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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 Acoela

Phylum Acoela

(p.66) 18 Phylum Acoela
Animal Evolution

Claus Nielsen

Oxford University Press

The Acoela is a small phylum which comprises approximately 400 species of small, mostly marine ‘worms’ that are completely ciliated. The acoels have an unquestioned monophyly and a well-documented phylogenetic position as one of the ‘basal’ metazoan groups. They were previously interpreted as ‘primitive’ platyhelminths, but more recent molecular studies place them at the base of the Eumetazoa, together with the nemertodermatids, and sometimes also with Xenoturbella, as the sister group of the Eubilateria. A few studies have not been able to establish a position together with the Platyhelminthes and the ‘basal’ position, which is supported by the low number of microRNAs. The acoels have a bilateral, creeping gastrulae with mesoderm, and their sister-group relationship with the Nemertodermatida is implied by their possession of the characteristic frontal organ.

Keywords:   worms, Acoela, acoels, Eumetazoa, nemertodermatids, Xenoturbella, Eubilateria, gastrulae, mesoderm, Nemertodermatida

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