Genus Polyplacocystis Mikrjukov, 1996



Diagnosis: Centrohelid heliozoa with a periplast comprising one or several types of spindle-form or flat siliceous elements (scales). The latter are placed in mainly tangential direction and have reticular or smooth texture of the upper surface. There are no internal septa in scales. Radial elements in the periplast are absent. A single stalked member is known; a mucous stalk is supported by very oblong scales.

Type species: P. symmetrica (PENARD, 1904)


Remarks: The genus includes P. symmetrica and P. ambigua with a reticular texture of the scale surface; P. ambigua differs by the shape differentiation of scales from internal layers of the periplast towards the external ones (Nicholls & Dürrschmidt 1985; Siemensma & Roijackers 1988).
Another part of the genus consists of three species with a smooth scale surface. The fresh-water species P. pallida [whose spindle-form scales have tubular ends and widening (unwrapped) central parts] and the marine species P. pedunculata (whose cell body has a long mucous stalk supported by very oblong narrow scales) stand out from other observed members of the group. All other isolated specimens reported from both fresh and marine waters.