Abstract
Pigmented granules in Aplysia neurons prepared in the dark contain material that appears to be composed of 50-angstrom globules and a precipitate, probably a calcium salt. On illumination the globules rearrange into paracrystalline arrays and membrane-like lamellae. The morphologic transformation may be related to calcium release from the granules, and the released calcium may mediate the light-evoked hyperpolarization described by others.