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(stigma) A pigmented structure in motile unicellular algae and in flagellated reproductive bodies of some multicellular algae. It consists of one or more concentrically arranged curved plates composed of orange-red carotenoid granules. The eyespot is situated close to the bases of the flagella. It functions as a light absorbing shield, which, depending on the position of the organism relative to the light source, prevents light from reaching photoreceptive areas at the bases of the flagella. This enables the organism to detect the direction of light and respond by movements of the flagella.
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