Abstract
The accessory eye of an African giant snail, Achatina fulica, is situated under an extension of the cornea of the main eye and invariably possesses a small lens. The accessory retina is composed of type I and type II sensory cells and nonpigmented supportive cells that closely resemble corneal cells. Type I cells contain photic vesicles, and their microvilli are comparable in length and density to those of the main retina. Type II cells bear stubby microvilli and various vesicles, but no photic vesicles. The ratio of type I to type II cells is about two to one in both accessory and main retinas. Certain electron-opaque cells scattered among corneal cells possess numerous photic vesicles and twisted microvilli that are confined to intercellular spaces. The cavity of the accessory eye, being separated from that of the main eye by two boundary layers, is filled with debris, such as granules, vesicles, and whorls.
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