Searching for the stem species of the Bilateria

Some recent molecular phylogenetic studies suggest a regrouping of the bilaterian superphyla into Deuterostomia, Lophotrochozoa (Lophophorata, Spiralia and Gnathifera) and Ecdysozoa (Cycloneuralia as the remaining Aschelminthes, and Arthropoda). In some of these trees Platyhelminthes have a more derived position among the Spiralia. On the other hand, taxa within or close to the Platyhelminthes have been singled out as possible plesiomorphic sister groups to all other Bilateria (Acoela and Xenoturbellida). For both proposals there exists conflicting evidence, both when different molecular features are compared and when molecular and phenotypic characters are used. In this paper we summarise the phenotypic models that have been proposed for the transition between diploblastic and triploblastic organisation (Planula- Phagocytella- Archicoelomate-, Trochaea-, Gallertoid-, Coeloplana-, Colonial- concept). With very few exceptions such models construct a vermiform organism (acoelomate/pseudocoelomate or coelomate) at the base of the Bilateria while the finding of similarities in the genetic regulation of segmentation in vertebrates and arthropods has stimulated the search for larger, more complexly designed ancestors. Because of the possible significance of vermiform organisation for understanding the origin of the Bilateria, we present new data concerning the development and evolution of the complex body wall muscle grid of platyhelminths and new findings on their stem cell system (neoblasts). We show that studying the various features of the development of the body wall in a variety of basal platyhelminths (eg. Acoela, Macrostomida) with cytological and molecular techniques would provide essential data for discussions of the diploblast/triploblast transition, because it is the central element in the biology of vermiform organisms.