Fine structure and function of ocelli and sagittocysts of acoel flatworms

Abstract

Ocelli of several species of acoels flatworms in the genera Convoluta, Praesagittifera, Symsagittifera, and Amphiscolops were examined by transmission electron microscopy. Each ocellus consisted of a slightly concave pigment cell containing spindle-shaped platelets and several sensory cells, one in close contact with the pigment cell and the others separated from it by the former. These ocelli appeared unlike photoreceptors in most turbellarians in their lack of either rhabdomeres or cilia and in having platelets in the pigment cell. In species of Amphiscolops, the pigment cell had no other secretory inclusions than the platelets; the other species had characteristic pigment granules in addition to the platelets. In Praesagittifera naikaiensis (Yamasu) the primary sensory cell was penetrated by branching processes from the other sensory cells; A. langerhansi(?) may have similar processes, but the other species did not. Experimental ablation and subsequent regeneration of the ocelli of P. naikaiensis produced behavior consistent with their being photoreceptive. Two kinds of sagittocysts, one up to 40 µm long and other 8 µm long, were found in P. shikoki Kostenko & Mamkaev. These were thick-walled secretory products with a protrusible filament, and their association with the reproductive organs of the worm indicate a role in copulation.