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 pigment-containing cell. But the median eyespot in M. eversum may have a single cell differentiated into a central sensory protion 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 3 sp. 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 microvilli extending in opposite directions from a central portion. In S. manteri each eyespot has 2 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.