Within the Nematoda, a range of reproductive systems occur.
Most nematodes are diecious, and have males and female adults that reproduce sexually.
Parthenogenetic reproduction and hermaphroditic reproduction also occur in some nematode species. Typically there is one testis in the male and two ovaries in the female, although in some nematode species a different morphology occurs. Possessing one genital tube is termed monodelphic, while possessing two genital tubes is termed didelphic. Sexual differentiation may occur as early as the second-stage juvenile, but only adult nematodes show fully developed sexual differentiation and sexual characters. Oogenesis starts at the ovary tip, in the germinal zone.