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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 Spiralia (Lophotrochozoa) is a clade consisting of five phyla or groups of phyla whose interrelationships have proved difficult to resolve: Schizocoelia, Platyhelminthes, Gastrotricha, Gnathifera, and Polyzoa. The presence of ciliated ectoderm and primary larvae in the Spiralia appears to be plesiomorphic. Compared to the Ecdysozoa, which is easy to define, the Spiralia shows a wide range of plesiomorphies. The clade was named after its spiral cleavage, which occurs in representatives of most of the phyla of the clade and exhibits strongly conserved blastomere fates. Fertilisation may take place anywhere on the egg, for example in some polychaetes, and some embryos, particularly those of molluscs and annelids, are characterised by cross-like patterns of more conspicuous blastomeres.

Keywords:   spiral cleavage, Spiralia, Schizocoelia, Platyhelminthes, Gastrotricha, Gnathifera, Polyzoa, blastomere, molluscs, annelids

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