These spore-producing organisms have long been classes as protozoans but recent genetic studies suggest they possess a strong affinity with the cnidarians, and ultrastructural study of the characteristic polar capsule of their spores shows them to be a typical nematocyst. Many myxozoans are quite pathogenic and spread rapidly among hosts in crowded aquaculture conditions.
These are intercellular parasites of the tissues of fishes. Large multinucleated trophozoites feed on surrounding tissue. Eventually, a large number of multicellular spores are formed and released. The spores contain polar capsules used to penetrate the tissues of the next host and allow a sporoplasm to enter the wound. The sporoplasm migrates to its preferred site and becomes a trophozoite. The spores are surrounded by two or three valves.
The spore can have a variety of shapes. Internally are one or more polar capsules which appear under the light microscope as coiled structures. In addition there is an amoeboid sporoplasm.