A little-studied group of ciliates comprising an order of the Holotrichia. The majority occur as commensals on marine crustaceans. This association is generally more intimate and complex than that of the chonotrichs found on related hosts. Their life histories may become exceedingly complicated, and they appear to bear a direct relationship to the molting cycles of their hosts. Apostomes are particularly characterized by the presence of a unique rosette in the vicinity of an inconspicuous mouth opening and the possession of only a small number of ciliary rows wound around the body in a spiral fashion. Foettingeria (see illus.) is an example commonly encountered.