Euglenids are marine and freshwater green or colorless flagellate organisms best known by their type genus, the euglena. Around 1,400 species of euglenids have been discovered so far, and it is possible that at least twice that many are waiting to be discovered.
The colorless Euglenids usually feed on bacteria or smaller flagellates. There are some species that are bright red, this is because of the pigment called astaxanthin. They can live as an autotroph or mesotroph when it is put in darkness for any amount time, but when re-introduced to light will it get its chlorophyll back. One important structure is an organelle that is sensitive to light called the stigma. Using the stigma for guidance, they move using a flagellum toward light it needs for photosynthesis.
Euglenids are either green colored or colorless. The outer part of a cell consists of a hard layer called a pellicle. Some euglenids have chloroplasts that contain the photosynthetic pigments chlorophyll a and b. The other ones are heterotrophic so they are able to absorb their food and ingest it.
Most euglenids have two flagella that come out of flagellar pockets. Some have more then two, and some have barely any at all. There is mostly a leading flagellum, which is usually beats only at its tip, and one behind, which might be attached to the side of the cell. Euglenids also have contractile vacuoles, which means that they can force out water, which is diffused into the cell; if they don't do that they could burst from all the excess water. They also have things called pyrenoid bodies, this means it can store starch, it can also be used as a spare energy source. 
Sexual reproduction does not exist to the Euglenids. Asexual reproduction occurs by mitosis and then cytokinesis. Euglenids reproduce asexually by dividing longitudinally. They said that the process of this can be sloppy because of the movement in the chromosomes when it is being divided is irregular than other cells which have mitosis. Through all of this it ends up giving you two genetically identical organisms. 
Euglenids are commonly found in freshwater. They live in between grains of sand and the interface between mud and the water column. It can be found in almost anywhere where there is fresh or brackish water. It lives the best where there is a lot of rich organic waste. There are between 800-1000 different species of Euglena in the British Isles.