The Ultrastructure and Evolutionary Significance of the Cerebral Ocelli of Mytilus Edulis, the Bay Mussel

Purpose of the Study: The cerebral ocelli of bivalves have not been previously examined by electron microscopy; only their pallial eyes have been studied at this level. This study examined the ultrastructure of cerebral ocelli for the purpose of assessing the evolutionary significance of these and other cerebral ocelli of protostomous invertebrates. Methods: Ocelli were excised from living mussels and immediately fixed in glutaraldehyde, post-fixed in osmium tetroxide, dehydrated in ethanol, and embedded in Epon. Next, the ocelli were thin-sectioned and then photographed on a Zeiss EM 9A. Results: The ocellus consists of a cup shaped retina that is composed of interdigitating pigment and sensory cells. There is no cornea or lens. The pigment cells contain pale membrane-bound granules of shading pigment and have stubby microvilli at their apices. The sensory cells have axons at their bases and long undulating arrays of rhabdomeric microvilli at their apices. Conclusion: The cerebral ocelli of Mytilus edulis have features that conform to the similarities among the cerebral ocelli of other molluscs, sipunculids, annelids, and onycophorans; these similarities indicate that the cerebral eyes of these forms are homologous.