[1] Both produce spores without the complex structures found in similar groups (such as polar filaments or tubules).

Haplosporid spores have a single nucleus and an opening at one end, covered with an internal diaphragm or a distinctive hinged lid.

After emerging, it develops within the cells of its host, usually a marine mollusc or annelid, although some infect other groups or freshwater species.

Paramyxids develop within the digestive system of marine invertebrates, and undergo internal budding to produce multicellular spores.

A 2009 study concluded that Haplosporidium species form a paraphyletic group and that the taxonomy of the haplosporidians needs a thorough revision.