Fine Structure of Photoreceptors in Three Types of Ocellate Cercariae

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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.

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