Comparison of the nervous system of the rhabdocoel Mesostoma ehrenbergii with that of the polyclad Notoplana acticola

The neuromuscular organization of the free-living marine polyclad Notoplana acticola (Boone) can be compared with that of the fresh water rhabdocoel Mesostoma ehrenbergii (Focke). These examples act as outgroups for each other, belonging to different clades of the Turbellaria and similarities between the two can be taken as synapomorphies to indicate features that might be primitive and general features of flatworm nervous systems. The fluorescent dye Lucifer Yellow was used to fill cells in the brains of the two species and the neuronal anatomies were photographed or drawn. Many different cell types have been found in both species. The predominant cell types in Mesostoma were heteropolar bipolar and heteropolar multipolar cells with isopolar bipolar and isopolar multipolar cells in much lower numbers; only a few unipolar cells were found. In Notoplana, many of the cells penetrated were also heteropolar bi- or multipolar cells. There were also neurons and architectural features that were morphologically very similar to each other, in the two animals, such as the BRA cells and commissural tracts. These shared features suggest that many of the peculiarities of polyclad nervous systems may be general flatworm features.