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Animal EvolutionInterrelationships of the Living Phyla$
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Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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Phylum Chaetognatha

Phylum Chaetognatha

(p.304) 55 Phylum Chaetognatha
Animal Evolution

Claus Nielsen

Oxford University Press

The Chaetognatha, or arrowworms, is a small phylum consisting of roughly 120 species of marine, mainly holopelagic ‘worms’. Some species, mainly of Spadella and Paraspadella, are benthic. Protosagitta, from the Lower Cambrian (Chengjiang), is undoubtedly a chaetognath. The chaetognaths have an apparently unquestioned monophyly, but their position is not well established. The group has usually been placed in the Deuterostomia based on the fate of the blastopore, but the nervous system and its ontogeny is clearly protostomian. Molecular phylogeny indicates many different positions for the Chaetognatha: ‘between’ Protostomia and Deuterostomia, for example, or as sister group of Lophotrochozoa + Ecdysozoa. On the basis of structure, embryology, and the Hox genes, the chaetognaths are a basal protostomian group. However, there is no unequivocal evidence to support its sister-group relationship with the protostomes, spiralians, or ecdysozoans.

Keywords:   arrowworms, Chaetognatha, Spadella, Paraspadella, chaetognaths, Deuterostomia, Protostomia, protostomes, spiralians, ecdysozoans

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