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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 Placozoa

Phylum Placozoa

(p.39) 11 Phylum Placozoa
Animal Evolution

Claus Nielsen

Oxford University Press

The Placozoa is a phylum having only one described species, Trichoplax adhaerens, even as genetic studies suggest the presence of many species that should represent several genera or families. Recently, the Ediacaran fossil Dickinsonia has been proposed as a placozoan on the basis of fossil ‘trails’ that resemble tracks of material digested by Trichoplax. Trichoplax has a flat body consisting of one layer of diploid epithelial cells. The body plan has been interpreted either as an unfolded gastrula or as a two-layered organism called plakula, from which the other eumetazoans probably evolved through invagination. Trichoplax reproduces asexually by fission or by formation of spherical swarmers from the upper side, while sexual reproduction remains poorly understood. Evidence points to Trichoplax as the sister group of the neuralians.

Keywords:   neuralians, Placozoa, Trichoplax adhaerens, Dickinsonia, epithelial cells, gastrula, plakula, eumetazoans, reproduction

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