Observations on the fine structure of photoreceptor cells in the planarian Dugesia tigrina

The bipolar photoreceptor cell of the planarian Dugesia tigrina was studied by electron microscopy. The photosensitive structure within the pigment-enclosed eyecup is attached to the free end of a cell process. This process, the dendritic fiber, contains large vesicles, mitochondria, and numerous neurotubules. It leads into an expanded cell body located lateral to and outside of the opening of the eyecup. The cell body contains a nucleus, nucleolus, mitochondria, several Golgi complexes, multivesicular bodies, vacuoles, free ribosomes, and ribosomes associated with an extensive endoplasmic reticulum longitudinally oriented in the cell. The axon, containing vesicles and few neurotubules, extends to the cerebral ganglion and ends in a terminal axonal enlargement which may contain mitochondria, numerous synaptic vesicles, and glycogen granules.