A1. Nonmotile algae without chloroplasts: Cyanobacteriophyta
A2. Motile algae with chloroplasts and flagella
A3. Nonmotile algae with chloroplasts but without flagella
A4. Cells with paired, symmetric semi-cells: Desmids
B1. Cells rarely solitary; usually forming clumps or filaments
C1_ Cells oval or elliptical, enclosed in dense mucilage forming small colonies (occasionally solitary). Cells with one plate-like chloroplast: Mesotaenium
C2. Semi-cells smooth-walled, oval or broadly elliptical; cells linked by mucilage threads to form branched colonies: Cosmocladium
C3. Cells linked end-to-end to form filaments
D1. Cells with shallow sinus or unconstricted; cell length ≤ width
D2. Cells with shallow sinus or unconstricted; cell length > width
E1. Cells barrel-shaped, distinctly expanded at center: Bambusina
E2. Semi-cells spherical, cells peanut-shaped with widely open sinus: Cosmarium moniliforme
E3. Cells cylindrical; sinus absent or very shallow and indistinct
D3. Cells with moderately deep sinus
B2. Cells mostly solitary unless dividing
A5. Cells enclosed in glassy, ornamental frustules: Diatoms