Absence of light response in eyeless planaria.

Planaria Dugesia dorotocephala were bilaterally enucleated, decapitated caudal to the auricles, or sectioned at the level of the pharynx. None of these worms either altered or elicited any movement responses to stimulation by horizontal or overhead light even though they made such responses to mechanical stimulation. By comparison, all intact planaria responded vigorously to light stimulation but not to an elevation of water temperature. These results suggest that light receptors in these planaria might have evolved away from the body surface and are located in the ocelli.