Euglena eat green algae, amoebas, parameciums and rotifer. Euglena are single cell organisms so their food sources are small, microscopic organisms along with the energy they can create through photosynthesis. Their green color comes from the green algae they eat and the chloroplasts which play a part in photosynthesis, but some types can be red as well.
Euglena are classified as protists because they do not fit in the animal kingdom or the plant kingdom. When an euglena is feeding, it surrounds the prey through a method called phagocytosis. This method allows the organism to surround its prey and absorb it. As these organisms can feed as both a plant and an animal, it was listed as unclassifiable on the two kingdom biological identification system created by Linnaean. The discovery of these organisms that did not go into either category later spurred the creation of a their division in the system called the Kingdom of Protista.
The euglena are normally found in the largest concentrations on water. When a pond, lake or still body of water has a green or sometimes red film over the top of it, it is highly likely that this film will be made up of euglena. Seeing these organisms in water can be a good indicator that the water is polluted.