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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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Animal Evolution

Claus Nielsen

Oxford University Press

The Eumetazoa (Gastraeozoa) is a clade consisting of a very well-defined monophyletic group, with the exception of the Placozoa (Trichoplax) and the ctenophores. Both morphological and more recent molecular studies suggest that Trichoplax is the sister group of the Neuralia (Cnidaria + Bilateria). Most molecular studies place the ctenophores in the basal metazoans, or in various other basal positions. The eumetazoans are distinguished by their characteristic epithelia, which have polarised cells joined together by cell junctions, and rest on a basal membrane with various structural molecules, including collagens, fibronectins, and proteoglycans. Like the sponges, the ontogeny of the eumetazoans begins from the zygote through a variety of embryological and larval stages before they attain adulthood. The new concept of early animal evolution posits that the eumetazoans originated from an organism with a sponge-grade life cycle, more specifically a homoscleromorph-like ancestor.

Keywords:   evolution, Eumetazoa, Trichoplax, ctenophores, epithelia, ontogeny

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