All roundworms have. Type roundworms general characteristics of the type
Type of roundworms - nematodes, very numerous and diverse. This type of living organisms unites almost 25 thousand species, united in 31 orders and 3 classes.
The sizes of roundworms differ in variety - from a few micrometers to several meters.
The most microscopic roundworms have a size of no more than 80 micrometers. Nematodes have a whole, unsegmented, filamentous, or spindle-shaped body. Some worms are barrel-shaped or lemon-shaped.
The body of roundworms is a developed muscular system, and is covered in some species with a smooth, in others - ringed nine-layer cuticle (skin). This allows them to adapt to different conditions habitats: in water, in the earth, in 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.
Nervous system plays important role in the activity of the internal organs of nematodes, so it is quite developed.
Inside the spinal and ventral chords are located parallel nerve fibers, connected by transverse jumpers, and assembled into single nerve trunks. The first such jumper is located in the throat of the worm. From them, nerve fibers depart to the muscles and other organs. On the sides are sensitive sensory nerves.
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 surroundings, 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 is similar in structure to a through tube. It begins with the mouth, then follows the esophagus, which passes into the anterior, then the middle intestine and ends with the posterior intestine, which goes out on the abdomen from the tail end of the worm.
The main features of roundworms are that they have:
- The whole is a hollow space of the body filled with connective tissue, like other worms, but liquid. Whole is called primary cavity body.
- The caudal part of the intestine, ending with the anus.
The roundworm's mouth leads into an easily expanding pharynx, provided with lips. Along the edges of the mouth opening there are teeth with which the worm grinds food. The pharynx opens the entrance to the midgut and acts as a kind of pump.
In the process of contraction of the radial muscles, food is sucked into the intestine. The movement of food is also facilitated by intracavitary fluid, which forms pressure in the intestines.
Roundworms lack hematopoiesis, and respiratory system. But gas exchange still occurs. We can say that the worms "breathe" the cuticle, or integument of the body. The energy of the nematode is obtained from glycogen, which is broken down in the internal organs of the worm.
The decay products are excreted from the body by the primary cavity fluid. Nutrients after processing in the intestines also enter this fluid, and are delivered to other parts of the body.
The excretory system is represented by two closed ducts that lead to the abdomen of the nematode. The decomposition products formed in the body of the roundworm first enter the coelom fluid, from it into the ducts of the excretory system, from where they are excreted.
A feature of roundworms can be called their sexual division into male and females. The genitals of both have an elongated tubular shape. In the paired genital organs 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 the sexually mature male introduces spermatozoa from the vas deferens into the female genital slit with needles formed from the cuticle.
body integuments
The body is formed by a skin-muscular sac covered with a cuticle. The cuticle cannot be stretched, so roundworms periodically shed it and increase in size.
muscles
Roundworms have 3 types of muscles: dorsal, abdominal and longitudinal, providing bending of the worm in different directions.
In the digestive system of worms, evolutionary changes have occurred, characterized by the appearance of the hindgut and anus. As part of their digestive system, there are 3 sections - anterior, middle and posterior. As part of anterior section has a mouth, pharynx and esophagus. Digestive processes take place in the middle section.
Respiratory system
Roundworms do not have a respiratory system. The entire surface of the body is used for gas exchange.
Circulatory system
There is no circulatory system. Transport of substances is carried out by hemolymph.
Excretory organs are protonephridia or skin glands called cervical. Some roundworms do not have excretory organs.
Nervous system
The nervous system has a ladder type. It consists of a peripharyngeal nerve ring with six nerve trunks that are connected with jumpers.
sense organs
Roundworms are endowed with organs of touch and smell. Free-living representatives have primitive eyes.
Regeneration
Roundworms are not capable of regeneration.
reproduction
Reproduction is sexual with internal fertilization. Most representatives of the type Roundworms are dioecious. Some have sexual dimorphism. The development of roundworms is indirect, there is a larval stage.
Classification
Type Roundworms are divided into 5 classes:
Gastrociliary.
Actually roundworms, or nematodes.
Hairy.
Rotifers.
Scrapers.
How are roundworms different from flatworms?
The difference between these types:
1. Flatworms have flat shape body. characteristic of roundworms round form body.
2. The digestive system of flatworms is non-permeable: there is no anus. Roundworms developed an anus, and consequently their herbal system became permeable.
3. Roundworms reproduce only through the sexual process, and flatworms both sexually and asexually.
4. All representatives of roundworms are dioecious. Flatworms are hermaphrodites.
5. Flatworms are capable of regeneration, but roundworms are not.
The type of roundworms or Nemathelminthes is one of the most numerous among the animal kingdom. There are over 20,000 species.
Turbellarians are considered their ancestors, but in the course of evolution, roundworms acquired special structural features, which allowed them to form a separate group.
The following classes of roundworms are distinguished:
- Class Nematodes;
- class ventral;
- Kinoryncha class;
- Hairy class;
- Class Rotifers.
Features of the structure of roundworms
Sizes 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 the absence of segmentation. A dense cuticle completely covers the body from the outside. The body cavity of roundworms is surrounded by a skin-muscular sac. It contains a liquid that fills the space between 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 in 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 opening, enters the pharynx and esophagus. digestion food bolus goes in the middle section, and after absorption nutrients, the remainder is expelled.
The movement of food is now carried out only in one direction, which contributed to better digestion.
excretory system
The excretory function is performed by the cervical gland, a large unicellular formation, from which a pair of lateral tubules departs. They open outward through the excretory pore.
Ammonia is able to leave the body of roundworms through skin by diffusion.
reproduction
Most representatives of the type are dioecious organisms. Often females and males have different appearance(phenomenon of sexual dimorphism). Development is direct, without a larval stage, and indirect with a change of hosts.
The reproductive system of roundworms is presented in the form of tubes. In males, this is more often one tube, which is divided into specialized departments. At the beginning, the testis is located, where the production of spermatozoa takes place, 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), spermatozoa are brought out.
The female genital tract consists of two paired tubes. They are blindly closed in the initial section, here are the ovaries, where there is a constant reproduction of germ cells. The gametes travel through the oviducts and enter the uterus, where internal fertilization occurs.
Roundworm eggs are surrounded by a thin shell rare cases she's fat. 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 association body. The ventral and dorsal nerves directly depart from the main ganglion and lie in the hypodermis, other nerve fibers are not connected with it.
The sense organs of roundworms are poorly developed. There is a chemical reception, organs of touch, free-living sea views have photosensitive cells.
At the distal end of the body of roundworms 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 ringed worms. Annelids differ from roundworms in having a closed circulatory system represented by the abdominal and dorsal vessels.
Comparison of flat and roundworms, their similarities and differences
The nutrition of nematodes depends on the lifestyle, some consume algae and bacteria, others feed on the host's body, there are also predatory species.
Significance in nature and human life
Roundworms are an integral part of food chains. Free-living individuals absorb bacteria, protozoa, and themselves become food for fish and crustaceans.
Species living in the ground feed on decayed organic matter, thus participating in soil formation.
Roundworms can infect fungi and plants, significantly reducing yields. The nematodes enter root system and lead to the death of the rhizome, thereby inhibiting plant growth. Cereal and vegetable crops (onion, potato, wheat nematodes) are most often infected.
The body of roundworms has a cylindrical shape, it is elongated in length with points at the ends. The movement of roundworms occurs due to contractions of the muscle fibers that make up their body.
The digestive system of nematodes is represented by the primary cavity in which the process of digestion takes place. The alimentary canal is divided into three sections: the middle, anterior and hindgut.
The foregut of roundworms begins with oral cavity and goes into the throat. This is where food absorption takes place. Digestion of food occurs in the midgut, absorption also takes place here. useful substances. hindgut ends at the anus.
The annular peripharyngeal ganglion, as well as the nerves extending from it, form the nervous system of roundworms. Nematodes have organs of touch and taste. Some types of free-living worms have photosensitive eyes.
Reproduction of roundworms
Roundworms belong to the species of dioecious animals. Reproduction occurs exclusively sexually. For some species of roundworms, sexual dimorphism is inherent - outward difference male from female.
The reproductive system of the female is represented by the oviducts, uterus, unpaired vagina and paired ovaries, the male has a vas deferens, an unpaired testis, and a copulatory apparatus.
Roundworms are characterized by internal fertilization with incomplete transformation (including the larval stage).
Pinworms parasitize the human large intestine. Females lay their eggs around the anus, which often causes itching. If hygiene is not observed, the eggs enter the human body. Often pinworms do not exceed 1 cm in length.
Origin of roundworms
Although the question of how and from whom roundworms evolved is still not completely closed, there is a fairly convincing theory that their ancestors were marine ciliary flatworms. Despite the differences in the structure of ciliary flatworms and roundworms (in particular, the absence of cilia, ring and diagonal muscles, the body is round in section with internal cavity where the ciliary has a solid jelly-like parenchyma), there is still a link - a primitive roundworm, belonging to the class of ventral, which lives in the bottom layers of water bodies. It just has the signs of both those and other animals.
The structure of roundworms
1. Roundworms are three-layer animals with bilateral symmetry.
2. The bodies of their worms resemble threads, spindles, barrels or lemons, depending on the species.
3. Worms are very different in size - from very tiny, not reaching a millimeter, to eight-meter giants.
4. Outer cover - cuticle, it may have transverse ring grooves or be equipped with attachment devices.
5. The next inner layer, hypodermis, does not consist of individual cells surrounded by walls, but of separate "pieces" of the cytoplasm with nuclei, between which cytoplasmic bridges are built. This type of tissue is called synthycium. Specific longitudinal ridges stretch in the hypodermis: ventral, dorsal, and a pair of lateral ones.
6. Roundworms only have longitudinal muscles. Thus, the cuticle, hypodermis and internal muscles make up skin-muscle sac.
7. Roundworms for the first time on the evolutionary path acquired primary cavity body - the so-called schisogoal which does not yet have an epithelial lining. Inside the cavity there is a fluid under pressure, with its help nutrients are redistributed.
Organ systems
3. excretory system - protonephridial. It is represented by unicellular or multicellular glands on the neck of the worm, lateral excretory ducts and pseudocoelomocyte cells.
4. Nervous system ladder type consists of a ganglionic ring located near the pharynx, two nerve trunks extending from it, and several more nerves that are connected by jumpers.
5. sense organs mostly poorly developed, there are chemoreceptors and various sensilla responsible for touch and smell.
6. Among the roundworms, hermaphrodite species were found, but in the vast majority they separate sexes with distinct sexual dimorphism.
7. Fertilization nematodes have internal, females different types capable of both laying eggs with larvae inside, and giving birth to "ready" larvae. Interestingly, larvae can hatch from eggs while still inside the host, before entering the environment.
