The Apostomatida are an order of ciliates of the class Oligohymenophorea. Individuals of the order are called apostomes. They are symbiotic with Crustacea. They are dormant and encysted on their host's exoskeleton for the majority of their life cycle. The life cycle of apostomes varies; some genera will multiply by fission, after which the larvae will develop within the crustacean hosts. Physical characteristics may include a small cytostome (mouth), often accompanied by a glandular rosette, and sparse, hairlike projections (cilia) arranged in spiral rows on the organism.