Neuroanatomy of the rhabdocoel flatworm Mesostoma ehrenbergii (Focke, 1836). I. Neuronal diversity in the brain.

We investigated the neuronal configurations of brain cells in the rhabdocoel flatworm Mesostoma ehrenbergii and compared them to those of the polyclad Notoplana acticola to determine neuronal synapomorphies for these different flatworm clades. The fluorescent dye Lucifer yellow was used to fill and visualize cells in the brain of M. ehrenbergii. The predominant cell types for both animals were heteropolar bipolar and heteropolar multipolar cells. Isopolar bipolar and isopolar multipolar cells were also found but in much lower numbers. Small amacrine-type multipolar cells that might be glial in nature were also found in an area of neuropil. A few unipolar cells were found that ranged from small amacrine-type cells to typical invertebrate-type unipolars. In addition, a number of morphologically identified cells found in different animals are described.