ABSTRACT
Before the 1970s, the phylogenetics of the Platyhelminthes was based on light microscopic data. However, it was during the early 1970s, that the era of transmission electron microscopy began, and what may now be called 'the traditional scheme of flatworm phylogeny' (see Ehlers 1985a; Westheide and Rieger 1996) is to a great degree, based on ultrastructural data. The change from light to electron microscopy has not, however, caused changes in the methods of phylogenetic inference. Ultrastructural characters can be assessed using the same homology criteria that were the theoretical basis of phyloge- netics from the very beginning of phylogenetic studies (Rieger and Tyler 1985). The development of effective methods for DNA sequencing caused a great increase in the number of studies using molecular data on phylogenetic questions. Although qualitative approaches to molecular phylogenetics do exist, the dominating methods of phylogenetic inference from molecular data are statistical in nature. Phylogenies that result from these studies mostly do not agree with tradi- tional views in many important aspects, but no morphologist can now disregard molecular data when studying phylogeny. How one should deal with these controversies is an important question that will be discussed in this chapter.
