Family: Colonies consisting of a fixed number of cells (Illustrations of The Japanese Fresh-water Algae, 1977).
Genus: Colony spherical; 8-16 cells of similar size, close together, mutually compressed; 2-layered gelatinous envelope; cells with 2 equal-length flagella; 2 contractile vacuoles anterior; nucleus central (Illustrated Guide, 1985).

P. morum 2 clips dividing cells col. 30 x 25 μm Binnuma River Omiya Saitama, 2000

P. morum 3 clips dividing cells col. 30 x 25 μm Binnuma River Omiya Saitama, 2000