A simple nervous system consists of a ring of nervous tissue around the pharynx that gives rise to dorsal and ventral nerve cords running the length of the body. Nematodes move by contraction of the longitudinal muscles. Because their internal pressure is high, this causes the body to flex rather than flatten, and the animal moves by thrashing back and forth. No cilia or flagellae are present. Nematodes do not have flame cells.
Most nematodes are dioecious.
Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them. The sperm are unique in that they lack flagellae and move by pseudopodia, like amoebas. Development of fertilized eggs is usually direct. Nematodes are almost unbelievably abundant. One study reported around 90,000 individual nematodes in a single rotting apple.