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Nematodes are almost unbelievably abundant. The number of described species is around 12,000, but the true number may be closer to 500,000. One study reported around 90,000 individual nematodes in a single rotting apple. Some species are generalists, occurring across wide areas and in many habitats; others are much more specialized. Nematodes have colonized nearly every conceivable habitat on earth.
Many nematodes are free living and act as decomposers and predators on microorganisms. But nematodes also include parasitic species, a number of which affect humans directly or indirectly. These include the common roundworms, which probably infest more than half the world's humans.
Most nematodes are dioecious. Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them.
Roundworms (nematodes) are surrounded by a strong, flexible cuticle. Their body plan is simple.They run in the longitudinal direction only. A true coelom is lacking, instead, nematodes have a "pseudocoel". The cavity of the pseudocoel is small, being mostly filled with an intestine and oviducts or testes. A simple nervous system consists of a ring of nervous tissue around the pharynx.
