Proposed life cycle for the X-cell parasite from flatfish in northern Japan. The proposed life cycle consists of four stages. (I) An infective stage, either flagellated or equipped with suitable apparatus for host penetration/infection, which is released into the environment to infect a fish host. (II) A proliferative cycle starts in the fish which develops into large epidermal pseudotumours. (III) As the pseudotumours mature, sac-like extremities are lost from the host fish and fall to the seabed where the X-cells are released into the substrate. (IV) In the substrate, either a free-living development occurs or the X-cells find an alternate host where development proceeds.