Roundworms, or nematodes, are amazing creatures, the presence of which we practically do not feel in our lives. They are invisible and yet they are the second most diverse 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 have spread everywhere and, like "gray cardinals", being in the shadows, meanwhile play one of the main roles in all ecosystems.
General information about nematodes
Nematodes combine worms that are round in cross section and often filiform-elongated. All of them belong to the group of molters (the class of protostomes). More than 24 thousand species of free-living and parasitic nematodes have now been described. This is the second most diverse group of animals after insects. Based on the rate at which new species are being identified and described, scientists estimate the actual number to be in the millions. All species are combined into 2829 genera, and they, in turn, into 267 families and 31 orders.
Nematodes are divided into free-living, parasitic and commensal. The first mastered not only the soil, but also water bodies (fresh and s alty), they are an important part of the ecosystem. In addition to omnivorous (non-specialized) species, they also include pronounced food specialists. For example, the acetic eel, as the name implies, feeds on acetic acid. Many species have become commensals and parasites of animals belonging to all major systematic groups, including protozoa. Their existence has been known since the ancient Carboniferous.
Size and body shape of nematodes
The size of free-living nematodes, as a rule, is small, up to 1 cm. Among the parasitic species, there are real giants. So, the horse roundworm reaches a length of 40 cm, and the female Placentonema gigantean (a worm that parasitizes in the placenta of the sperm whale) - 8 m. At the same time, trichinella, the owners of which are carnivorous animals, including humans, have microscopic dimensions. The infective larva of males reaches 1.16 x 0.06 mm, and the female - 1.36 x 0.06 mm. All nematodes are sexually dimorphic, with females always slightly larger than males.
The nature of the movement of free-living nematodes is determined by their anatomical features. Roundworms have a filamentous or fusiform body, unsegmented. Less common in females are lemon-shapedshape or barrel-shaped. The body is round in cross section, has bilateral symmetry with elements of a two-beam, and the head shows signs of a three-beam.
The coloration of free-living nematodes is unremarkable. 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.
Building features
Unlike flatworms, mesenchyme is almost completely absent in the body of nematodes, the space between the subepithelial 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 for 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, due to the indivisibility of the internal space of the animal, the whole body always participates in it.
All nematodes are devoid of respiratory and circulatory systems, as well as flagellar cells, with the exception of some sensory organs.
Digestive system
Nematodes of all kinds have a tube-like digestive system. It begins with the oral cavity, then passes into the esophagus, anterior, middle intestine and ends with the back. The mouth is terminal, rarely shifted to the dorsal or ventral side. It is surrounded by lips and leads into the pharynx, which has a trihedral,lumen that expands with contraction. It is used to suck food. The pharynx has a complex structure and, depending on the lifestyle of nematodes (predators, parasites), can be equipped with various “weapons”. The digestive system ends with the posterior intestine, which opens in males with a cloaca, and in females with an anus.
Mostly free-living nematodes feed on algae, bacteria, detritus, but there are also predators among them. For example, mononkh-one-tooth. In 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 the 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 pseudocoelomite 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 nervous system of nematodes is represented by the peripharyngeal nerve ring andseveral 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 to it.
In free-living nematodes (size, coloration, character of movements - discussed above), the sensory organs are represented by sensilla: labial papillae, tactile setae, male supplementary organs, olfactory amphids, phasmids (sensory glandular organs), as well as terminal tail 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. As a rule, the life cycle consists of four juvenilestages and one adult. The transition between them occurs at the moment of molting.
