actinomyxidian, any parasitic microorganism of the class Actinomyxidia (Actinosporea), phylum Myxosporidia (Myxospora). It inhabits the alimentary canal of certain aquatic worms. The characteristic spores develop in the host’s gut after the union of large and small gametes. The spores contain three polar filaments (possibly anchoring devices) enclosed in capsules and one to many infective sporozoites. The representative species Triactinomyxon ignotum lives in the marine worm Tubifex and has spore valves that are elongated into three large hooks.