Evolution of the eye
Eyespots are found in nearly all major animal groups. The Euglena'''s eyespot, called a stigma, is at the front. Its red pigment shades a collection of light sensitive crystals. Together with the leading flagellum, the eyespot allows the organism to move in response to light to do photosynthesis, and to predict day and night. These movements are the main circadian (daily) rhythms.