Fine structure of the eyes in two species of dalyelliidae (turbellaria rhabdocoela)

SUMMARY The eyes of the Dalyelliidae, Dalyellia viridis (G. Shaw, 1791) and Gieysztoria diadema (Hoisten, 1907), have been examined with the electron microscope. They are made up of a pigmented, bean-shaped cup, the rim of which projects inward as an unpigmented cytoplasmic lamina representing the so-called lens as described by the light microscopists. The concavity of the pigment cup contains the photoreceptor processes of two retinal cells, whose cell bodies lie on the external dorsal part of the cup. The more proximal part of the dendritic process of the photoreceptor is characterized by numerous neurotubules, vesicles and mitochondria; the more distal part by long microvilli. The microvilli of the anterior photoreceptor cell are arranged in two groups oriented at right-angles to each other; those of the posterior one are all directed backwards. The soma, rich in multivesicular bodies and Golgi apparatuses, continues with an axon that penetrates the brain. The lens corresponds to the cytoplasmic extens...