Scientific classification
BŁtschli 1884

The euglenids are one of the best-known groups of flagellates, commonly found in freshwater especially when rich in organic materials, with a few marine and endosymbiotic members. They are included in the Euglenozoa, distinguished from other such forms by several features, most notably the presence of microtubule-supported proteinaceous strips underneath the cell membrane. These form a pellicle, which gives the cell its shape and varies from rigid to flexible, in the latter case allowing it to perform an inching motion called metaboly. Many euglenids have chloroplasts, but others are primitively or secondarily colourless.

Primitively colourless euglenids typically feed on bacteria, smaller flagellates, and the like. There is a well developed cytostome supported by microtubules which are often packed together to form two or more rods, functioning in ingestion. There is typically one leading flagellum, which is usually rigid and beats only at its tip, and one trailing, which may or may not be attached to the side of the cell. The common genus Peranema is typical of these forms; others include Heteronema, Petalomonas, and Entosiphon.

Euglenids presumably acquired chloroplasts from some ingested green alga. These contain chlorophylls a and b, giving them a bright green color, and are bound by three membranes. Often they are associated with granules of paramylon, a storage carbohydrate that is unique to the group. Most coloured euglenids have a stigma, or eyespot, which is a small splotch of red pigment on one side of the flagellar pocket. This shades a collection of light sensitive crystals near the base of the leading flagellum, so that the two together act as a sort of directional eye. The cytostome is vestigial, although nutrients may still be obtained by absorption. In many cases exposure to certain chemicals or prolonged absence of light may kill off the chloroplasts without otherwise harming the euglenid, and there are a number of species which are secondarily colourless, typically treated in separate genera.

A few coloured euglenids have two roughly equal flagella, as the marine genera Eutreptia, and some forms have four. In most, however, the trailing flagellum is shortened so that it does not emerge from the flagellar pocket, making the cell effectively uniflagellate. The emergent flagellum typically undergoes a complex looping motion which pulls the euglenid along a slightly helical path. These include the common genera Euglena, Phacus, and the colourless Astasia. A few, such as Trachelomonas, produce organic loricae which encase the cell. There is one genus, Colacium, in which the mature cells are non-motile and form branched colonies supported by mucous stalks, but otherwise there is no tendency towards the complex forms found in most other algae.

The euglenids were first defined by Otto BŁtschli in 1884 as the flagellate order Euglenida. They were treated by botanists as the algal division Euglenophyta, a double-placement they retained until the flagellates were broken up. Both names are still used to refer to the group, though the formal taxon Euglenida is often restricted to those closely related to Euglena.

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