Phacus is a genus of unicellular excavates, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle.

Phacus are commonly found in freshwater habitats around the globe and include several hundred species that continue to be discovered to this day.

[2] 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.

The reason behind the separation was in order to create a group that correctly organizes their established morphological characteristics such as their rigid, flat, leaf-shape and small discoid chloroplasts with absent pyrenoids.

Ehrenberg, Hubner, Dujardin, and Klebs were some of the earliest researchers of Phacus, but they all had different diagnostic criteria when classifying these organisms.

Originally, Ehrenberg tried to classify three species he discovered as Euglena because of their color, but ultimately were moved to the genus Phacus by Dujardin due to the similarity they exhibited in other more prominent morphological characteristics such as shape and strip arrangement.

Since the establishment of the genus, there has been difficulty in classifying organisms due to morphology because many species of Phacus have physiological traits that closely resemble other related genera.

[5] Even the most minor difference in certain physiological properties caused great confusion in determining the proper classification for many species.

Since its inception, Phacus has undergone constant reclassification and considerable re-evaluation of different taxa in accordance with new morphological and molecular information.

Many species of this genus have been discovered in several countries, including Japan, the United States, Portugal, Brazil, Korea and the Philippines.

Phacus organisms are found in a range of freshwater environments (some more acidic or alkaline than others), prefer cooler temperatures, and on average exist in more neutral pH aquatic habitats.

[6] Many species of Phacus are considered to be euplanktonic (free-floating organisms or open water plankton) because they are commonly found together with other genera of Euglenophyta.

[8] These species include members of the genera Lepocinclis, Trachelomonas, Euglena, and many different kinds of algae, which are typically found in similar aquatic habitats.

The planktonic environments Phacus are generally found include swamps, ditches, trenches, ponds and even in many rice fields throughout North America and around the world.

They are a small part of phytoplankton communities, but do serve important purposes such as nutrient cycling and food web stability.

[9] In studies using beef extract to increase organic content of certain cultures, some species of Phacus were observed to have clear morphological changes different from the controls.

[10] This pocket acts as a sort of cytostome or ingestion organelle, allowing the organism to feed when bacteria enter inside.

Metaboly, which is the ability of some organisms to alter their shape, is not possible in Phacus due to the fusion of the genus’ pellicular strips.

Scientists who study the genus Phacus Dujardin, have also observed the presence of ellipsoid, tiny disk-shaped or flat-shaped paramylon grains.

Studies show that morphological changes observed in the genus are possibly due to the level of organic enrichment in their freshwater habitats.

Phacus is a member of the family Phacaceae,[15] the order Euglenales, class Euglenoidea, and finally the phylum Euglenozoa.

It appears that the number of those strips has either increased or decreased over time depending on certain species and that their arrangement (either helical or longitudinal) has also changed throughout evolution.

The changing numbers of strips and the clustered patterns associated are not actually adaptive themselves, but may have evolved due to the cell becoming more flat and more rigid over time.