ABSTRACT
The evolutionary origin of the flatworms remains a major unsolved puzzle in metazoan phylogenetics. While three clearly monophyletic clades are recognized among the flatworms, namely Acoelomorpha, Catenulida, and Rhabditophora (Figure 1.1), how or even whether these three are related to each other and what their relationships are to other invertebrate groups is controversial (Smith et al. 1986). Perhaps, as has been argued recently from morphological characters (Haszprunar 1996) as well as from base-sequence data of the 18S ribosomal gene (Ruiz-Trillo et al. 1999), the Acoelomorpha, or at least the Acoela within it, could be the most primitive group of all bilaterally symmetrical animals, maybe even quite far removed from the other flatworms (Ruiz-Trillo et al. 1999). Similarly, the Catenulida may stand entirely separate from other flatworms by virtue of the very peculiar nature of their reproduction, among other things – the female organs are suppressed in many species and conceivably these animals develop by parthenogenesis of their quite aberrant, large spermatozoa (Sterrer and Rieger 1974; Schuchert and Rieger 1990a).
