Patterns in the nervous and muscle systems in lower flatworms

In order to test the monophyly of the Plathelminthes and the phylogenetic relationships of Acoela and Nemertodermatida., studies on the neuroanatomy of these groups were performed using anti-serotonin (5-HT) and anti-FMRF related peptides (FaRPs) immunocytochemistry. The Catenulida + Rhabditophora seem to be monophyletic. Four synapomorphies are proposed for these taxa. The presence of: 1. a bilobed brain showing 5-HF- and FaRP immunoreactivity. 2. a distinct neuropile, showing 5-HT and FaRP immunoreactivity. 3. two main nerve cords (MC's) aligned by 5-HT-immunoreactive (IR) marker neurones, 4. FaRP-immunoreactivity in the stomatogastric nervous system (NS). No synapomorphies were detected between Acoela and other Plathelminthes. The IR patterns of Acoela are characterised by: 1. The presence of a 5-HT-IR commissural brain. 2. The presence of clusters of FaRP-IR cells not integrated into a brain of the flatworm type. 3. The absence of a regular orthogon. 4. The absence of serotoninergic marker neurones along the main nerve cords (MC's). 5. The absence of a stomatogastric FaRP positive nervous system (NS). No support was obtained for a taxon Acoelomorpha. Our data are compatible with the hypothesis that both the Acoela and the Nemertodermatida do not belong to the Plathelminthes.