Acanthocystis penardi  Wailes, 1925

Diagnosis: Periplast 9-130 μm, average 30 μm. Radial spine-scales long, 6-25 μm, sometimes distinctly separated into long and short spines, which differ only in length. Long spine-scales average three-thirds of the envelope diameter. Shaft 0.25-0.5 μm thick, straight, hollow and cylindrical, slightly enlarged and provided with 8-14 magical teeth. Basal disc 1.0-3.5 μm in diameter, with central shank. Tangential plate-scales 2.2-3.6 x 3.1-5.7 μm, oval, often with concave sides, without distinct structure, slightly thickened in the centre, sometimes with slightly thickened margin. Cysts with long, narrow scales, which have a longitudinal bead in the middle. Axopodia usually relatively short, rarely longer than twice the body diameter. Nucleus shape-changing and highly eccentric. Constantly moving, rarely resting. Large individuals sometimes with zoochlorellae. The habitus form can be very different. In their typical form, determination by light microscopy is not difficult.

Ecology: Widespread in various water types: moors, ponds, ditches and lakes; both in dystrophic and eutrophic waters; between aquatic plants and in sediments. Up to 35 m depth in Lake Geneva (Penard, 1904). Europe, Canada, Chile, Malaysia.