Evolution of invertebrate nervous systems: the Chaetognatha as a case study

Authors

  • Steffen Harzsch,

    1. Zoological Institute and Museum, Ernst-Moritz-Arndt University Greifswald, Johann-Sebastian-Bach-Str. 11/12, D-17487 Greifswald; and Department of Evolutionary Neuroethology, Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knöll-Str. 8, D-07745 Jena, Germany
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  • Andreas Wanninger

    1. Research Group for Comparative Zoology, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark
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Steffen Harzsch, Zoological Institute and Museum, Ernst-Moritz-Arndt University Greifswald, Johann-Sebastian-Bach-Str. 11/12, D-17487 Greifswald, Germany. E-mail: sharzsch@ice.mpg.de

Abstract

Harzsch, S. and Wanninger, A. 2010. Evolution of invertebrate nervous systems: the Chaetognatha as a case study. —Acta Zoologica (Stockholm) 91: 35–43

Although recent molecular studies indicate that Chaetognatha may be one of the earliest Bilaterian offshoots, the phylogenetic position of this taxon still is a matter of ongoing debate. In this contribution, we review recent attempts to contribute phylogenetic information on the Chaetognatha by analysing structure and development of their nervous system (neurophylogeny). Analysing this group of organisms also has a major impact on our understanding of nervous system evolution in Bilateria. We review recent evidence from this field and suggest that Urbilateria already was equipped with the genetic toolkit required to build a complex, concentrated central nervous system (CNS), although this was not expressed phenotypically so that Urbilateria was equipped with a nerve plexus and not a CNS. This implies that in the deep metazoan nodes, concentration of the ancestral plexus occurred twice independently, namely once after the protostome–deuterostome split on the branch leading to the protostomes (resulting in a ventrally positioned nerve cord) and once along the chordate line (with a dorsal nerve cord).

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