoring the beneficial qualities of the soil.
The earthworm is in fact very useful for humans. It makes the soil layers fertile and enriches them with all kinds of nutrients. However, the total number of individuals in many regions of Russia is rapidly declining. This happens due to the uncontrolled introduction of pesticides, fertilizers and mineral mixtures into the soil. Earthworms are also hunted by numerous birds, moles, and various rodents.
What do earthworms eat?
At night, the earthworm crawls to the surface and pulls the half-rotten remains of plants and leaves into its shelter. Also, its diet includes soil rich in humus. One representative of the species can process up to half a gram of soil per day. Considering that up to several million individuals can live simultaneously on an area of one hectare, they are capable of acting as irreplaceable soil converters.
After rain, you can see a large number of worms on the asphalt and soil surface, what makes them crawl out? Even the name “earthworms” indicates that they love moisture very much and become more active after rain. Let's consider several possible reasons why earthworms crawl out to the surface of the earth after rain.
Soil temperature
It is believed that worms crawl to the surface in search of warmth, since after rain the soil temperature drops by several degrees, which causes discomfort for them.
Changes in acid-base balance
Another theory says that the worms crawl to the surface due to a change in the acid-base balance of the soil after rain, it becomes more acidic, which negatively affects these diggers. According to researchers, emergency evacuation to the soil surface saves them from death in an acidic environment.
Lack of air
The third theory explains that after rain there is more oxygen in the top layer of the soil, so the worms crawl up en masse. Water enriches the upper layers of the earth with oxygen, and many species of worms love moisture and vitally need sufficient oxygen. And through the surface of the body, oxygen is absorbed best in a humid environment.
Trips
British scientist Chris Lowe suggested that worms crawl to the surface of the earth during rain in order to make a long journey to new territory. On the surface, worms are able to crawl much further than underground, and dry soil causes discomfort when moving, strong friction is created, and grains of sand stick to the surface of the worm’s body, injuring it. And after rain, the surface of the earth is highly moistened, which allows them to freely travel to new areas of the ground.
The sound of rain
Another scientist, Professor Joseph Gorris from the USA, suggested that earthworms are frightened by the noise of rain, since the vibrations it creates are similar to the sound of the approach of their main enemy, the mole. That is why some fishermen use a technique to lure bait to the surface: they insert a stick into the ground, attach a sheet of iron to its surface and pull it so as to create a vibration, which will be transmitted into the ground through the stick. When frightened, the worms climb to the surface of the earth and become easy prey for experienced fishermen.
Reproduction and lifespan of earthworms
The earthworm is a hermaphrodite. It has both female and male genital organs. However, it is not capable of self-fertilization. With the onset of warm climatic conditions required for reproduction, individuals crawl in pairs, touching each other with their abdominal region, and perform a kind of seed exchange. Afterwards, the muff is transformed into a cocoon, in which the eggs develop.
Some varieties are distinguished by asexual reproduction. The worm's body splits in two, with one part regenerating the front end and the other regenerating the back end. There are also species of worms that reproduce without spermatheca by laying spermatophores. The lifespan of worms can exceed ten years.