What makes up the musculoskeletal sac of nematodes. Free-living nematodes or roundworms: description, structural features. Significance in nature and human life

In the animal kingdom. More than 80 thousand species have been described, but in reality, there are many more. Judging by the number of their appearance, adaptability to the environment and good survival, experts came to the conclusion that this population has more than a million representatives. Nematodes have adapted to living in different sources, such as seas and reservoirs, soil, etc. Their presence in the human and animal body causes enormous damage, and the worms themselves can cause various diseases.

Digestive system of roundworms

Nematodes or, simply speaking, roundworms, unlike their relatives, are endowed with an anus. The intestine passes through the entire body of the helminth, taking the form of a straight tube. Food initially enters the pharynx (the initial part of the intestine), which is represented by muscular walls.

As a rule, breathing in roundworms occurs through the entire body, since there is no circulatory blood supply system. Accordingly, there is no need to transfer oxygen to all organs. The energy received from food is released due to the breakdown of glycogen - a substance of an organic nature.

Selection system

Nematode neurological map

The organs of touch and chemical perception are well developed. Nematodes have no sense organs.

Fertilization and reproduction system

Life cycle of a nematode

Roundworms go through five stages of development during their existence: four of them are larval and one adult. All transitions are associated with a change in environment or migration from one host to another.

Biohelminths

People are the main carriers great apes and other mammals. Biohelminths are carried by various blood-sucking insects.

Fillaria enter the bloodstream only when the carrier is maximally active. In the case of mosquitoes, this is evening and night time. In horsefly - morning and afternoon. When transferring filaria by midges or biting midges, their output does not have a specific period, and depends only on humidity.

The main types of roundworms

1. Wuchereria banctofti : in humans and monkeys, it is concentrated in the blood arteries and lymph nodes. Leads to stagnation of blood and lymph. It is also the cause of elephantiasis and allergization. The main vector is the mosquito.
2. Brugia malayi : definitive hosts are humans, some species of monkeys, the cat family. Pathogenicity and concentration similar Wuchereria banctofti. Also carried by mosquitoes.
3. Oncocerca volvulus : carried by midges, and the carrier of the helminth is a person. It is concentrated in the body under the skin of the head, chest, arms and legs. Promotes the formation of painful nodules. Concentrating in the area of ​​\u200b\u200bthe organs of vision, it can cause blindness.
4. loa loa : localized under the skin and mucous membranes in humans and monkeys, forms painful nodules and abscesses. Carried by horseflies.
5. Mansonella : concentrated in the body of the carrier, which is people, in the subcutaneous fat layer, the mesentery of the intestine and under the serous membranes. Carried by mosquitoes.

It is also worth mentioning the most common types of roundworms.

Ascaris

Roundworm

Roundworm

Nematodes(from Greek. nemas- “thread”) is a class of Roundworms (about \ (20 \) thousand species). Free-living nematodes live at the bottom of the seas and fresh waters, in damp earth and leaf litter (they are very small - from \ (0.05 \) to \ (50 \) mm).

Skin-muscular sac and primary body cavity

The body of the nematodes is spindle-shaped: it is narrowed towards the anterior and posterior ends.

Under a dense elastic shell - cuticle- lies a layer of epithelial cells. Below are longitudinal muscles- four single-layer tapes. This structure allows roundworms to crawl, bending the body.

The cuticle, epithelial cells and muscles form skin-muscle sac.

Between the musculocutaneous sac and the intestines is primary body cavity. It is filled with liquid, which, due to pressure, maintains permanent form body, promotes the distribution of nutrients throughout the body of the animal, the movement of decay products to the excretory organs.

Digestive system

Unlike ciliary worms and flukes, nematodes have an anal (anus) opening, and the intestines look like a straight tube passing through the entire body.

Gas exchange and metabolism

In free-living roundworms, gas exchange (oxygen consumption and carbon dioxide release) occurs through the integument of the body.

Due to the lack circulatory systems s nutrients and decay products in the body are carried by the primary body cavity fluid.

excretory system

The excretory system consists of two lateral blindly closed canals. They open outward through an excretory opening on the ventral side of the anterior part of the body. The walls of the channels are formed by one or several very long cells (their length can reach \(40\) cm). Formed in the body harmful substances enter the cavity fluid, then into the channels of the excretory system, and are brought out.

Nervous system

Nervous system nematodes are represented by the peripharyngeal nerve ring and longitudinal nerve trunks - the system stem type.

Type roundworms Nematodes biology classes systems structure body characteristic 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 seabed , V ground fresh and brackish reservoirs and finally in various soil types.

Roundworms External structure

The body of the nematode is spindle-shaped 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 there are never cilia.

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

Skin-muscular sac

The musculocutaneous sac 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 syncytium is formed with nuclei scattered in the cytoplasm. On inner surface the hypodermis forms thickenings in the form of four ridges running along the entire body of the worm. In the two lateral ridges of the hypodermis, the channels of the excretory system pass, in the dorsal and ventral ridges - the main nerve trunks.

Directly under the hypodermis 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 plasma processes branch off from the sarcoplasmic sac to the hypodermis and other muscle cells. Contractile fibers of muscle cells are adjacent to the hypodermis and form a cortical layer of the longitudinal muscles. The sarcoplasmic layer protrudes 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 flatworms. Roundworms with the help of muscles can only bend the body. Straightening of the body occurs when the muscles relax due to the pressure of the cavity fluid and the elasticity of the cuticle.

The longitudinal rollers of the hypodermis divide the muscular layer into four strips that stretch along the body.

body cavity

The skin-muscular 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 commonly called the primary body cavity.

The organs of blood circulation and respiration in roundworms, like in flatworms, are absent.

Digestive system

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

The mouth leads to an ectodermal pharynx lined with cuticles. The pharynx is sometimes incorrectly referred to as the esophagus. It has muscular walls. The lumen of the pharynx on a transverse section is usually triangular in shape. In many nematodes, the anterior part of the pharynx is armed with chitinous teeth. The back of the pharynx in some nematodes is swollen and is called the bulb or bulb.

The pharynx is followed by the endodermic midgut, the wall of which consists of a single layer of epithelium and does not have 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 hind gut is an ectodermic region and is lined with a cuticle.

excretory system

The excretory organs of nematodes 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 roundworm, two excretory canals pass in the lateral ridges of the hypodermis, closed at the posterior ends. These canals, approaching the anterior end of the body, bend on 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 run in 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 stellate, located in the body cavity, including two, four or six. Apparently, they play the role of "accumulation kidneys", 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 (the aerobic part of respiration) by nematodes.

Nervous system and sense organs

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

From the nerve ring, six nerves usually leave forward, heading to the sensory labial papillae.

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

The sense organs of nematodes 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, closed by a thinner cuticle, to which the nerve endings fit.

At the head end there are organs of chemical sense - amphids. They represent various shapes recesses in the skin, to which the endings of sensory nerves approach. Only a few free-living aquatic nematodes have organs of vision.

The constancy of the 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 complete the excretory organs. The muscle cells are also large and few in number. Small species may have as few as eight. In roundworm, the 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, while reaching large sizes, is the most important feature of nematodes. In addition to nematodes, this is characteristic of only a few groups close to nematodes (rotifers). The inability of nematodes to asexual reproduction and regeneration of lost parts.

reproductive system

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

The female genital organs of roundworms are paired. They begin as thin, filiform 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, in which the sperm that got there during copulation is stored. The eggs are fertilized in the uterus and initial stages their embryonic development. The uterus is connected into an unpaired short tube - the vagina, through which the eggs are laid. The female genital opening is placed on the ventral side, more often in the anterior part of the body (in roundworm in the anterior third of the body).

The male reproductive organs of roundworms are usually solitary, but there are also paired ones. In roundworm, the male reproductive apparatus is a gradually expanding tube, consists of a thin and longest section - the testis, which passes into a wider vas deferens and then into the ejaculatory canal, which opens with a genital opening into the hindgut. The posterior intestine turns into a cloaca, in which the copulatory apparatus is placed. The copulatory apparatus usually consists of two long setae - spicules, which are 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 additional formations, including the mentioned copulatory bursa.

The spermatozoa of nematodes are very peculiar, they have an amoeboid shape, which differs from the seed cells of representatives of all other animals. Spermatozoa of nematodes are able to move like small amoeba.

Nematode eggs are usually covered with a dense shell, or shell. So, roundworm eggs are covered with a layered, highly developed shell, which ensures their resistance during prolonged stay in 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 live birth: they give birth to fully formed larvae.

Development

The development of nematodes 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 crushing is characteristic, already at the fourth division of the egg (stage 16 blastomeres), the sexual germ and the germ of the midgut are separated. In the same way, the 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. A small worm-like larva emerges from the egg, which in most nematodes molts four times before turning into an adult worm. The number of molts may be smaller due to the fact that the first molts (one or two) occur before the larva emerges from the egg membranes.

Roundworms or nematodes are amazing creatures, whose presence in our lives we practically do not feel. They are invisible and yet they are the second species diversity group in the animal kingdom after insects. Thus, the number of free-living nematodes in one cubic meter of water or soil can exceed one million individuals. They spread all over the place and, like " gray cardinals”, being in the shade, meanwhile play one of the main roles in all ecosystems.

General information about nematodes

The size and shape of the body of nematodes

The nature of the movement of free-living nematodes determines their anatomical features. Roundworms have a filamentous or fusiform body, unsegmented. Less common in females are lemon-shaped or barrel-shaped. The body is round in cross section, has bilateral symmetry with two-beam elements, and the head shows signs of three-beam.

The coloration of free-living nematodes is not remarkable. Body color ranges from translucent to milky white with hints of yellow or pink. In the photo above, a deep-sea nematode from the order Desmodorida.

Structural features

Unlike flatworms, mesenchyme is almost completely absent in the body of nematodes, the space between the subepithemial longitudinal muscles and the intestine is filled with the primary body cavity (pseudocoelom). The cavity fluid creates a strong pressure, which, together with the cuticle, acts as an antagonist of the longitudinal muscles. This system maintaining the shape of the body is called the hydroskeleton. The nature of the movement of free-living nematodes is directly related to it. For them, only serpentine movement is possible. Moreover, because of the indivisibility of the animal, the whole body always participates in it.

All nematodes are deprived of the respiratory and circulatory systems, as well as flagellar cells, with the exception of some sensory organs.

Digestive system

Mostly free-living nematodes feed on algae, bacteria, detritus, but there are also predators among them. For example, mononkh-one-tooth. This predatory worm a large and sharp spike projects upward from the oral cavity, sensitive pyramids are developed on the head, and nerve papillae near the mouth. When they are irritated, the muscles of the esophagus instantly contract, and the victim is drawn into oral cavity.

Features of the excretory system

The excretory system is rather primitive. There is an assumption that its main organs are a unicellular (less often multicellular) cervical gland, or lateral intracellular channels (renettes), as well as pseudo-coelomite cells. The latter do not have ducts, their function is the isolation and utilization of metabolic products. The renette consists of a voluminous body and an excretory duct that opens outward in an adjustable pair. In addition, ammonia from the body of free-living nematodes can be released through the body wall by diffusion.

In the photo above, a representative of the class Adenophorea (order Desmoscolecida).

Nervous system

The nematode is represented by a peripharyngeal nerve ring and several longitudinal nerves. The first is a single circular ganglion and, in all likelihood, plays the role of an associative organ. The nerve ring is located at the level of the middle of the pharynx and the dorsal ring is tilted forward. The dorsal nerve and ventral nerve trunk depart from it. The remaining longitudinal nerves are not directly connected with it.

In free-living nematodes (size, coloration, character of movements - discussed above), the sensory organs are represented by sensilla: labial papillae, tactile bristles, male supplementary organs, olfactory amphids, phasmids (sensory glandular organs), as well as terminal caudal glands, the secret of which is required for attachment to the substrate. All of these organs are chemo- and mechano-, less often photoreceptors, or have mixed sensitivity.

Development of nematodes

The vast majority of nematodes are dioecious animals, but there are also hermaphrodites. As a rule, they lay eggs, live birth occurs less often. In male nematodes, the posterior end of the body is bent to the ventral side and has a complex copulatory apparatus on it. They have two testes with vas deferens and one ejaculatory canal. Nematode sperm have a diverse structure, flagella are absent, and motility is amoeboid. The genital organs of females are represented by a single or double set, consisting of ovaries, oviducts and uterus, as well as a vagina.

Reproduction of nematodes is not accompanied by metamorphosis. Usually, life cycle consists of four juvenile stages and one adult. The transition between them occurs at the time of molting.

Question 1. What is the body shape of a nematode?
The shape of the body of nematodes is spindle-shaped, since their body usually tapers towards both ends. The cross section of the body is round.

Question 2. What are the structural features of the nematode.
In the structure of the nematode, the following features can be noted:
- the body is non-segmented, covered with a dense cuticle;
- the skin-muscular sac contains a weakly expressed annular and well-developed longitudinal muscles in the form of four ribbons (bending and unbending the body in the dorsal-abdominal direction, nematodes can crawl forward, lying on their side);
between the muscle layer internal organs there is a cavity; this primary body cavity performs the functions internal environment of the body and hydroskeleton, provides intestinal movement independent of the walls of the body and participates in metabolism and their transport;
- the mouth opening is located at the front end of the body;
- excretory system it is represented by unicellular skin glands that secrete soluble metabolic products;
- circulatory and respiratory systems No;
- the nervous system is formed by a near-pharyngeal nerve ring with several nerve trunks;
- reproductive system represented by the ovaries and testes; as a rule, nematodes are dioecious;
- The sense organs are poorly developed.

Question 3. What is a cuticle? What is its meaning?
The cuticle is a dense multilayer non-cellular formation on the surface of the body. It is a kind of external skeleton, which creates a support for the muscles. The protective role of the cuticle is also important: it protects the body from mechanical damage and toxic substances.

Question 4. What is the role of the body cavity?
The body cavity plays important role in exchange processes. Through it, the digested substances and food are transported from the intestines to the muscles and the reproductive system; the removal of metabolic products to the excretory organs is also partially carried out. Thus, the fluid that fills the body cavity takes on the function of the internal environment of the body, like blood.

Question 5. How is the nervous system of nematodes arranged?
The nervous system of nematodes is formed by a near-pharyngeal nerve ring surrounding the anterior part of the esophagus. Several short branches extend forward from the ring; six trunks are sent back, and two of them, passing along the median dorsal and abdominal lines, are more powerful than the others. Both main nerve trunks are interconnected by numerous jumpers, which look like thin half-rings encircling the body.

Question 6. What departments make up the digestive system of roundworms?
Digestive system begins mouth opening located at the anterior end of the body. The intestines form a straight tube that runs through the entire body. Its anterior section is subdivided into the oral cavity and pharynx. The pharynx is followed by a poorly differentiated intestine ending in the anus.

Question 7. Describe the development of roundworm.
Getting into the faeces environment, eggs with access to oxygen in humid conditions and with enough high temperature(about 25 ° C) develop, and a larva forms under their shell. The latter in egg shells enters the human digestive system. IN small intestine it is released from the membranes, drilled into the wall of the intestine and enters the bloodstream. With the blood flow it is transferred to the liver, heart and through the pulmonary arteries to the lungs. In the lungs, the larva enters the bronchi, causing inflammation, accompanied by a cough. With sputum, the larvae enter the oral cavity, and then swallowed again with saliva. Adult sexually mature roundworms develop in the intestine.

Question 8. How can you get infected with ascaris?
Common house flies play an important role in spreading roundworm eggs and infecting people with them. Eggs can enter the human body from unwashed hands, from contaminated water, from unwashed vegetables and fruits.

Question 9. What hygiene measures should be observed to prevent ascariasis?
Hygienic measures to prevent ascariasis: compliance with the rules of personal hygiene; disposal of faeces used as fertilizers; sanitary improvement of dwellings (water supply, sewerage); cleaning drinking water; systematic medical examinations.

The solution contains answers to the questions of the educational edition and is made in an easy-to-read PDF format.