![]() On the origin of metameric segmentation and some other morphological questions. in Animal Evolution: Genes, Genomes, Fossils and Trees (eds Telford M. Deuterostomic development in the protostome Priapulus caudatus. Xenacoelomorpha is the sister group to Nephrozoa. Animal phylogeny and its evolutionary implications. Die systematische Einteilung des Tierreichs. (eds) Embryology, Constructing the Organism (Sinauer Associates, 1997). This challenges the previous evolutionary emphasis on extant blastoporal behaviours to explain the origin and diversification of bilaterian animals. We further assess these principles in the spiral-cleaving group Annelida, and propose that the developmental relationships of mouth and blastoporal openings are secondary by-products of variations in axial and mesoderm development. We show that the differential activity of Wnt signalling, together with the timing and location of mesoderm formation, correlate with the differential behaviour and fate of the blastopore. ![]() To identify the underlying mechanisms, we compared the development of two brachiopod species that show deuterostomy ( Novocrania anomala) and protostomy ( Terebratalia transversa), respectively. Although this distinction preluded the classification of bilaterian animals in Deuterostomia and Protostomia, and has influenced major scenarios of bilaterian evolution, the developmental basis for the appearance of these different embryonic patterns remains unclear. The mouth opening of bilaterian animals develops either separate from (deuterostomy) or connected to (protostomy) the embryonic blastopore, the site of endomesoderm internalization.
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