The phylum Euglenophyta is named for the common genus Euglena. Euglena and other euglenoids are best known for their characteristic undulating, shape-changing motion called metaboly. Many euglenoids can also glide and swim using their flagella. Members of this group are either unicellular or colonial. Of the 40 genera, about 2/3 of these are heterotrophic and have colorless plastids or none at all. A third of the genera are able to photosynthesize, and have green plastids with chlorophyll a and b and carotenoid pigments. The cytoplasm of Euglena and other euglenoids contains many paramylon starch storage granules. The euglenoid cells are covered by a pellicle composed of ribbonlike, woven strips of proteinaceous material that cover the cell in a helical arrangement from apex to posterior. Freshwater euglenoids have a contractile vacuole. Euglenoids sense light using a red pigmented eyespot or stigma and the paraflagellar body located at the base of the emergent flagella.   Most of the 900 euglenoid species inhabit freshwater environments. Euglenoids live in hard or soft water habitats of varied pH and light levels - mainly marshes, swamps, bogs, mires, and other wetlands with an abundance of decaying organic matter. Populations thrive under high nutrient levels, and are therefore useful bioindicators of such conditions. The decomposing organics release short chain fatty acids, sugars, vitamins B1 and B12, and the other organic compounds needed by euglenoids. This group may also be found in marine or brackish sand and mud flats, farm ponds, the digestive tracks of small aquatic creatures, and the interfaces of air and water, and water and sediment. Euglenoids usually reproduce asexually.