The body is covered by a flexible cuticle, which is shed four times during the growth. There are four bundles of longitudinal muscles. The nematodes have sinusoidal movements and the body takes support against particles and water films. In the body cavity the tubular gonads and the intestine are found. Special respiratory and circulatory organs are missing.
Many species have both males and females and internal fertilization.
However, in the soil fauna several species lack males and reproduce as hermaphrodites or by parthenogenesis. The mouthparts are adapted to different food sources, which among soil living forms can be roots, fungal hyphae, bacteria or other animals. In the food web of the soil the nematodes are found in several positions. Some eat bacteria, some browse on fungi or fine roots, others are true root feeders and still others are predators on microscopic animals. The relative abundance of various species constitute a fauna structure, which is partly dependent on soil factors and climate.