Phacus is a genus of unicellular euglenoids, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, and have a single flagellum that extends the length of their body. They are morphologically very flat, rigid, leaf-shaped, and contain many small discoid chloroplasts.

Phacus are commonly found in freshwater habitats around the globe and include several hundred species that continue to be discovered to this day. There are 564 species of Phacus in the database, but only 171 have been accepted taxonomically. It is a large and complex genus, with ongoing species revisions continuing to the present.

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