a heavily pigmented region in certain one-celled organisms that apparently functions in light reception (photoreception). The term is also applied to certain light-sensitive cells in the epidermis (skin) of some invertebrate animals (e.g., worms, starfishes).

In the green one-celled organism Euglena, the eyespot is located in the gullet, at the base of the flagellum (a whiplike locomotory structure). A cup-shaped mass of pigment rods shields a sensitive area of the flagellar base from light coming from the direction of the opposite end of the organism. The light-sensitive region apparently influences flagellar motion in such a manner that the organism moves toward light.