Genus: Highly flattened; pellicle firm; body form constant (Kudo, 1966). Chloroplasts small, discoid, pyrenoids usually absent; most species flat and leaf-shaped; often with ridges or fins running helically or longitudinally (Illustrated Guide, 1985). Species: About 30-50 μm long, 10-20 μm wide; circular in cross section; with caudal prolongation; pelicle obliquely ridged; flagellum as long as the body (Kudo, 19660). Cell body 28-55 μm long, 10-21 μm wide two paramylons thick disc-shaped, parietal (An Illustrated Guide to Freshwater Zooplankton in Japan, 2000).

50 μm 100 μm 150 μm; x 400

Phacus pyrum (Ehrenberg) Stein: Cell body long ellipsoid or pear-shaped; anterior rounded; a long spine at posterior end; pellicle spiral with ridges; 2 ring-shaped paramylons; (7)-15.6-21 μm diam., 27-30 μm long (Illustrations of The Japanese Fresh-water Algae, 1977).