Roundworms, or nematodes, are amazing creatures whose presence in our lives we hardly feel. They are invisible and nevertheless are the second most diverse group in the animal kingdom after insects. So, the number of free-living nematodes in one cubic meter of water or soil can exceed one million individuals. They spread everywhere and, like “gray cardinals”, being in the shade, meanwhile, play one of the main roles in all ecosystems.
General information about nematodes
Nematodes combine roundworms in the cross section and often a threadlike shape. All of them belong to the group of molting (the class of primary animals). Now described over 24 thousand species of free-living nematodes and parasitic. This is the second most diverse group of animals after insects. Based on the pace of identification and description of new species, scientists estimate their real number in a million. All species are united in 2829 genera, and they, in turn, in 267 families and 31 orders.
Nematodes are divided into free-living, parasitic, and commensals. The former have mastered not only the soil, but also water bodies (fresh and salty), they are an important part of the ecosystem. In addition to omnivorous (non-specialized) species, they include pronounced food specialists. For example, acetic acid, as the name implies, feeds on acetic acid. Many species became commensals and parasites of animals belonging to all large systematic groups, including the simplest. Their existence is known from the ancient Carboniferous.
Dimensions and body shape of nematodes
The size of free-living nematodes, as a rule, is small, up to 1 cm. Among the parasitic species, real giants are found. So, horse roundworm reaches 40 cm in length, and female Placentonema gigantean (a parasitic parasitic worm in the placenta placenta) - 8 m. At the same time, trichinella hosts of which are carnivores, including humans, are microscopic. The invasive larva of males reaches 1.16 x 0.06 mm, and the female - 1.36 x 0.06 mm. All nematodes have sexual dimorphism; females are always slightly larger than males.
The nature of the movement of free-living nematodes is determined by their anatomical features. Roundworms have a filiform or spindle-shaped body, not segmented. Less common in females are lemon-shaped or barrel-shaped. The body in the cross section is round, has bilateral symmetry with two-beam elements, and the head shows signs of a three-beam.
The coloration of free-living nematodes is not remarkable. Body color ranges from translucent to milky white with shades of yellow or pink. In the photo above, a deep-sea nematode from the Desmodorida order.
Structural features
Unlike flatworms, the mesenchyme is almost completely absent in the body of nematodes, the space between the subepitemial longitudinal muscles and the intestine is filled with the primary body cavity (pseudo-whole). The cavity fluid creates a strong pressure, which, together with the cuticle, acts as an antagonist of the longitudinal muscles. This system of 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 undifferentiation of the internal space of the animal, the whole body always participates in it.
All nematodes are devoid of the respiratory and circulatory systems, as well as flagellum cells, with the exception of some sensory organs.
Digestive system
Nematodes of all kinds have a through digestive system similar to a tube. It begins with the oral cavity, then passes into the esophagus, front, middle intestine and ends in the back. The mouth is terminal, less often shifted to the dorsal or ventral side. It is surrounded by lips and leads to the pharynx, which has a trihedral, widening with contraction lumen. 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 hind gut, which opens in males with cloaca, and in females with an anus.
Basically, free-living nematodes feed on algae, bacteria, and detritus, but there are predators among them. For example, mononkh-odnozub. In this predatory worm, a large and sharp spike protrudes from the mouth with the tip up, 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 quite primitive. There is an assumption that its main organs are the unicellular (less often multicellular) cervical gland, or the lateral intracellular canals (renets), as well as pseudo-cellulitis cells. The latter do not have ducts, their function is the isolation and disposal of metabolic products. Renetta consists of a volumetric telome and excretory duct opening outward by 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 a peri-pharyngeal nerve ring and several longitudinal nerves. The first is a single circular ganglion and, in all probability, 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 inclined forward. The dorsal nerve and ventral nerve trunk depart from it. The remaining longitudinal nerves are not directly connected with it.
In free-living nematodes (size, color, nature of movements - discussed above), sensory organs are represented by sensilla: labial papillae, tactile bristles, complementary organs of males, olfactory amphids, phasmids (sensory-glandular organs), and 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.
Nematode development
The vast majority of nematodes are dioecious animals, but hermaphrodites are also found. As a rule, they lay eggs, less often live birth occurs. In male nematodes, the posterior end of the body is bent to the abdominal side and it has a complex copulative apparatus. They have two testes with vas deferens and one ejaculatory canal. Nematode sperm have a diverse structure, no flagella, and amoeboid motility. The genitals of females are represented by a single or double set consisting of ovaries, oviducts and uterus, as well as the vagina.
Reproduction of nematodes is not accompanied by metamorphosis. As a rule, the life cycle consists of four juvenile stages and one adult. The transition between them occurs at the time of molting.