What kind of life do roundworms lead? How do roundworms - nematodes - work? Threat to humans

The phylum roundworms, or nematodes, presumably originated from turbellarians. Evolving, this class acquired a peculiar structure, which is strikingly different from the structure of flatworms. This fact forces us to consider nematodes as a separate specimen of the animal world. Since the relationship of nematodes with groups higher up has not been proven, they are considered a lateral branch of the family tree of animals. This phylum has more than 10,000 species of organisms.

IN general characteristics roundworms, attention is focused on the external structure. From a medical point of view, roundworms are of great interest, since only they contain forms that are pathogenic for the human body.

This unique structure allows them to crawl freely and bend their body in different directions. Characteristics of the type of roundworms show that they lack the circulatory and respiratory systems. These organisms breathe through the cover of their body.

Digestive system

The digestive system of roundworms resembles a tube, that is, it is continuous. Starting from the oral cavity, it gradually passes into the esophagus, then into the foregut, middle and hind intestine. The hindgut ends at the anus on the other side of the body.

Many representatives of roundworms have a terminal oral opening, in some cases it is shifted to the ventral or dorsal side.

Selection system

Breeding system

The nematode has a reproductive system with a tubular structure. These organisms are heterosexual. Males have only one tube, different sections of which perform different functions. The narrowest section is the testis, which, in turn, is divided into two sections - reproduction and growth. Next is the vas deferens, and the channel for the eruption of the seed.

Females have a 2-tube reproductive system. One tube, ending in a dead end, plays the role of an ovary; it is filled with germ cells capable of reproduction. This organ flows into a larger section, which plays the role of the oviduct. The largest section of the female reproductive system is the uterus. The two uteri, connecting to each other, form the vagina, access to which is open on the front of the body.

Females and males differ significantly in external characteristics. Males, as a rule, are smaller and the back of the body in many is twisted towards the belly. In most species of nematodes, reproduction is viviparous - females carry an egg in the uterus until the larva hatches from it.

Nervous system

The nervous system of roundworms is a nerve ring, from which nerve trunks branch. Of these, the ventral and dorsal trunk are the most developed.

Life cycle

Nematodes in the human body cause diseases called nematodes, many of which pose a serious threat to health. There are classes of roundworms that are most common among humans.

Roundworms

The egg that the roundworm produces ends up in a person with unwashed vegetables or berries, onto which they fell, respectively, from the ground. The larva hatches from the egg and begins its journey through the human body. It has the ability to pass through the intestinal walls, penetrates the blood vessels, and enters the liver, atrium and lungs with the blood flow. To develop safely, roundworms need oxygen, so the larvae migrate to the pulmonary alveoli, and from there to the bronchi and trachea.

The waste products of roundworms are very toxic, so patients may experience severe headache, constant fatigue, outbursts of irritability. In addition, ascariasis often provokes intestinal obstruction.

Very common helminths, small nematodes white. The size of males is no more than 3 mm, females reach a length of 12 mm. Infection with pinworms can occur due to non-compliance with hygiene rules, so children who visit children most often become victims of enterobiasis. kindergarten. The patient is tormented severe itching, he scratches the skin until it bleeds, pinworm eggs remain on the hands and under the nails, after which they are transferred to household items and food.

The structure of roundworms of this species is such that they cling tightly to the walls of the intestine and feed not only on its contents, but also on blood. The toxins released by pinworms can cause headaches, insomnia, fatigue and dizziness, as well as allergies.

Through the blood vessels, the crooked head enters the heart, from there to the lungs, upper respiratory tract and pharynx. Together with saliva, they penetrate the esophagus, then the stomach, and their destination is the duodenum. This type of nematode can enter the body in two ways - either with contaminated food and water, or by penetrating through the skin. Soon after entering the body, the patient begins to suffer from pain in the duodenum, indigestion, fatigue, headache, depression, impaired memory and attention. If not treated promptly, this disease can be fatal.

How to combat the penetration of nematodes into the body? Preventive measures are quite simple, but nevertheless require strict adherence:

• do not neglect the rules of personal hygiene, wash your hands as often as possible hot water with soap;
• carefully process all vegetables, fruits and berries before eating (to protect yourself, you need to immerse them in boiling water for 3 seconds, or for 10 seconds in hot water, then rinse thoroughly with cold water);
• It is not recommended to use human and pig feces that have not undergone the composting process as garden fertilizer;
• cut nails of adults and children as often as possible, change bed linen and underwear daily.

Nematodes are an integral part of nature, and it is impossible to eliminate them, but with the help of simple measures you can protect yourself from their invasion of the body.

The phylum of roundworms or Nemathelminthes is one of the most numerous among the animal kingdom. There are more than 20,000 species.

Turbellaria are considered their ancestors, but during the course of evolution, roundworms acquired special structural features, which allowed them to form a separate group.

The following classes of roundworms are distinguished:

1. Class Nematodes;
2. Class Gastrociliaceae;
3. Kinorhyncha class;
4. Class Hairworms;
5. Class Rotifers.

Structural features of roundworms

Dimensions vary from 80 microns to 8 meters. Females are usually larger than males. All representatives have bilateral symmetry.

The body shape is often cylindrical or fusiform, characterized by a lack of segmentation. A dense cuticle completely covers the outside of the body. The body cavity of roundworms is surrounded by a skin-muscular sac. It contains fluid that fills the space between the internal organs. Serves as a support and performs a transport function.

Digestive organs

The digestive system of roundworms has a more progressive structure. Roundworms differ from flatworms by having an anus. They also have a separate hindgut.

The digestive tract is divided into three parts: anterior, middle and posterior. Food is absorbed through the mouth and enters the pharynx and esophagus. Digestion of the food bolus occurs in the middle section, and after absorption of nutrients, the remains are excreted.

The movement of food is now carried out only in one direction, which has contributed to better digestion.

Excretory system

The excretory function is performed by the cervical gland, a large single-celled formation from which a pair of lateral tubules extend. They open to the outside with an excretory pore.

Ammonia is able to leave the body of roundworms through the skin by diffusion.

Reproduction

Most representatives of the phylum are dioecious organisms. Often females and males have different appearance(the phenomenon of sexual dimorphism). Development can be direct, without a larval stage, or indirect with a change of hosts.

The reproductive system of roundworms is presented in the form of tubes. In males it is often one tube, which is divided into specialized sections. At the beginning is the testis, where sperm are produced, which enter the ejaculatory canal through the vas deferens. It opens into the distal part of the intestine - the cloaca. With the help of copulatory organs (cuticular needles), sperm are brought out.

The female reproductive tract consists of two paired tubes. They are blindly closed in the initial section; here are the ovaries, where the constant reproduction of germ cells occurs. Gametes travel through the oviducts and enter the uterus, where internal fertilization occurs.

The eggs of roundworms are surrounded by a thin shell, in rare cases it is thick. There are species capable of viviparity.

Nervous system

It is represented by the peripharyngeal ring and longitudinal nerves. The ring is located in the pharynx and serves as the main associative organ. The ventral and dorsal nerves arise directly from the main ganglion and lie in the hypodermis; other nerve fibers are not connected to it.

The sense organs of roundworms are poorly developed. There is chemical reception, organs of touch, free-living marine species have photosensitive cells.

At the distal end of the body of roundworms there are tail glands, the secreted fluid of which is necessary for attachment to the substrate.

Also, roundworms in their life cycle often use intermediate hosts for full development. This is not typical for annelids. Annelids differ from roundworms by the presence of a closed circulatory system, represented by ventral and dorsal vessels.

Comparison of flat and roundworms, their similarities and differences

The nutrition of nematodes depends on their lifestyle, some consume algae and bacteria, others feed on the host’s body, and there are also predatory species.

Meaning in nature and human life

Roundworms are an integral part of food chains. Free-living individuals absorb bacteria and protozoa, and themselves become food for fish and crustaceans.

Species living in the ground feed on rotted organic matter, thus participating in soil formation.

Roundworms can infect mushrooms and plants, significantly reducing crop yields. Nematodes penetrate the root system and lead to the death of the rhizome, thereby inhibiting plant growth. Cereals and vegetable crops (onion, potato, wheat nematodes) are most often infected.

Roundworms are a group of living organisms classified according to phylum rank. The taxonomy of this type is ambiguous. According to some (older) sources, the phylum includes five classes (Nematodes, Gastrociliaceae, Cynorynchians, Hairworms, Rotifers). According to modern sources, roundworms are equated to nematodes and include two or three of their classes. Hairworms, rotifers, and gastrociliformes are considered as separate types; Kinorhynchus is a class belonging to a group that currently does not have a specific systematic rank. Only nematodes are discussed below.

The appearance of roundworms can be considered the next major evolutionary step that occurred on Earth after flatworms. According to a widely accepted hypothesis, nematodes evolved from ciliated worms (a class related to flatworms) or had a common ancestor with them.

The following major events occurred in nematodes aromorphoses:

The appearance of an anus (anal) opening in the intestine.

Formation of a body cavity filled with fluid.

Dioecy in most species of roundworms.

The structure of nematodes

The body length of roundworms (nematodes) varies, from less than a millimeter to slightly less than 10 meters. The body is unsegmented, usually fusiform or filiform, pointed at both ends, and has the shape of a circle in cross section. The symmetry of the body should be considered bilateral, although there are elements of radial (two-ray and in the head part three-ray).

The body wall of nematodes, like that of flatworms, is formed by skin-muscle sac. However, in roundworms it consists only of longitudinal muscles (four cords), hypodermis (epithelium) and cuticle. There are no transverse or dorso-abdominal muscles, like those of flatworms. In this regard, nematodes can only bend their body. Cuticle performs protective function, has a noncellular structure, formed from secretions of the hypodermis. It can be either smooth or ringed (but the worm itself does not have a ringed structure!). The hypodermis forms longitudinal ridges pressed into the body cavity (dorsal, abdominal, left and right).

In roundworms, in the process of evolution, arose primary body cavity, called pseudo-goal. This cavity does not have an epithelial lining; it is simply a space between the skin-muscular sac and the internal organs, filled with fluid.

The digestive system of nematodes has the form of a through tube, in which three sections are distinguished: anterior, middle and posterior. The tube has two holes: oral and anal(anal). The appearance of a second opening in the intestine is considered a major aromorphosis in the animal world. In the digestive system they secrete oral cavity, esophagus, foregut, midgut and hindgut. In many species, the mouth is surrounded by three lips, followed by a muscular pharynx. The pharynx opens into the midgut.

The excretory system is formed by one huge excretory cell and phagocytic cells. The excretory cell has processes with channels that penetrate the body of the roundworm and opens outward with one hole. Along the channels in the cavity there are phagocytic cells that capture decay products and transport them into the channels.

The nervous system of nematodes consists of cephalic ganglia, peripharyngeal ring, longitudinal trunks, connected by transverse bridges. Of the longitudinal trunks, the dorsal and ventral trunks achieve the greatest development.

Eat organs of touch and chemical sense, some species have primitive eyes.

In nematodes, as well as in flatworms, no circulatory and respiratory systems . Gas exchange occurs across the entire surface of the body. In addition, in a number of species respiration occurs without the participation of oxygen (glycolysis). Nutrients from the intestine are distributed to the cells of the body through a fluid that fills the pseudocoel.

1 - mouth; 2 - intestine; 3 - anus; 4 - excretory cell; 5 - testis; 6 - nerve ring; 7 - dorsal nerve trunk; 8 - abdominal nerve trunk; 9 - excretory opening.

Most representatives of nematodes are characterized by dioeciousness(some individuals contain only the female reproductive system, others only the male). Both the female and male reproductive systems have a tubular structure. Moreover, the male one has an unpaired structure (one testis, vas deferens, ejaculatory duct opening into the hindgut). In the female reproductive system, not only the ovaries and oviducts are paired, but also the uterus. Only the vagina is unpaired. Typically, females differ from males, i.e. sexual dimorphism.

Individual development in many species occurs with incomplete transformation (there are larval stages, but no metamorphosis).

Type roundworms Nematodes biology classes systems structure body characteristics signs representatives photo difference cavity

General characteristics of nematodes

Free living roundworms distributed throughout the globe. They are found in a wide variety of habitats: on the seabed, V ground fresh and brackish reservoirs and finally, in various soil types.

Roundworms External structure

The body is fusiform or filiform, round in cross section, undivided, covered with a layer of dense and elastic cuticle, on which various tubercles and spines are often located, but never cilia.

The mouth is located at the very anterior end of the body and is often, for example in roundworms, surrounded by three lips seated with sensitive papillae. Not far from the posterior end of the body, on the ventral side, the anus is located. The section of the body behind the anus is called the tail.

Skin-muscle bag

The dermal-muscular sac of roundworms consists of the cuticle, hypodermis and one layer of longitudinal muscles.

Below the cuticle is the hypodermis. In many nematodes it consists of large epithelial cells. In roundworms, the boundaries between individual cells disappear and a syncytium is formed with nuclei scattered in the cytoplasm. On the inner surface, the hypodermis forms thickenings in the form of four ridges running along the entire body of the worm. The channels of the excretory system pass through the two lateral ridges of the hypodermis, and the main nerve trunks pass through the dorsal and ventral ridges.

Directly below the hypodermis there is only one layer of longitudinal muscle cells. These are very large cells, consisting of an elongated spindle-shaped contractile fiber, in the middle part of which there is a sarcoplasmic sac containing the cell nucleus. Numerous plasmatic processes branch off from the sarcoplasmic sac and go to the hypodermis and other muscle cells. The contractile fibers of muscle cells are adjacent to the hypodermis and form the cortical layer of the longitudinal muscles. The sarcoplasmic layer extends into the body cavity.

Due to the presence of only longitudinal muscle cells, the movements of nematodes are much more uniform compared to the movements of flatworms. Roundworms can only bend their body using muscles. Straightening of the body occurs when muscles relax due to the pressure of the abdominal fluid and the elasticity of the cuticle.

Longitudinal ridges of the hypodermis divide the muscular layer into four stripes stretching along the body.

Body cavity

The skin-muscle sac limits the body cavity in which the internal organs are located, namely the digestive and reproductive systems. This cavity, preserved in the animal, is usually called the primary body cavity.

Roundworms, like flatworms, lack circulatory and respiratory organs.

The mouth, located at the anterior end of the body, is usually surrounded by six outgrowths - lips (the roundworm has three), on which there are sensitive papillae.

The mouth leads into an ectodermic pharynx lined with cuticle. The pharynx is sometimes incorrectly called the esophagus. It has muscular walls. The lumen of the pharynx in a transverse section is usually triangular in shape. In many nematodes, the anterior part of the pharynx is armed with chitinous teeth. The posterior part of the pharynx in some nematodes is swollen and is called a bulb or bulb.

Following the pharynx is the endodermic midgut, the wall of which consists of single-layer epithelium and has no muscle fibers. The midgut lies freely in the body cavity and does not form bends. Unlike flatworms, digestion in nematodes is entirely extracellular and occurs in the cavity of the midgut. The short hindgut is an ectodermic section and is lined with a cuticle.

Excretory system

The excretory organs are represented by skin glands, which also perform the function of osmoregulation. Protonephridia are absent. The excretory organs are located in the hypodermis and are formed by one or two, rarely many cells. In the roundworm, two excretory canals pass through the lateral ridges of the hypodermis, closed at the posterior ends. These canals, approaching the anterior end of the body, bend to the ventral side, merge together into a short unpaired canal and open on the ventral side with an excretory opening. This entire organ is one giant cell: the excretory canals pass through its processes, and the nucleus is located in the cytoplasm of the left canal. The excretory organs of nematodes are called cervical glands. Free-living nematodes, in addition to them, have terminal, or tail, excretory skin glands.

In addition, the excretory function is performed by large phagocytic cells, often star-shaped, located in the body cavity in the number of two, four or six. Apparently, they play the role of “storage buds”, capturing insoluble metabolic products - excreta and bacteria - from the body cavity. In addition, hemoglobin was found in them. It is assumed that phagocytic cells serve as centers that regulate the consumption of free oxygen (aerobic part of respiration) by nematodes.

Nervous system and sensory organs

In nematodes, the nervous system is located in the hypodermis. It consists of a nerve ring that surrounds the pharynx and to which adjoin small clusters of nerve cells that form ganglia.

Six nerves usually extend forward from the nerve ring to the labial sensory papillae.

Several longitudinal trunks extend back from the ring (ascarids have six). Of these, the most developed is the abdominal trunk, located in the abdominal ridge of the hypodermis. The peripharyngeal nerve ring, the associated ganglia and the ventral nerve trunk form the central nervous system of nematodes. In addition to the abdominal trunk, the hypodermis usually contains a dorsal, two lateral and four thinner longitudinal trunks. Longitudinal trunks can arise directly from ganglia adjacent to the nerve ring. In the roundworm, the abdominal and dorsal trunks are connected by semi-circular commissures located asymmetrically on the right and left sides.

Sense organs are poorly developed. The organs of touch are represented by papillae, papillae and setae, which are concentrated mainly at the anterior end of the body. They are usually associated with pores in the cuticle, covered by a thinner cuticle, to which the nerve endings approach.

At the head end there are organs of chemical sense - amphids. They are variously shaped depressions in the skin to which the endings of sensory nerves approach. Only a few free-living aquatic nematodes have visual organs.

Constancy of cellular composition

When studying the histological structure of various organs of nematodes, it turned out that all organs are formed by a small number of very large cells. One or two giant cells form the excretory gland, four very large phagocytic cells complement the excretory organs. Muscle cells are also large and few in number. Small species may have only eight of them. The roundworm's central nervous system consists of only 162 cells. The formation of new cells in nematodes stops at the early stages of development, and then the number of cells remains constant throughout life. The constancy of the cellular composition with a small number of cells reaching large sizes is the most important feature nematodes Apart from nematodes, this is characteristic of only a few groups close to nematodes (rotifers). The constancy of the cellular composition is associated with the inability of nematodes to reproduce asexually and regenerate lost parts.

Reproductive system

Roundworms are dioecious, and hermaphrodites are quite rare among them. Nematodes have pronounced sexual dimorphism. Males are usually smaller in size, the posterior end of their body is somewhat twisted (ascaris), and some species have a leathery fold at the posterior end - a copulatory bursa, which plays a role in mating (pile, etc.). The genital organs have a simple structure and are represented by elongated, sometimes very long, tubes.

The female genital organs of roundworms are paired. They begin with thin filamentous ovaries, which are very thin tubes filled with radially arranged germ cells. The ovaries imperceptibly pass into wider sections - the oviducts. These are hollow tubes that do not contain developing eggs. The oviducts open into thicker muscular tubular organs - the uterus, which store the sperm that got there during copulation. Fertilization of eggs occurs in the uterus and initial stages their embryonic development. The uteri are connected into an unpaired short tube - the vagina, through which eggs are laid. The female genital opening is located on the ventral side, often in the front part of the body (in roundworms in the front third of the body).

The male genital organs of roundworms are usually single, but they can also be paired. In the roundworm, the male reproductive apparatus is a gradually expanding tube, consisting of a thin and longest section - the testis, which passes into a wider vas deferens and then into the ejaculatory canal, which opens with the genital opening into the hindgut. The hind intestine becomes the cloaca, which houses the copulatory apparatus. The copulatory apparatus usually consists of two long setae - spicules, formed in the lateral pockets of the cloaca. During copulation, the spicules are pushed out by the male and inserted into the genital opening of the female, thereby holding her. The copulatory apparatus includes many accessory formations, including the mentioned copulatory bursa.

Spermatozoa are very unique; they have an amoeboid shape, which makes them different from the seminal cells of representatives of all other animals. Nematode sperm are able to move like small amoebas.

Nematode eggs are usually covered with a dense shell, or shell. Thus, roundworm eggs are covered with a layered, highly developed shell, which ensures their resistance during long-term exposure to the external environment. In most nematodes, fertilized eggs are laid outside and develop in the external environment. Only in a few species (Trichinella) the entire process of embryonic development of eggs occurs in the uterus. At the same time, females are characterized by viviparity: they give birth to fully formed larvae.

Development

The development of roundworms occurs in a very peculiar way and in many forms with transformation. Crushing of eggs, complete, uniform, takes place according to the bilateral type. An extreme degree of determinism of fragmentation is characteristic; already at the fourth division of the egg (stage of 16 blastomeres), the sexual primordium and the primordium of the midgut are separated. In the same way, outer integument, pharynx, muscles, etc. develop from single blastomeres. This development is called mosaic, and it determines the constancy of the cellular composition of adult forms. The egg hatches into a small, worm-like larva, which in most nematodes molts four times before developing into an adult worm. The number of molts may be smaller due to the fact that the first moults (one or two) occur before the larva emerges from the egg shells.

• there is a primary cavity in the body;
• the helminth has an anus and a posterior part of the intestine;
• type annelids are dioecious individuals.

The main signs of roundworms:

1. They have a non-segmented body, round in cross section. Its surface is three-layered and consists of meso-, endo- and ectoderm. The worm has a skin-muscle sac.
2. All types of annelids have a pseudocoel - this is the primary body cavity filled with liquid. It gives the body additional elasticity and performs the functions of a hydroskeleton. This liquid is also responsible for metabolic processes. This is where all the internal organs are located, forming the digestive, nervous, excretory, muscular and reproductive systems.
3. The structure of roundworms is such that they lack a respiratory and circulatory system.
4. The peculiarities of roundworms are that they digestive system represented by a through tube starting with the oral opening. The mouth is surrounded by cuticular lips. At the end of the digestive tube is the anus. The entire tube is divided into three sections. Pinworms have a special extension of the esophagus called the bulbus.
5. As for the nervous system, it consists of the peripharyngeal ring, cephalic ganglia and nerve trunks (abdominal, dorsal and two lateral trunks). The most developed abdominal and dorsal spines. They are connected by special jumpers.
6. No matter how many species of roundworms there are, all of them have poorly developed senses. As a rule, they consist of tactile tubercles and special organs of chemical sensation.
7. The excretory system of a roundworm consists of a small number of excretory phagocytic cells. They accumulate metabolic products and foreign substances that enter the body cavity.
8. Nematodes are roundworms that have a tubular structure of the genital organs. The number of female genital organs is usually paired. The male, on the contrary, has unpaired genitals. Its reproductive apparatus consists of a testis and a vas deferens, which passes into the ejaculatory canal and opens into the posterior part of the intestine. The body structure of the female is somewhat different. Its reproductive apparatus consists of paired ovaries, from which there are two tubular oviducts and a paired uterus. It unites into a common vagina.

We have listed general signs, characteristic of representatives of the type roundworms. However, the external structure of individual individuals may differ slightly. So, if we describe the group of roundworms, the representatives of this class are as follows:

• roundworms;
• whipworm;
• pinworms;
• hookworm;
• trichinella;
• guinea worm.

Roundworms

This is a fairly large helminth, the female of which can grow up to 40 cm, the male - about 20 centimeters. The roundworm has a cylindrical body narrowed at the ends. The body of the male is twisted in a spiral from the rear end towards the abdomen.

Important! People become infected by eating unwashed fruits and greens.

The life cycle of roundworm in the human body is as follows:

1. After the egg is in the intestine, its shell is dissolved by digestive juices, and a larva emerges from it.
2. It enters the bloodstream through the intestinal walls. Then it migrates through the liver to the right atrium, ventricle and lungs.
3. From there, through the pulmonary capillaries, the larva enters the bronchi and trachea, provoking a cough.
4. During a cough, it is swallowed again and again ends up in the intestines. Here it reaches the state of a sexually mature individual, lives and reproduces for up to a year.

Pinworms

Important! The eggs become infectious within a couple of hours of being laid. If, while scratching this area, eggs get under the patient’s nails, he will re-infect himself if hygiene is not observed.

Since the lifespan of a sexually mature pinworm reaches 58 days, self-healing of the patient may occur if re-infection does not occur. As for males, they die immediately after mating with females and exit the body naturally (with feces).

The anterior filament-like elongated part of the worm body is thinner than the posterior one. It contains the esophagus. The posterior end of the male specimen is thickened and spirally twisted. This is where the intestines and reproductive system are located. The eggs of this worm look very much like a barrel with plug-shaped lids at the ends. They are light transparent and reach 50 microns in length.

Trichinella

This worm is a biohelminth. Its life cycle is as follows:

The worm lives in the small intestine, where it can live up to five years. It is classified as a geohelminth. Hookworm migrates in the human body like roundworms. The eggs pass through the feces into the external environment, where every other day they hatch into larvae that feed on feces. Once they reach the filarial stage, the larvae become infective.

Sometimes infection occurs through the oral route, but more often the filariae penetrate through the skin. From the intestines, the larvae migrate to the blood vessels and lungs. Then they rise through the bronchi into the trachea, from where, during the cough reflex, they are thrown into the mouth and swallowed. After this, they settle in the duodenum.