Phylogenetischer status und bedeutung der mesenchymaten Bilateria. Principles of Comparative Anatomy of Invertebrates (Univ. On the origin and evolution of the lower Metazoa. Vol II: Platyhelminthes and Rhynchocoela. Die Organisation der Turbellaria Acoela (von Wilhelm Engelmann, 1891). New ideas on the origin of bilateral animals. Evolution of deuterostomy – and origin of the chordates. Evolution of the bilaterian larval foregut. Polychaete trunk neuroectoderm converges and extends by mediolateral cell intercalation. H., Zelada-Gonzales, F., Burgtorf, C., Wittbrodt, J. Dorsal or ventral: similarities in fate maps and gastrulation patterns in annelids, arthropods and chordates. Die entstehung der metamerie der wirbellosen. On the early phylogeny of the Metazoa: the bilatero-gastrea theory.

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 exceptional case study allows us to exclude many developmental variables that might influence gastrulation (for example, yolk, egg size, cell number) and that often become significant when comparing blastoporal behaviours of embryos of distantly related and rapidly evolving lineages. After fertilization, the embryos of these two brachiopod species undergo radial cleavage, gastrulation by invagination and form a planktonic larva that eventually metamorphoses into the adult 22, 23 ( Supplementary Fig. transversa share a comparable ecology and reproductive strategy (similar yolk content, developmental timing, lecithotrophy, indirect life cycle), but display deuterostomic 22, 24 and protostomic 23 development, respectively ( Fig. Brachiopods are marine, sessile, filter-feeding invertebrates that belong to the clade Spiralia 3 that, together with the Ecdysozoa and Chaetognatha, form the Protostomia 3. 1a,j), on which previous experimental embryological work had been conducted 22, 23 ( Supplementary Fig. To identify the developmental basis of different blastoporal fates, we strategically selected two brachiopod species, N. 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.

Blastopore that becomes mouth