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Apostomatida (class Ciliatea, subclass Holotrichia) An order of ciliate protozoa in which mature forms have spirally arranged body cilia. The organisms live as parasites or commensals and often have two hosts, one of which is commonly a marine crustacean (e.g. Foettinga lives inside the gastrovascular cavity of sea anemones; motile young leave the sea anemone and encyst on the outside of small crustaceans; when the crustaceans are eaten by sea anemones the young emerge from their cysts).