The myxozoans are parasitic protozoans that inhabit primarily the tissues and organ cavities of ectothermic vertebrates, especially fish. They also live in some annelids. Their cells contain flattened mitochondrial cristae, but no flagella, as they are parasites. Myxozoans contain several complex polar capsules inside valved spores. These have several developmental stages, including a multinucleated plasmodial stage and a multicellular stage. Inside each polar capsule are several solid polar filaments that provide anchorage on host cells to be infected. (Unlike in other types of parasites, the polar filaments of myxozoans do not provide a passage for the sporoplasm to enter new hosts.) The two myxozoan phyla contain at least 12,000 species.

Phyla of the Myxozoans
• Phylum Myxosporidia (Myxospora)
• Phylum Paramyxidia