Photoreceptors in species of the Macrostomida (Plathelminthes): ultrastructural findings and phylogenetic implications

Abstract 

The submicroscopic anatomy of intracerebral and pericerebral photoreceptors in six species of the Macrostomida is described. Cylindromacrostomum notan-dum, Paramyozonaria simplex and Macrostomum hystricinum marinum possess two rhabdomeric intracerebral photoreceptors each consisting of two pigmented cup cells and three (C. notandum and P. simplex) or two sensory cells (M. hystricinum marinum). In C. notandum and P. simplex two of the sensory cells are equal in size, while the third one is much smaller. This organisation is hypothesised as an autapomorphy of the Dolichomacrostomidae. Photoreceptors with two mantle cells are also known for Microstomum spiculifer. Since only one cup cell exists in representatives of nearly all other high-ranked taxa of the Rhabditophora, it is concluded that the characteristic ”two cup cells in rhabdomeric photoreceptors” has evolved in the stem lineage of the taxon Macrostomida or Macrostomorpha, respectively. In Myozona purpurea and Psammomacrostomum turbanelloides rhabdomeric intracerebral photoreceptors of a special type were encountered. These light-sensing organs consist of numerous cells forming an ellipsoid. The surface membranes of these cells are elongated to form filiform extensions which are tightly intertwined with each other. Pericerebral ciliary aggregations consisting of cells with an internal cavity into which axonemata of modified cilia project were observed in all species mentioned above and in Bradynectes sterreri as well. Such putative light-perceiving organs are widespread within taxa of the Plathelminthes Rhabditophora and have been hypothesised either as homologous characteristics or as analogous ones. With increasing examples being described it becomes likely that pericerebral ciliary aggregations are an apomorphic ground pattern characteristic of the Rhabditophora.