An order of the protozoan class Myxosporidea (subphylum Cnidospora). It is characterized by the production of spores with one or more valves and polar capsules, and by possession of a single sporoplasm with or without an iodinophilous vacuole. Myxosporidians are mainly parasites of fishes. They infect all parts of the body, including the heart and brain, and often induce considerable pathological changes in the host tissue.