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Orthonectida ( /ˌɔrθɵˈnɛktɪdə/) is a small phylum of poorly-known parasites of marine invertebrates (Hanelt et al. 1996) that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.

The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms. They are gonochoristic, with separate male and female individuals (Barnes 1982). When they are ready to reproduce, the adults are released from the host, and sperm from the males penetrates the bodies of the females to achieve internal fertilisation. The resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts. Once it finds a host, the larva loses its cilia and develops into a syncitial plasmodium larva. This, in turn, breaks up into numerous individual cells that become the next generation of adults (Barnes 1982).

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