Phacus is a genus of unicellularexcavates, of the phylumEuglenozoa (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. Currently, there are 564 species of Phacus in the database, but only 171 have been accepted taxonomically. The genus was established in 1841 and since then major discoveries have led it to become an extremely large group containing hundreds of species with varied physiological characteristics. Contemporary studies agree that Phacus is not monophyletic or holophyletic, but is actually polyphyletic. Unfortunately, the phylogenetic relationships within the genus are currently poorly understood.
The genus name is believed to have originated from the Greek word Phakos, meaning lentil or lens. This may be due to the general round or oval shape of the many species that are part of this genus. Its origins date back to the nineteenth century, in France, where it first received publication and establishment by Dujardin. The genus name is currently treated in literature as masculine.