Initial morphological diversity as a criterion in deciphering turbellarian phylogeny

The most profound structural variety in morphofunctional systems and morphogenetic mechanisms, i.e. the highest morphological diversity, is observed in those groups where these systems and mechanisms are evolutionarily most primitive. Here, such variety can involve the basic body plan of a given phylum and the types of morphogenesis characteristic of it. This correlation provides a new criterion of evolutionary primitiveness, namely, the criterion of initial morphological diversity.The highest morphological diversity among turbellarian groups is observed in the order Acoela. Acoel turbellarians are archaic in most of their features, apparently being a group near the base of the turbellarian phylogenetic tree. Among other turbellarians there are a few groups that also are archaic in some few features (above all, the Catenulida), although on the whole they are more advanced than the Acoela. The Turbellaria as a whole is notable for its morphological diversity in comparison with other classes of the Scolecida.