The zygotes undergo a specific number of divisions and the adult contains a fixed number of cells. Very numerous in both species and individuals. Found in fresh water, marine, moist soil, tissues of plants, and tissues and body fluids of animals. A complete digestive tract is present and nutrients are transported through the body in the pseudocoelomic fluid. A tough transparent cuticle forms the outer body covering.
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.