The fine structure of rhabdomeric photoreceptors ofCirrifera aculeuta, Monocelis fuscaandNematoplana coelogynoporoidesis described. The existence of one receptor cell and one pigmented or unpigmented mantle cell is thought to be a basic feature of the Neoophora at least. The existence of a pigment shield formed by many cells is derived, in addition to one receptor and one unpigmented mantle cell, in the eyes ofMonocelis fuscu.