The Myxozoa (etymology: Greek: μύξα myxa "slime" or "mucus" + thematic vowel o + ζῷον zoon "animals") are a group of aquatic parasitic animals. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a myxosporean spore usually ranges from 10 μm to 20 μm whereas that of a malacosporean spore can be up to 2 mm. Myxozoans can live in both fresh and marine habitats.