[Urbilateria, a complex organism?].

In order to establish the portrait of Urbilateria, the common ancestor of triblastic metazoan, this paper focuses on the antero-posterior segmentation frequently considered as characterising the bilaterian bauplan. The synthesis presented here describes the morphological, anatomical and functional aspects of this organisation. Furthermore it analyses the conditions of its emergence during the ontogenesis of Annelids, Arthropods and Chordates and identifies its genetic bases. The provided data exhibit the unitary character of the segmentation modalities among protostomian and deuterostomian organisms. This process occurs in two phases, involving a posterior proliferative zone after the gastrulation. It shows the similarity of the expression patterns of orthologous genes, the implication of comparable signalisation and regulation pathways. The congruence of the results obtained at both structural and molecular levels reinforce the segmental organisation conception of the common ancestor of Bilaterians.