The euglenids (Euglenophyta, Euglenida) are one of the best-known groups of flagellates, and belong to the Euglenozoa. They are distinguished by a number of features, most notably the storage of food as the carbohydrate paramylon, and the presence of a series of microtubule-supported proteinaceous strips underneath the cell membrane that form a pellicle that gives the cell its shape. This varies from rigid to flexible, in the latter case allowing the cell to perform an inching motion called metaboly.
Many euglenids are primitively colorless and feed on bacteria and smaller flagellates. More needs to be said about these. One neat thing is that many, for instance Peranema and Heteronema, swim with the anterior flagellum held rigid, beating only at the tip.
At some point, euglenids aquired chloroplasts. These are typically bound by three cell membranes, so presumably they were retained from some ingested alga, with the outermost membrane corrseponding to a normal vacuole. As far as pigmentation is concerned, the chloroplasts contain only chlorophyll a. Photosynthetic euglenids have generally lost the cytostome but are still capable of feeding heterotrophically, and in fact the chloroplasts can be killed off without damaging the host cell. A few genera of euglenids are secondarily colorless - Astasia and Khawakinea, which properly belong within Euglena, and Hyalophacus, which properly belongs within Phacus.
Colored euglenids do not generally have the leading-trailing biflagellate pattern found in the primitively colorless forms and most other euglenozoa. In Euglena, Phacus, and their kin, the trailing flagellum is shortened so that it does not even leave the flagellar pocket, making the organisms effectively monoflagellate; in Eutreptia the two flagella are roughly equal in length and function. In these forms the flagella tend to undergo a looping motion, pulling the cell through the water in a slightly-helical path.
Most colored euglenids have an stigma, or eyespot (lost in secondarily colorless forms), which is a small splotch of red pigmentation shading the flagellar pocket. At the base of the leading flagellum is a collection of light sensitive crystals, so together the two act as a sort of directional eye. This is in fact where the group's name comes from: Gr eu+glêne, eyeball.
A few euglenids, eg Trachelomonas, live encased within a rigid lorica, which is sometimes attached to the substrate; unlike a cell wall this can easily be broken without harming the cell. There is one form, Colacium, that is aflagellate and non-motile in its adult stage, forming branched colonies held together by mucilage, but other than this no euglenids have made any move towards the sort of complex organization found in other algae.
Euglenids are common in freshwater, especially when it is rich in organic materials - often present in sufficient numbers to give the water a greenish color. A few, like Eutreptia, are marine, and some forms even live commensally inside animals.