Comparative morphological studies on the visual systems in a binocular and a multi-ocular species of freshwater planarian

The visual systems of Bdellocephala brunnea Ijima & Kaburaki, a species with two eyes, and Polycelis sapporo (Ijima & Kaburaki), a species with multiple eyes, were investigated using light and electron microscopy. The eye of the binocular species consisted of 40–50 photoreceptor cells and 6–12 pigmented eyecup cells. The eye of the multi-ocular species was smaller and consisted in most specimens of one photoreceptor cell and one pigmented eyecup cell. The ultrastructure of the photoreceptor cells and of the pigmented cells was similar in the two species. Despite differences in numbers of constitutive cells, the arrangement of functional elements in the ocelli of these planarians is the same.