A phylum of microscopic wormlike animals that live as parasites inside the gonads of various marine invertebrates, including polychaete worms, bivalve molluscs, echinoderms, and flatworms. Some 45 species are known. The adult body typically measures less than 300 µm long and is covered in ciliated jacket cells; most internal organ systems are absent. Mating occurs between free-swimming males and females outside the host, and the fertilized eggs develop into ciliated larvae, which locate and enter the appropriate hosts via their genital ducts. Inside the gonad, each larval cell then develops and enlarges to contain multiple nuclei, which give rise to a new generation of tiny male and female adults inside a single cell. The presence of this amoeba-like plasmodium causes loss of fertility of the host gonad. Subsequently, the young orthonectids leave the host to become a new free-swimming sexual generation. Formerly regarded as closely related to Dicyemida, the evolutionary affiliations of Orthonectida remain uncertain.