Cnidosporidia

Cnidosporidia 

a class of parasitic protozoa. They are characterized by spores that contain an amoeboid embryo and one or more nematocyst-like polar capsules. The polar capsules serve to attach the parasite to the tissues of host organisms and assure broad distribution of the cnidosporidians. They are parasitic in the tissues and organs of a number of invertebrate animals and fish, causing dangerous illnesses and frequently leading to the death of the animals. There are three orders, Myxosporidia (parasitic in fish), Microsporidia (parasitic in various invertebrates and fish), and Actinomyxidia (parasitic in oligochaete worms), which comprise more than 1,000 species.