IT has long been known that certain chitons (Mollusca, Polyplacophora) possess eyes or ocelli1. These ocelli occupy a unique position, being set dorsally in the outer most layer of the shell. No eyes from the soft parts of adult animals have yet been described2. Light microscope investigations distinguished several types of organelles within the shell. These are basically the aesthete3, the intrapigmental ocellus4, and the extrapigmental ocellus1,5. All chitons yet examined seem to possess one or more of these types. Several chitons have been shown to respond to light stimuli6–8 and it is assumed that these shell organelles function as light receptors6,7,9. Electron microscopy of the aesthetes of several American species10,11 has failed to reveal any structures recognizable as typical of photoreceptors.