These protozoans are intracellular parasites of invertebrates, although some infect vertebrates. They are capable of killing their invertebrate hosts.
When a spore is ingested, it extrudes a tubular polar filament which penetrates host intestinal cells and allows a sporoplasm to enter the cell through the tube. The sporoplasms quickly begins spore formation, which then ruptures the host cell. Spores pass in the feces, to be ingested by another host.
The spore bears a resemblance to the myxozoan spore, but the resemblance is superficial. The microsporean spore is unicellular. The polar filament is hollow and is not contained within a separate capsule. The spore walls are not divided into separate valves.