A1. NONMOTILE ALGAE WITHOUT CHLOROPLASTS: CYANOBACTERIA
A2. NONMOTILE ALGAE WITH CHLOROPLASTS
A3. MOTILE ALGAE WITH CHLOROPLASTS AND FLAGELLA
B1. MOTILE COLONIES WITH TWO OR MORE CELLS
B2. MOTILE SINGLE-CELL ALGAE
C1. CELLS WITH ONE VISIBLE FLAGELLUM
C2. CELLS WITH TWO EQUAL LENGTH FLAGELLA
D1. CELLS ENCLOSED IN FIRM, TRANSPARANT, SPHERICAL MUCILAGE LAYER
D2. CELLS ENCLOSED IN RIGID LORICA
D3. CELLS NOT ENCLOSED IN LORICA OR SURROUNDED BY SPHERICAL OR OVAL MUCILAGE
E1. NARROWLY ELLIPTICAL OR SPINDLE-SHAPED CELLS; FLAGELLA OFTEN DIRECTED BACKWARDS; CELLS OFTEN NONMOTILE AND EMBEDDED IN MUCILAGE OF OTHER ALGAE: Chlorogonium
E2. CELLS SLIPPER-SHAPED, OVAL, TEARDROP-SHAPED; ASYMMETRIC; TWO NEARLY EQUAL FLAGELLA INSERTED BELOW APEX (SUBAPICAL): Cryptomonads
E3. SPHERICAL, OVAL, OR ELLIPTICAL CELLS WITH TWO EQUAL FLAGELLA INSERTED AT THE APEX (LARGE, DIFFICULT GROUP): Chlamydomonas and similar
C3. CELLS WITH TWO DISSIMILAR FLAGELLA
C4. CELLS WITH MORE THAN TWO FLAGELLA
A4. CELLS DIVIDED INTO MIRROR-IMAGE SEMI-CELLS: Desmids (see key at cedar.wwu.edu/cedarbooks/14/ )
A5. CELLS ENCLOSED IN GLASSY, ORNAMENTAL FRUSTULES: Diatoms (see key at diatoms.org)