stigma

The stigma (or eyespot apparatus) is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids.

It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light (positive phototaxis) or away from the light (negative phototaxis).

Eyespots are the simplest and most common eyes found in nature, composed of photoreceptors and areas of bright orange-red pigment granules.