Original: Together with the leading flagellum, the eyespot allows the organism to move in response to light, often toward the light to assist in photosynthesis, and to predict day and night, the primary function of circadian rhythms.
Simplified: Together with the leading flagellum, the eyespot allows the organism to move in response to light to assist in photosynthesis, and to predict day and night, the primary function of circadian rhythms.