Myxozoa are microscopic metazoan parasites with a reduced body. The dimensions of the myxospore, the typical myxozoan stage in fish hosts, range between one hundredth and two hundredths of a millimeter. Myxospores consist of several cells, which are transformed to shell valves, nematocyst-like polar capsules with coiled extrudible polar filaments, and amoeboid infective germs. Myxospores develop in plasmodia (trophozoites), which can be very large and polysporic (generally histozoic in host tissue) or small and mono- or disporic (coelozoic in organ cavities). Myxozoans are parasites of fish, worms (oligochaetes and polychaetes) and bryozoans. A few representatives were found as parasites of amphibians and reptiles, and recent findings confirmed the ability of myxozoans to infect mammals and birds. Humans as potential hosts for myxosporea were also reported, however, myxospores were detected in fecal samples and probably just passed through the digestive tract.
Myxozoa contains two classes: Malacosporea and Myxosporea. Malacosporea includes only two genera (Tetracapsuloides and Buddenbrockia) with a total of three described species. Myxosporea includes about 2200 species in 60 genera.
Myxozoans were considered to be protists for more then one hundred years until the early nineties of 20th century. Then, the phylogenetic analysis of the primal myxosporean SSU rDNA sequence confirmed earlier hypotheses that myxozoans are multicellular organisms and placed Myxozoa inside Metazoa. However, SSU rDNA data failed to find the correct position of the Myxozoa within metazoan taxa. Myxozoan SSU rDNA appeared to be a fast-evolving sequence resulting in long-branches in phylogenetic trees. Therefore, the SSU rDNA data are insufficient to decide whether Myxozoa are closely related either to Bilateria, Cnidaria, or other taxa.