ABSTRACT
The Acoelomorpha Ehlers, 1984, comprising Acoela and Nemertodermatida, were traditionally considered monophyletic and were included within the monophyletic Platyhelminthes (Ehlers 1985a,b; Rieger et al. 1991). However, a conflicting view existed regarding Acoelomorpha as one of the three groups (Acoelomorpha, Catenulida and Rhabditophora) constituting the non-monophyletic Platyhelminthes (Smith et al. 1986). Recent morphological and molecular data suggest that Acoela could be the sister group to the Bilateria (Haszprunar 1996; Ruiz-Trillo et al. 1999). The position of Nemertodermatida was even more difficult to determine in molecular studies (Littlewood et al. 1999a; Ruiz-Trillo et al. 1999). Therefore the search for additional characters is pertinent to our understanding of the phylogeny of the Acoelomorpha. This study summarizes data obtained so far from our recent studies on brain and sperm structure, and attempts to clarify the interrelationships of the Acoela and the Nemertodermatida on the one hand and the phylogeny within the Acoela on the other hand.
