The Myxozoa are a group of microscopic, parasitic animals. Originally they were considered protozoa, and included with other non-motile forms in the group Sporozoa. However, as their distinct nature became clear they were removed to their own phylum. They are now generally considered to have developed from multicellular animals, and are classified with them.

Many Myxozoa have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. Infection occurs by valved spores. These contain one or two sporoblast cells, and one or more polar capsules, containing filaments that anchors the spore to its host. The sporoblasts are then released as a motile form called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores.

The polar capsules are very similar in structure and appearance to the stinging cells of Cnidaria. On account of this the Myxozoa have been generally held to be extremely reduced cnidarians, and in particular have been considered close relatives of Polypodium , with some genetic support. More recent studies of Hox genes, however, point to an origin among the Bilateria. This has been given strong support by the discovery that Buddenbrockia , a worm-like parasite of bryozoans up to 2 mm in length, belongs among the Myxozoa. Genetically it is almost indistinguishable from the other forms, and it has Myxozoan-like spore capsules, but it retains a bilateral body form with longitudinal muscles. This serves as a missing link between the Myxozoa and their multicellular ancestors.

Myxozoa are split into two classes, Malacosporea and Myxosporea. The outdated subgroup Actinosporea is now recognized as a life cycle phase of Myxosporea.

Some species:

• Class Malacosporea
  • Buddenbrockia plumatellae
  • Tetracapsuloides bryosalmonae , an important parasite of salmon
• Class Myxosporea