Eye spot. It's a) a sense organ that lets the Euglena move away from the light and b) the receptor by which it harvests light for photosynthesis.
Not a plant, it lacks a cell wall, it's an animal, one of two we know of that photosynthesizes.
THere's about 800 species found in both fresh and saltwater.
Difficult to find a pure culture in the aquarium or pond water but they are kept by people that maintain a lot of live foods and are an excellent first food for fish.
