A phylum of unicellular flagellates of ancient eukaryotic lineage with unclear taxonomy. They lack a CELL WALL but are covered by a proteinaceous flexible coat, the pellicle, that allows the cell to change shape. Historically some authorities considered them to be an order of protozoa and others classed them as ALGAE (some members have CHLOROPLASTS and some don't).

A large group of flagellated EUKARYOTES found in both free-living and parasitic forms. The flagella are present in pairs and contain unique paraxonemal rods.