Acanthocystis penardi Wailes, 1925
Diagnosis: Periplast 9-125 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 needles 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 scales 2.2-3.6 μm x 3.1-5.7 μm, oval, often with concave sides, without distinct structure, slightly thickened in the centre, sometimes with slightly thickened margin. Cyste 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 zoochlorella. The habitus form can be very different. In their typical form, determination by light microscope is not difficult.
Ecology: Widespread in various water types: moors, ponds, ditches and lakes; both in dystrophic and eutrophic waters; between aquatic plants and on sediments. Up to 35 m depth in Lake Geneva (Penard, 1904). Europe, Canada, Chile, Malaysia.