ИСТИНА

Comparative analysis provides evidence that the last common ancestor of cnidarians and triploblastic bilaterians was a bilateral animal. So, the bilateral symmetry is therefore older than triploblastic condition. As ctenophores branched off before the cnidarian/triploblastic bilaterian bifurcation, they keep the axial symmetry. The common cnidarian/triploblastic bilaterian ancestor was presumably a mobile benthic coelenterate with elongated mouth, metameric coelenteron pouches and metameric tentacles. It crawled on the oral surface which evolved into the ventral side of triploblastic bilaterians. Ediacaran metameric bilaterians could be considered to be kin to such common cnidarian/triploblastic bilaterian ancestor. Protobilaterians were comparatively complicated organisms having the through gut, coelom, segmentation and supposedly metameric limbs. New finding let us revive the old ideas of Sedgwick and Van Beneden on the origin of metamery from the cyclomery in new appearance. This hypothesis enables to explain emerging of quite sophisticated forms in Cambrian, i.e. Cambrian explosion phenomenon. Forms with through gut, coelom, segmentation, and metameric limbs occur in all four principal groups of triploblastic bilaterians (Deuterostomia, Ecdysozoa, Lophophorata, and Trochozoa). It seems to be much more likely that the last common ancestor of triploblastic bilaterians had already had all mentioned characters than to imagine that through gut, coelom, segmentation, and metameric limbs appeared in bilaterian evolution three or four times independently.