Roundworms lack an organ system. General characteristics of roundworms

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, adaptation to the environment and good survival, experts have come 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 humans and animals causes enormous damage, and the worms themselves are capable of causing various diseases.

Roundworm digestive system

Nematodes or, simply put, roundworms, unlike their congeners, are endowed with 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 the muscular walls.

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

Allocation 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

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

Biohelminths

The main carriers are humans, apes and other mammals. Biohelminths are carried by various blood-sucking insects.

Phyllaria enter the bloodstream only when the carrier is maximally active. In the case of mosquitoes, this is evening and night time. The horsefly has morning and afternoon. When fillyaria are transferred 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 concentrates in the blood arteries and lymph nodes. Leads to stagnation of blood and lymph. It is also the cause of the appearance of elephantiasis and allergies. The mosquito is the main vector.
2. Brugia malayi : the final owners are humans, some species of monkeys, the feline family. Pathogenicity and concentration is similar Wuchereria banctofti... Also transmitted by mosquitoes.
3. Oncocerca volvulus : carried by midges, and the carrier of the helminth is a person. Concentrates in the body under the skin of the head, chest, arms and legs. Promotes the formation of painful nodules. Concentrating in the area of ​​the organs of vision - 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 : concentrates in the body of the wearer, which is people, in the subcutaneous fat layer, the mesentery of the intestine and under the serous membranes. Carried by biting midges.

The most common types of roundworms are also worth mentioning.

Ascaris

Roundworm

Roundworm

Roundworms are three-layered undivided animals with bilateral (double) symmetry, the body of which is covered with a skin-muscular sac, and the space between the internal organs is filled with fluid.

Systematics. The Roundworm type is divided into classes: Gastrotrichs, Nematodes, Hairworms, Scrapers, Rotifers. The most famous and important is the Nematoda class, which will mainly be discussed.

Body shape. Most of the roundworms have a strongly elongated body, round in cross section. The ends of the body are usually pointed. However, spherical, lemon-shaped and bean-shaped forms come across. It is customary to distinguish between the esophageal region, the middle and the tail of the body.

Musculocutaneous sac. Outside, the body of nematodes is covered with a dense multilayer cuticle. Most often, it has a ringed structure. This layer has an important support and protective value.

Under the cuticle, the nematode skin itself is located - the cellular or syncytial epithelium, which is called the hypodermis. The hypodermis forms on the dorsal and ventral sides, as well as on the sides, longitudinal thickenings - hypodermal ridges. Large nerve trunks are located in the dorsal and abdominal ridges, and the channels of the excretory system in the lateral ones. The hypodermis plays an important role in the respiration of nematodes.

Musculature in nematodes is represented by one layer of longitudinal muscle cells adjacent to the hypodermis. Most often, the muscle layer is divided into dorsal and abdominal bands, which provide mobility of the body of the worms. There are also smaller muscle groups that provide mobility of internal organs - esophageal, intestinal, anal, vulvar, spicular and some others. These muscles go from the body wall to the corresponding organs.

Movement. Due to the structure of the muscular system, the movements of nematodes are extremely imperfect. These animals can lengthen or shorten the body only very slightly if all the muscles act simultaneously; Roll up in a ring-like manner if only one muscle works and bend serpentinely if the muscles work alternately. Some soil nematodes are capable of jumping several millimeters.

Body cavity. Unlike flatworms, in which the space between the body wall and internal organs is filled with parenchyma, in roundworms this space forms a cavity filled with fluid. This cavity has no walls of its own and is limited by a layer of muscles and internal organs. Such a cavity is called primary (synonyms: protocoel, hemocoel, schizocel). The primary cavity is filled with liquid under significant pressure (high cavity turgor), which maintains the shape of the nematode (support function). The products of metabolism accumulate in the liquid, which are subsequently excreted from the body (excretory function).

Digestive system nematodes are through and consists of three sections - anterior, middle and posterior intestine.

The anterior gut begins with the mouth opening surrounded by the lips. The mouth can contain teeth, a spear, or a stylet. The oral cavity is followed by the esophagus, the structure of which is quite diverse. It can be straightforward or subdivided. In a number of species, there are swellings on the esophagus - metacorpal and cardial bulbs. A muscle crushing apparatus is often located inside the cardiac bulb, which homogenizes food. Between the bulbs, a small isthmus is noticeable - the isthmus - which is encircled by a nerve ring. The esophagus has a cuticular lining. Some groups have esophageal glands that secrete enzymes.

The midgut is a tube of unilamellar epithelium. In this section, food is digested and absorbed.

The posterior part of the intestine is lined from the inside with a cuticle and opens at the posterior end of the body in females with the anus, and in males - into the cavity of the cloaca.

Excretory system... One or two large cells of the cervical gland are located in the esophageal part of the body. These cells form one or two excretory canals that stretch in the lateral ridges of the hypodermis along the entire body. In the front part, two channels are connected to an unpaired duct, which opens outward with an excretory pore. Many species have several so-called phagocytic cells that capture foreign substances from the cavity fluid. These cells are probably associated with excretory canals.

Nervous system... The central part - the nerve ring - encircles the esophagus. The ring consists of nerve fibers and a few nerve cells. Six nerve trunks extend from the ring forward to the organs of the esophageal part of the body. Several trunks also stretch back. Moreover, they are located in the hypodermis. Usually one or two nerve trunks are most developed. The close connection between the nervous system and the skin-muscular sac allows some authors to talk about the skin-muscle-nerve sac in nematodes.

Sense organs. Nematodes have developed tangoreceptors (tactile), chemoreceptors (chemical sense), and photoreceptors. Tangoreceptors are represented by papillae (small elevations on the cuticle) and bristles. These organs are located mainly at the head end, and in males and in the tail area. Chemoreceptors are represented by amphids - a kind of depressions in the cuticle, which have a different structure. Amphids are located on the lips and on the sides of the head. Free-living aquatic nematodes sometimes have paired pigment spots at the head end, equipped with a small lens. These are peculiar light-sensitive organs.

The reproductive system. Nematodes are dioecious organisms, while males are easily distinguished from females (sexual dimorphism). Males are usually smaller; their tail is hooked to the ventral side.

The reproductive system of males and females has a tubular structure. In some species, the genital tube is unpaired ( monodelph ), while others have a pair ( didelph ). In the latter case, both tubes have a common excretory duct.

In males usually a monodelphous reproductive system, it consists of a gland - testis, and excretory ducts - vas deferens and ejaculatory duct... Amoebae spermatozoa form in the testes and enter the ducts. The ejaculatory canal, together with the hindgut, open at the posterior end of the body into the cloaca. Males also have a copulatory apparatus. It consists of one or two chitinous spiculus and knuckle... Curved spicules along the groove of the shank move outward and are fixed in the genital opening of the female. In many species, males have paired wide outgrowths on the sides of the tail, forming sexual bursa... During mating, the male clasps the female's body with these outgrowths.

Reproductive system of females usually didelph. Each of the two genital tubes consists of an ovary, an oviduct, and a uterus. Both uterus lead into a common vagina, which opens with a genital opening - the vulva. The vulva is most often located in the front of the body.

A necessary condition for the manifestation of the vital activity of nematodes is the presence of drip-liquid moisture. Some species tolerate prolonged drying (10 or more years).

Meaning. Inhabiting almost all biocenoses, nematodes are of great importance. They decompose dead organisms and take part in soil formation. The high abundance of nematodes in soil and bottom silt determines their significant role in food webs.

Morphology scrub is very peculiar. The most characteristic feature of all scrapers is the presence of a proboscis at the anterior end of the body, armed with hooks and capable of being drawn into a special vagina. Under the cuticle is the hypodermis, which is a syncytium with a system of cavities - lacunae. Muscle cells also fuse with each other. Special muscles set in motion the proboscis and some parts of the reproductive system. There is a muscle cord - ligament , which extends from the proboscis sheath to the posterior end. The body cavity is primary. The nervous system consists of one ganglion and nerve trunks extending from it. The sense organs are extremely poorly developed and are represented only by small tactile papillae. The intestines of the scrapers are absent and absorption of food occurs through the integument.

Scrapers are dioecious animals. The testes in males are paired, usually compact and connected to the ligament. The vas deferens depart from the glands, which merge into the unpaired ejaculatory canal. In this organ, the ducts of the cement glands open, the secretions of which clog the genital opening of the female during mating. The female gonads - the ovaries - are also paired and located inside the ligament. However, even in young females, the ovaries disintegrate into clumps of eggs. The ligament breaks in some types of scrapers, and the eggs fall into the body cavity. Outward they are brought out through a special complex apparatus. A special uterine bell swallows eggs; at the same time, mature ones are passed into the uterus and are subsequently taken out, and immature ones are pushed back into the body cavity.

Development cycle scraper occurs with a change of owners. In some, eggs ripen in water. Others have it on land. For further development, the eggs of "aquatic" species must enter the body of an intermediate host — usually a crustacean; in "soil" species, insects serve as such a host. A larva forms in arthropods - acantor which is encapsulated and turned into acanthella with a proboscis screwed inward. When such an arthropod is eaten by the final host, Acanthella turns into an adult worm. In the developmental cycle of "aquatic" species of worms, additional hosts are often present - fish that eat crustaceans and serve as food for the final hosts - predatory fish.

Type Roundworms: structural features... The name says a lot about the structure of representatives of this type - in cross section they have a round shape. Their body is fusiform and narrowed at the ends, you can clearly see the head at the front end. The symmetry type of the body is bilateral.

Their body consists of a musculocutaneous sac, which is separated from the internal organs by a primary cavity, or pseudocele. The skin on top is covered with a dense layer of cuticle, which is produced by the epithelium of the outer integument.

Roundworms: the structure of the digestive system... The digestive tract of this group of organisms is through. The mouth opening is located at the front end of the body and is surrounded by lips and muscle tissue. It leads to the muscular pharynx, which is used to actively suck in food. Food then enters the middle, and then into the hind intestine. Metabolic products are excreted through the anus in females, or into the cloaca in males.

Roundworms: respiration and secretion... roundworms are enough. This includes the large unicellular cervical gland. Metabolic products accumulate in the cavity fluid, after which they move to the gland through two longitudinal lateral channels and are removed to the outside. The cervical gland has a separate excretory opening that opens into the abdominal opening. Some metabolites, such as ammonia, are excreted directly through the body wall.

As for respiration, organisms do not have specially designed organs for this. Oxygen is absorbed by the intestinal tissues. In the event that you have to live in conditions of oxygen starvation, roundworms can switch to anaerobic breathing.

Roundworms: reproduction... Representatives of this belong to dioecious organisms. At the same time, a rather noticeable sexual dimorphism is also observed - in the male, the posterior end is bent over the abdominal part of the body.

The female reproductive organs are represented by the ovaries, uterus and oviducts. The male has testes, vas deferens, and an ejaculatory duct. The sperm of the male are introduced into the vagina of the female through it. Interestingly, the sperm do not have flagella, have limited mobility and are amoeboid in shape.

How roundworms differ from flatworms? In fact, the differences between these two types of animals are quite noticeable. To begin with, it is worth mentioning the shape of the body - in cross-section, Flatworms have a flattened body. Another significant difference is the primary body cavity, which is absent in representatives of Flatworms. In addition, a through digestive system already appears in Nematodes. It's worth noting that Flatworms are hermaphrodites.

The type of roundworms is nematodes, which are very numerous and varied. This type of living organisms unites almost 25 thousand species, united in 31 orders and 3 classes.

They differ in the variety and size of roundworms - from a few micrometers to several meters.

The most microscopic roundworms are no more than 80 micrometers in size. Nematodes have a solid, unsegmented, filiform, or fusiform body. Some worms are in the shape of a barrel or lemon.

The body of roundworms is a developed muscular system, and is covered in some species with a smooth, in others - with a ringed nine-layer cuticle (skin). This gives them the opportunity to adapt to different living conditions: in water, in the ground, in the organisms of animals and humans.

Under the cuticle is the subcutaneous layer - the hypodermis, which forms 4 chords around the circumference of the body:

• Dorsal - on the back.
• Ventral - on the abdomen.
• 2 side.

The nervous system plays an important role in the activity of the internal organs of nematodes, therefore it is quite developed.

Inside the spinal and abdominal chords, there are parallel nerve fibers connected by transverse bridges, and collected into single nerve trunks. The first such bridge is located in the worm's pharynx. From them, nerve fibers leave to muscles and other organs. Sensory sensory nerves are located on the sides.

The sense organs in roundworms are poorly developed, and are represented by bristles in the abdominal part of the body. With these bristles, nematodes sense their environment, their location. Small dimples serve as organs of smell. Large, to some extent developed species of worms have the simplest organs of vision.

Digestive organs

The digestive system of roundworms in structure resembles a through tube. It begins with the mouth, then the esophagus follows, which passes into the anterior, then the middle intestine and ends with the posterior intestine, which extends outward on the abdomen from the tail end of the worm.

The main features of roundworms are that they have:

• The whole is the hollow space of the body, filled not with connective tissue, like in other worms, but with fluid. The whole is called the primary body cavity.
• The tail of the intestine, ending with the anus.

The mouth of the roundworm leading to the easily expanding pharynx is supplied with lips. There are teeth along the edges of the mouth opening, with which the worm grinds food. The pharynx opens the entrance to the middle intestine and acts as a kind of pump.

In the process of contraction of the radial muscles, food is sucked into the intestines. The movement of food is also facilitated by the intracavitary fluid, which forms pressure in the intestine.

Roundworms lack hematopoiesis, and there is no respiratory system. But gas exchange still takes place. We can say that the worms "breathe" with the cuticle, or body integument. The energy of the nematode is obtained from glycogen, which is broken down in the internal organs of the worm.

Decay products are excreted from the body by the primary cavity fluid. Nutrients after processing in the intestines also enter this fluid, and with it they are delivered to other parts of the body.

The excretion system is represented by two closed ducts, which are displayed on the nematode abdomen. The decomposition products formed in the body of the roundworm first enter the coelomic fluid, from it into the ducts of the excretory system, from where they are removed.

A feature of roundworms can be called their sexual division into males and females. The genitals of both have an elongated tubular shape. In the paired genitals of the female, represented by a double uterus, as well as two ovaries with oviducts, from several tens to several thousand eggs are formed, after a sexually mature male with needles formed from the cuticle introduces spermatozoa from the vas deferens into the female genital slit.

The systematization of roundworms is still open. Initially, all worms (round, flat, annelid) were combined into one type - worms. In this taxonomy, the class of roundworms was subdivided into two subclasses :

• Adenophorea (Adenophorea) ;
• Secernentea .

The difference between the subclasses consisted in the presence or absence of phasmids - tactile organs. After combining roundworms into a separate type group, a classification was proposed that distinguishes six classes :

• Nematoda ;
• Gastrotricha ;
• Kinorinchi ;
• Hairy (Gorciiacea) ;
• Rotifers (Rotatoria) ;
• Scrapers (Acanthocephala) .

Subsequently, the gastrointestinal, rotifers, hairworms, and side-scrapers were separated into separate types, and the kinorinchs became a class of the Cycloneuralia clade. The class of Nematodes or, in fact, Roundworms has also been revised. Modern taxonomy (2011) highlights three classes of Roundworms :

• Chromadorea ;
• Enoplea ;
• Dorylaimea .

Rice. 1. Human roundworm is a representative of chromadoria.

Some experts consider the Dorylaimea class to be a subclass of Enoplea.

The classes include 31 orders, over 200 families, and about 3000 genera. Currently, about 30 thousand species have been described, but the variety of nematodes suggests about a million species.

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