New ideas on the origin of bilateral animals

Comparative anatomy and embryology provide impressive evidence that the ventral side of all Bilateria (except Chordata) originates from the blastoporal surface, while the mouth and anus develop, respectively, from the anterior and posterior extremities of an elongated blastopore. From the point of view of paleontology, some Vendian multicellular animals represent transitional forms between Radiata and Bilateria. Vendian Bilateria are metameric organisms with a symmetrical or asymmetrical arrangement of segments; they can be considered as bilaterally symmetrical coelenterates crawling on the oral surface. In the recent Cnidaria, homologues of the genes “Brachyury,” “goosecoid” and “fork head” are expressed around the mouth. In the recent Bilateria these genes are expressed along the elongated blastopore and around the mouth and anus. These data corroborate the validity of the idea of amphistomy and the homology between the ventral surface in Bilateria and oral disk in coelenterates. It is supposed that the ancestors of Bilateria were crawling on the oral surface (=ventral side) and gave rise to both Fanerozoic Cnidaria and triploblastic Bilateria. This allows us to suggest the origin of Bilateria from Vendian bilaterally symmetrical coelenterates with numerous metameric pockets of the gastral cavity. Such ancestors gave rise to both Cnidaria and Bilateria. Apparently the primary Bilateria were complicated organisms having a coelom and segmentation, which allows us to explain the great diversity of highly organized organisms (arthropods, mollusks, and others) in the Cambrian era. An idea is proposed that Ctenophora are the only group of recent Eumetazoa that retain primary axial symmetry.