#orthonectida

Nematomorpha: Horsehair Worms. Until adulthood, all Horsehair Worms are parasites of various insect species. They are mostly freshwater, with a few saltwater species. As the larvae grow into adulthood, many can alter the behavior of their insect hosts by compelling them to fall into bodies of freshwater. When it senses water, the adult worm emerges to spawn. These parasites are important in controlling insect populations, and in some cases, providing a food source for fish. Without them, the effects would cascade down the food chain and lead to shifts in the ecosystem's balance.

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Orthonectida is a small phylum of poorly known parasites which are some of the simplest animals. Hosts include flatworms, polychaete worms, bivalves, and echinoderms, and orthonectids are small enough (0.05 mm to 0.79 mm in length) to freely swim around within the bodies of their hosts.

Adult orthonectids are slug or worm-like, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. Most species have male and female individuals, with females being larger than males, but some are hermaphroditic. They leave their host to reproduce, and the female dies after giving “birth.” Once a larvae finds a new host, it becomes a plasmodium which breaks into individual cells which will grow into the next generation of adults, absorbing nutrients within the host’s body.

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Phylum: Orthonectida

...a small (20 spp.) phylum of poorly researched parasites of marine invertebrates. Orthonectids are simply microscopic animals that  consist of a single layer of ciliated cells that surround a mass of sex cells. They swim freely inside their flatworm/polychaete/echinoderm or bivalve hosts and have two separate sexes. Once they are ready to reproduce they will leave their host and will the male's sperm will penetrate directly into the female's body. The zygote will develop into a ciliated larva that will escape from its mother to seek out a new host. Once it finds one it will lose its cilia and develops into a plasmodium larva. Which will break up into numerous cells that will become the next generation.

Phylogeny

Animalia-(mesozoa)-Orthonectida