Longitudinal muscles are present and provide for whip-like movements. Dioecious with females larger than males. Sexual reproduction only, with internal fertilization. Female may produce 100,000 or more resistant eggs per day. Like rotifers, nematodes have a fixed number of cells as adults.
Nematodes fill various roles in the community.
Free-living forms are important in decomposition and nutrient cycling. Plant parasitic forms are important agricultural pests.