[Origin of bilateral-symmetrical animals (Bilateria)].

The paper is an attempt to attack the old problem of the origin of Bilateria by the methods of evolutionary tetrad (i.e. combination of comparative anatomy, comparative embryology, paleontology, and molecular biology). Three groups of theories of classical comparative anatomy (planulod-turbellarian, archicoelomate, and metameric) are discussed. Comparative embryology brings out clearly that the ventral side of embryo comes from the blastoporal region in all groups of Bilateria (except Chordata, where the blastoporal region corresponds to the dorsal side that is come out of the upside-down morphology of chordates) and mouth and anus comes from the anterior and posterior ends of elongated blastopore. From the point of view of paleontology, some of vendian metazoans demonstrate transitional conditions between the Radiata and Bilateria. Vendian bilaterians are metameric organisms with normal or asymmetric position of segments and could be pictured as "bilateral coelenterates" creeping on the oral surface. In Cnidaria, the expression of homologues of "Brachyury", "goosecoid", and "fork head" genes are revealed in the circular region around the mouth. In Bilateria, these genes are expressed along the elongated blastopore and around the mouth and anus. These results support the old conception on the amphistomic origin of mouth and anus as well as the homology between the oral disc of cnidarians and ventral side of Bilateria. The combination of four mentioned approaches enables us to propose the conception of the origin of Bilateria from vendian bilateral coelenterates with numerous metameric pouches of gastral cavity. Bilaterian ancestors crawled on the oral disc (= ventral side). These ancestors gave rise to both phanerosoic cnidarians and triploblastic bilaterians. Cnidarian ancestors attached to bottom by the aboral pole with the resulting degradation of aboral nerve ganglion. Bilateral symmetry of anthozoans is considered to be primitive feature for cnidarians. In case of triploblastic Bilateria, the elongated blastopore closed in the middle and subdivided into mouth and anus (amphistomy) and gastral pouches separated from the central part of gastral cavity and transformed to metameric coelomic chambers. The primary bilaterians are supposed to be complicated organisms having coelom and segmentation. The complexity of primary Bilateria provides an explanation for the abundance of highly organized organisms (arthropods, mollusks etc.) in Cambrian time. It is postulated that Ctenophora is the only group recent eumetazoans with primary axial symmetry.