| Abstract: | The hypothesis of dorso-ventral axis inversion in Chordata based on recent molecular biology data is discussed from the viewpoint of evolutionary morphology. It is supposed that, as a result of such an inversion, the nerve cord located ventrally in most Invertebrata found itself on the dorsal side of Chordata. However, an analysis of the contemporary hypotheses explaining chordate origin (dipleuruloid and hemichordate) shows that neither of them provides convincing evidence of dorso-ventral inversion. A new hypothesis agreeing with both the molecular and morphological data is suggested. According to this hypothesis, the divergence of Protostomia and Deuterostomia occurred early in the origin of Bilateria. The initial stage (for a common ancestor) is assumed to be some turbellarians, in which several nerve cords extend back from the apical organ in a radial-symmetrical pattern. In the process of subsequent oligomerization, the ventral nerve cords become more developed in Protostomia, while in Deuterostomia the dorsal nerve cords have priority. It is emphasized that reaching an understanding between molecular biologists and evolutionary morphologists is a necessity. |
