Fine Structure of Photoreceptors in Three Types of Ocellate Cercariae

Abstract

Each paired photoreceptor in cercariae of Macravestibulum eversum, Crepidostomum sp., and Skrjabinopsolus manteri consists of one or two sensory cells with microvilli, and one separate pig- ment-containing cell. But the median eyespot in M. eversum may have a single cell differentiated into a central sensory portion around which a ring of pigment forms late in development. Moreover, the zone of microvilli impinges on a separate unicellular lens which is not present in the paired eyespots of any of the three species studied. In M. eversum, the pigment cell of each paired eyespot forms a deep cup which contains an anterior and posterior sensory cell with their microvilli apposed and interdigitating. In Crepidostomum sp., the pigment cup is shallower and contains a single sensory cell with parallel micro- villi extending in opposite directions from a central portion. In S. manteri, each eyespot has two sensory cells but they are on opposite sides of the pigment cell. Thus, eyespots in cercariae differ inter se as well as from the photoreceptors of miracidia. Among the features common to both free- living and many parasitic flatworms are photo- receptors which are closely associated with the brain and consist of separate sensory and pigment-containing cells. The pigment cell forms or participates in the formation of a more or less distinct cup occupied by a part of the sensory cell whose distal or peripheral region consists of numerous microvilli. The portion of the sensory cell just external to the cup may consist of a bulbous, refractile region, which is commonly described as a lens. From it the cell narrows to become the axon which leads to the brain.