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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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(p.62) 16 BILATERIA
Animal Evolution

Claus Nielsen

Oxford University Press

The Bilateria is a clade comprised of two well-separated groups, Acoelomorpha and Eubilateria. The acoelomorphs have the structure of a bilateral gastraea, while the eubilaterians have a tube-shaped gut. Recent evidence suggests that the acoelomorphs are the sister group of the ambulacrarians, implying massive gene losses. The Bilateria and the Coelenterata, two main groups within the Eumetazoa, have long been compared with each other. The Bilateria is characterised by bilateral symmetry, whose origin appears to be intimately linked with a change of life style. In Nematostella, Hox genes appear to be involved in bilateral patterning along the primary (anterior–posterior) axis) and along the directive axis. However, more recent studies show that the evolution of both the expression of bone morphogenetic protein genes and Hox genes has occurred along separate lines in bilaterians and cnidarians. Hence, ‘true bilaterality’ is a bilaterian apomorphy.

Keywords:   acoelomorphs, Bilateria, Acoelomorpha, Eubilateria, eubilaterians, Coelenterata, Hox genes, evolution, bilaterians, bilaterality

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