ABSTRACT
According to a reconstructed phylogenetic system for flatworms, the stem form of the Bilateria gave rise to the Platyhelminthes and the Eubilateria (Figure 22.1) (Ax 1984). While the monophyletic or polyphyletic status of the Platyhelminthes and the relationship to the protostome and deuterostome lines is under discussion (see Smith et al. 1986; Koopowitz et al. 1995; Haszprunar 1996; Tyler 2001, this volume), recent general phylogenetic schemes advocate a flatworm-like common ancestor for all metazoan phyla (Barnes et al. 1988; Willmer 1990).
