A1. Nonmotile algae without chloroplasts: Cyanobacteriophyta
A2. Nonmotile algae with chloroplasts
B1. Cells usually solitary
B2. Nonmotile colonies with two or more cells
B3. Filamentous algae including pseudofilaments (see epiphyte links for epiphytic filaments)
C1. Cells form pseudofilaments where they are aligned end-to-end but not joined across the endwall
C2. Unbranched filamentous algae
C3. Branched filamentous algae
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