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Nematodes: general characterization of nematodes
Medical expert of the article
Last reviewed: 04.07.2025
Nematodose - diseases caused by parasitic roundworms nematodes. They are common on all continents. In the world about 3 billion people are infected with nematodes.
Nematodes have an elongated, cylindrical body shape. The cross-section of the body is round. The size of nematodes ranges from 1 mm to 1 m and more.
Externally, nematodes are covered with a skin-muscle sac formed by the cuticle, hypodermis and one layer of longitudinal muscles. The cuticle is multilayered, it serves as an external skeleton, protects the body of nematodes from mechanical damage and chemical effects. Under the cuticle lies the hypodermis, which is a symplast and consists of a layer underlying the cuticle - the subcuticle and longitudinal ridges, the number of which varies from 4 to 16 or more. Metabolic processes actively occur in the hypodermis and intensive biosynthesis occurs. Under the hypodermis lies one layer of longitudinal muscles, divided by hypodermis ridges into several longitudinal bands. Nematode movements are limited. The body bends only in the dorsoventral plane due to the fact that the abdominal and dorsal muscle bands act as antagonists. Inside the skin-muscle sac is the primary body cavity, which has no special lining and contains cavity fluid and internal organs. The cavity fluid is under high pressure, which creates support for the muscles (hydroskeleton) and plays an important role in metabolic processes. In some nematodes, this fluid is toxic.
The digestive, excretory, nervous and reproductive systems are well developed. The respiratory and circulatory systems are absent.
[ Digestive system of nematodes
The digestive system is represented by a straight tube, which is divided into three sections - anterior, middle and posterior. It begins with the mouth, located at the anterior end of the body. Most nematodes have a mouth surrounded by three lips. Some species have a mouth capsule armed with teeth, plates or other cutting elements. The mouth is followed by the pharynx and a cylindrical esophagus, which in some species has one or two expansions (bulbs). The esophagus is followed by the midgut, which passes into the posterior, ending in the anus. Some species of nematodes do not have an anus.
Excretory system of nematodes
The excretory system is represented by 1-2 unicellular skin glands, replacing protonephridia. Two long lateral canals extend from the gland, located along the entire body of the nematode in the lateral ridges of the hypodermis. At the back, the canals end blindly, and in the front part they join into one unpaired canal, opening outward, sometimes near the anterior end of the body. Nematodes have special phagocytic cells (1-2 pairs), in which various insoluble metabolic products are retained and accumulated. They are located in the body cavity along the lateral excretory canals in the anterior third of the body.
Nematode nervous system
The nervous system is represented by the peripharyngeal nerve ring, surrounding the anterior part of the esophagus. Nerve trunks extend forward and backward from the ring. Six short nerve branches extend forward. Six trunks also extend backward, among which the most powerful are the dorsal and ventral, passing in the hypodermal ridges. Both main nerve trunks are connected to each other by numerous commissures, which look like thin semicircles encircling the body alternately on the right and left sides. The sense organs are poorly developed. There are organs of touch and chemical sense.
Reproductive system of nematodes
Nematodes are dioecious and exhibit external sexual dimorphism. Females are larger than males. Some males have a posterior end that is twisted toward the ventral side. The male has one tubular testicle that passes into the vas deferens, followed by the ejaculatory duct that opens into the posterior section of the intestine. Males have a cloaca. Near the cloaca, males have copulatory spicules. In some nematodes, in addition to the spicules, males have a copulatory bursa, which is an expanded and flattened wing-shaped lateral part of the posterior end of the body.
In females, the reproductive system is paired, tubular, and consists of ovaries, oviducts, uterus, and vagina. The narrowest, blindly closed sections of the tube are the ovaries. They gradually pass into wider sections that function as oviducts. The widest sections of the uterus are connected to each other and form an unpaired vagina that opens outward on the abdominal side in the anterior third of the nematode's body. Sexual reproduction and internal fertilization are characteristic of nematodes.
Development of nematodes
Most nematodes lay eggs, but there are also viviparous species. The formation and maturation of larvae most often occurs in the external environment. In some species, the development cycle can be completed in one host organism. In most species, the larva develops in the egg up to the invasive stage in the external environment and emerges from it in the intestine of the host that swallowed the egg. During the development process, the larvae molt several times.
In some nematodes, the larvae, having emerged from the egg in the external environment, are capable of leading a free life in the soil. There are rhabditiform and filariform larvae. Rhabditiform larvae have two expansions (bulbus) in the esophagus, while in filariform larvae, the esophagus is cylindrical. The larvae can actively penetrate the skin of the host, and not only through the mouth.
The development cycles of nematodes are varied. Most nematodes are geohelminths. They develop directly, without changing hosts. The larvae of many geohelminths typically migrate through the organs and tissues of the host to the final location, where they reach sexual maturity. Some geohelminths develop without larval migration. Geohelminths that affect humans cannot parasitize animals. Nematodoses caused by these helminths are classified as anthroponous diseases. Other types of nematodes are classified as biohelminths. They develop indirectly. They need an intermediate host. These can be blood-sucking insects, crustaceans, or the same organism can serve successively as the final and then intermediate host.
Human infection with biohelminth nematodes occurs both through the alimentary route when eating an intermediate host, and as a result of transmission by a carrier.
Most nematodes that parasitize humans live in the human digestive system in their mature stage. Some are localized in the lymph nodes and vessels, in the connective tissue, under the skin of the extremities, in the subcutaneous fat.