Nematode

The nematodes (Nematoda), also called nematodes (ancient Greek νῆμα nema, German 'Faden') or eelworms, are a phylum of the animal kingdom with the most species to date. More than 20,000 different species have been described so far. They are probably the most individual-rich group among multicellular animals: According to one estimate, they make up about 80% of all multicellular animals. They are mostly relatively small, white to colorless, filamentous worms that live in moist media. The majority of species feed on microbes, but there are also predatory and numerous parasitic species that infest plants, animals and humans.

Nematodes have successfully adapted to almost every terrestrial and aquatic ecosystem, including extreme habitats such as deeper regions of the Earth's upper crust and the polar regions. In many habitats, they often represent the largest group in the metazoan fauna in terms of both number of individuals and species diversity.

Physiology

Nutrition

The food varies and ranges in free-living species from bacteria and algae to fungi, carrion, faeces and predatorily preyed upon animals. There are often small appendages on the mouth that are used for feeding or groping. The food is pulled in there and crushed by strong muscles. The food then passes from there into a simple long intestinal tract where it is processed and digested. Nematodes do not have a vascular system with which to distribute food components throughout the body. Instead, the nutrients are processed in the intestinal tract, and from there they pass directly through the walls to the body cells where they are needed.

The intestine may also contain endosymbiotic microorganisms (bacteria and fungi) that are required for the breakdown of certain food components, e.g. for the degradation of cellulose. In addition, endogenous cellulases have been found in a few species such as Bursaphelenchus xylophilus and the beetle-dwelling Pristionchus pacificus. For the origin of their cellulase genes, a horizontal gene transfer, starting from their endosymbionts, was proclaimed.

Breathe

Oxygen uptake functions similarly to digestion. Since the nematodes have no respiratory organs and no vascular system, the oxygen is absorbed through the skin and diffuses directly to the body cells.

Reproduction

Reproduction is sexual, usually with two separate sexes. Males are typically smaller than females and often have a characteristically curved tail. However, self-fertilizing hermaphrodites, such as Caenorhabditis elegans, are not uncommon. Parasitic species often have a rather complicated life and reproductive cycle with alternation of generations, which may be accompanied by host change or organ change in the host. Here, the microfilariae, i.e. the larvae of the worm, can be ingested by mosquitoes and passed on to other final hosts in the next developmental stage.

Skinning

The nematodes moult and are therefore classified as moulting animals (Ecdysozoa), as well as on the basis of RNA studies within the primordial mouths (Protostomia). In free-living species, development usually takes place directly with four moults in the course of growth.

Habitats

Nematodes occur almost everywhere, in the sea, in freshwater and in terrestrial ecosystems. They are generally considered to be "common and omnipresent" and are often represented by more species and individuals in an ecosystem than any other group of multicellular animals (Metazoa). They are very significant in soils, where they occupy several of the lower trophic levels. Furthermore, nematodes have also colonized extreme habitats. The species Halicephalobus mephisto has been found in South Africa in fissure waters at depths of up to 3.6 kilometres, making it the deepest multicellular animal living in the Earth's crust. Others inhabit the soils of the McMurdo Dry Valleys in Antarctica, where they are able to survive extremely adverse conditions, where the interaction of extreme cold, salinity and drought means that there is effectively no soil water available, by entering a dormant stage (anhydrobiosis). There are also a significant number of parasitic species, both in plants (see for example turnip greens) and in animals, including humans. Nematodes that parasitize humans and live in their intestines include, for example, the roundworm (Ascaris lumbricoides), the whipworm (Trichuris trichiura), the medina worm (Dracunculus medinensis), the pinworm (Enterobius vermicularis) and the dwarf pinworm (Strongyloides stercoralis), whereas the filariae Wuchereria bancrofti, Brugia malayi and Loa loa live in the lymphatic vessels and subcutaneous fatty tissue respectively. in the subcutaneous fatty tissue.

Most free-living nematodes are microscopic and belong to the meiofauna. Only some animal parasites can become significantly larger.