Belonocystis spinifera (Brown, 1918) n. comb.
Syn.: Belonocystis tubistella Rainer, 1968
Diagnosis: Cells spherical, with fine filopodia without any extrusomes, filopodia can wave like a slow flagellum; nucleus eccentrically located, with a prominent nucleolus; one to four functional contractile vacuoles are present; periplast composed of radiating spines with a broad tent-shaped base and a long hollow shaft that was widest near the base and tapered to a very fine distal point; base of the shaft divided into four curved arms that form a basal tent-like structure covered by a membranous sheet, perforated with circular to elliptical pores in close proximity forming a mesh-work appearance to the spine base.
Dimensions: Nicholls: cells 8-16 µm diameter; nucleus about 4 µm diameter; spines 10-15 µm long, shaft 0.1-0.3 µm diameter wide near the base; scale base 2-3 µm wide, pores in the sheet mesh-work structure forming the bases 0.1-0.3 µm diameter.
Ecology: fresh water; North America, Europe. In my samples from different watertypes it is usually a common organism.
Remarks: The fine filopodia are difficult to see, even with high magnification phase-contrast microscopy. In the living, uncollapsed state, the top portion of the spine base appears funnel-shaped with sloping sides that abruptly straighten to nearly parallel sides where scale bases abut one another.
Amazingly, Penard never described this species, as it is easy to spot and by no means very rare.
Literature: Rainer, 1968, , Mayer, 1997, Nicholls, 2012.