Light-receptors in the form of small dark spots disposed in groups, such as:
cerebral, when situated over or alongside the cerebral organ;
frontal, when strewn over the region between the cerebral organ and the anterior margin of the body; marginal, when occurring along the body margin; submarginal, marginal eyes well separated from body margin;
tentacular, when lying in or around the tentacles, or where tentacles would be, if present.