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Eyespot
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Labeled diagram of a Euglena, a unicellular protist, showing internal structures such as the nucleus, nucleolus, mitochondrion, chloroplasts, stigma (eyespot), flagellum, contractile vacuole, photoreceptor, stored carbohydrate, endoplasmic reticulum, and Golgi apparatus.
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Also called:
stigma
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Labeled diagram of a Euglena, a unicellular protist, showing internal structures such as the nucleus, nucleolus, mitochondrion, chloroplasts, stigma (eyespot), flagellum, contractile vacuole, photoreceptor, stored carbohydrate, endoplasmic reticulum, and Golgi apparatus.
Euglena Illustration of Euglena anatomy.

Eyespot

biology
Also known as: stigma
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Britannica Editors
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•History

Eyespot, a heavily pigmented region in certain one-celled organisms that apparently functions in light reception. 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.

This article was most recently revised and updated by Mic Anderson.