Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Lagenodifflugia

Shell pyriform, with a constriction of the neck, cross section is circular or slightly compressed, composed of mineral particles bounded by a structured organic cement network. In the region of the constriction, the shell is divided in two parts by a diaphragm-like part of the shell wall with a single central opening. Aperture is terminal, circular. Ecology: freshwater, Sphagnum. Type species: Lagenodifflugia vas (Leidy, 1874) Medioli et Scott, 1983