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A1. Nonmotile algae without chloroplasts: Cyanobacteriophyta
A2. Nonmotile algae with chloroplasts
B1. Cells usually solitary
B2. Nonmotile colonies with two or more cells
C1. Cells flattened, forming ribbon-like chains, short cylinders, or plate-like groups of cells
C2. Colonies spherical, oval, or elliptical
C3. Colonies various shapes but not flat, spherical, oval, or elliptical
D1. Cells enclosed in rigid vase-like or cylindrical lorica: Epipyxis
D2. Cells spherical or broadly oval
E1. Cells with one or more long, nonmotile pseudocilia
E2. Pseudocilia absent; colonies lacking distinct mucilage layers; mucilage may be present, but may be difficult to see unless stained
E3. Pseudocilia absent; colonies surrounded by distinct mucilage layer or connected by mucilage strands; mucilage visible without staining
D3. Cells club-shaped, fusiform, cylindrical, crescent-shaped, or spindle-shaped (not spherical or oval)
B3. Filamentous algae including pseudofilaments (see epiphyte links for epiphytic filaments)
B4. Cells living inside zooplankton or other aquatic animals (endozoic) or attached to aquatic plants (epiphytic)
A3. Motile algae with chloroplasts and flagella
A4. Cells divided into paired semi-cells that are approximate mirror images of each other: Desmids
A5. Cells enclosed in glassy, ornamental frustules; many species exhibit gliding movement: Diatoms