In order to test the monophyly of the Plathelminthes and the phylogenetic r
elationships of Acoela and Nemertodermatida., studies on the neuroanatomy o
f these groups were performed using anti-serotonin (5-HT) and anti-FMRF rel
ated peptides (FaRPs) immunocytochemistry. The Catenulida + Rhabditophora s
eem to be monophyletic. Four synapomorphies are proposed for these taxa. Th
e presence of: 1. a bilobed brain showing 5-HF- and FaRP immunoreactivity.
2. a distinct neuropile, showing 5-HT and FaRP immunoreactivity. 3. two mai
n nerve cords (MC's) aligned by 5-HT-immunoreactive (IR) marker neurones, 4
. FaRP-immunoreactivity in the stomatogastric nervous system (NS). No synap
omorphies were detected between Acoela and other Plathelminthes. The IR pat
terns of Acoela are characterised by: 1. The presence of a 5-HT-IR commissu
ral 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 wit
h the hypothesis that both the Acoela and the Nemertodermatida do not belon
g to the Plathelminthes.