Light microscopy of Planaria, a common flatworm measuring a few millimetres in length, gliding through pond water. It moves by beating cilia located on its ventral surface, allowing it to slide over a mucus film. It has two eye spots, or ocelli, which are sensitive to light and dark, and are used to move away from light. This is a freshwater planarian, although other types can inhabit saltwater, and some are even terrestrial in suitably moist environments. Recorded under Nomarski differential interference contrast illumination.