ULTRASTRUCTURE OF PHOTORECEPTORS IN PROMONOTUS-MARCI (PLATHELMINTHES,PROSERIATA, MONOCELIDIDAE)

Promonotus marci possesses two light-sensing organs not visible with t he light microscope. Both organs take a pericerebral position. Each ph otoreceptor consists of two sensory cells lying opposite each other. T hese sensory cells are enveloped by a single cup or mantle cell which lacks pigment granules. No additional outer, multicellular pigment shi eld is present. The light-sensing organelles are formed by microvilli- like extensions of the surface membrane of both sensory cells. These m icrovilli interdigitate in the centre of the eyes. The photoreceptors correspond to those of all other Proseriata Lithophora in the existenc e of a mantle cell free of pigment granules. The absence of an additio nal pigment shield is a plesiomorphy compared to other Monocelididae i nvestigated up to now.