Family: Unicells; cell wall present; two flagella (Illustrations of The Japanese Fresh-water Algae, 1977).
Genus: Two flagella of equal length inserted close together into an anterior papilla of variable shape usually directed anteriorly; chloroplast form variable; usually 2 contractile vacuoles; cell division producing 2-8 zoospores; sexual reproduction usually isogamous, anisogamy and oogamy known but rare; > 500 species, mostly fresh water (Illustrated Guide, 1985). A large chloroplast; stigma; a single nucleus; two contractile vacuoles anterior (Kudo, 1966).