Nematodes, or roundworms (genus Nematoda, Nematodes)
It’s a type of Nemathelminthes worm. Currently, about 80,000 described species of nematodes exist. Still, the estimates of real diversity, based on the rate of new species description (in particular, specific parasitic insects), suggest the existence of about a million species.
Non-symbiotic (free-living) nematodes live in salt and fresh water, and soil. Many of them have become parasitic and cause nematode diseases of plants cause or nematodosis in animals and humans. The best-known human parasites among the roundworms are ascaris, pinworms, trichinella, hookworms, guinea worms.
Body length ranges from 80 microns to 8.4 m (this is the length of a parasite living in the sperm whale placenta). The body of the nematode is fusiform. In the cross-section, the body is round, at the base, it has bilateral symmetry of the body with biradiate elements. The anterior end of the body (head) also shows signs of triradial symmetry.
Roundworms have developed skin-muscular sac. The body is covered with a thick elastic cuticle allocated external epithelium (hypodermis). In parasitic forms, the epithelium can have a syncytial structure. There are no cell boundaries in it, and it consists of a cytoplasmic mass with nuclei included in it. Under the hypodermis, there is a layer of longitudinal muscles, dividing by the hypodermis ridges into 4 strands.
More or less large forms have a primary body cavity – a pseudocoelom, between the skin-muscular sac and the body’s internal organs. In the shallow marine nematode, the body cavity is almost absent, and the slit space between the body wall and organs is filled with an extracellular matrix.
Except for some sensory organs, nematodes lack flagellar cells.
The digestive system of roundworms is like a tube, hollow. In several parasitic units the intestine transformed into a lumenless trophosome. Mouth is terminal or rarely shifted to the ventral or dorsal side. Mouth surrounded by lips and leads into a muscular pharynx. With its help, the nematodes suck up food. The pharynx has a complex structure, and in many groups of preying and parasitic nematodes it can be strengthened with structures. The pharynx opens into the endodermal midgut. The throat opens in endodermic origin midgut. The digestive system ends with the posterior intestine, which opens with the anus in females, and the cloaca in males.
Nematodes do not have specialized respiratory organs. The absorption of oxygen occurs through the entire surface of the body. The parasites are anaerobes.
It is assumed that the principal organs of the excretory system of nematodes are unicellular (or more rarely two-or multicellular), cervical glands, or Renette glands, and large cell pseudo-coelomocytes. In fact, the excretory canals are derivatives of the excretory cell. Pseudo-coelomocytes do not have ducts – they isolate and utilize metabolic products. In addition, ammonia can be released from the nematode body by diffusion through the body wall.
The nervous system consists of a parapharyngeal nerve ring and several longitudinal nerves. The nerve ring is located at the middle of the pharynx and is inclined forward with its dorsal edge (in some groups, the tilt is reversed). Structurally nerve ring is a single circular ganglion and serves as the main body of the association organ. The ventral nerve trunk and the dorsal nerve originate from it. The remaining longitudinal nerves are not directly connected with it. The ventral nerve trunk contains the body of neurons; other longitudinal nerves do not have bodies and are bundles of neurons in the trunk ventral appendages. All longitudinal trunks pass intraepithelially – in the ridges of the hypodermis. The sense organs are represented by numerous sensilla: tactile setae, labial papillae, male supplementary organs, olfactory amphids, and sensory glandular organs – phasmids. These sense organs are mechano-, chemo-, or less often photoreceptors or have mixed sensitivity and are always associated with glandular cells. The main organs of distant chemical reception are amphids – complex paired organs at the anterior end of the body, which have a variety of shapes. Other head sensory organs include the head sensilla, subordinate in their arrangement to radial symmetry and arranged in three or two rows. In some free-living species, internal mechanoreceptors, metanemes have been found.
The vast majority of nematodes have a distinct external sexual dimorphism. In males, the posterior end of the body is bent vertically and has a complex copulatory apparatus. Various supplementary organs and (in rhabditid nematodes) bursae help hold the female during copulation. Sperm are inserted using spicules that protrude from the cloacal opening. The internal genital organs are originally paired and have a tubular structure. Females have a single or double set of ovary, ovarian tube, and uterus; the vagina is always one. Males have one or two testes with spermoduct and impair ejaculatory duct. Nematode sperm have an extremely diverse structure, are devoid of flagella, and have amoeboid (but not actin) motility.
Development occurs without metamorphosis. Usually, four juvenile stages and one adult are distinguished in the life cycle. The transition between stages is carried out during molting. Since some of the molts can occur in the egg membranes, the number of free stages can be reduced. In rhabditid nematodes, the so-called Dauer stage is common – a modified third juvenile stage that plays a dispersal role and survives adverse environmental conditions.
Roundworms/nematodes can be effectively treated with Vermox (Mebendazole).