FULL ANSWER
The euglena's eyespot is not an eye, but it can react to changes in the brightness of a light source. The eyespot is important because it helps the euglena move toward light, the source of energy for photosynthesis, states Biology Cabinet.
The euglena is one of the few organisms besides plants that can carry out photosynthesis to produce food for itself. The euglena turns to where the eyespot detects the smallest amount of light, which is the direction in which the transparent eyespot is most covered by the black pigmented region, according to Dr. Paul of Johnson County Community College