They occur as parasites in animals, man and plants but also as free-living forms in soil, and bottom sediments of lakes and seas. The nematodes are the most abundant group of multicellular animals in these habitats. One square-meter of soil, sea or lake bottom may contain several millions of nematodes.
Nematodes occur as parasites in most species if animals including man.
They also occur as parasites and patogens in a great number of plant species and create extensive crop losses especially in tropical areas. More than 700 species have so far been found in Sweden. Most nematodes are microscopic transparent animals often less than 1 mm long. The body is covered by a flexible cuticle, which is shed four times during the growth. There are four bundles of longitudinal muscles.