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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.238) 42 ECDYSOZOA
Animal Evolution

Claus Nielsen

Oxford University Press

The Ecdysozoa is a clade whose ‘defining’ apomorphy is the moulting of the cuticle (ecdysis), although a few annelids have also shown complete or partial moulting. Ecdysis appears to be regulated by the hormone 20-hydroxyecdysone in arthropods, even as other hormones are involved in the ecdysis of nematodes. There are two recognised groups, Panarthropoda and Cycloneuralia, which are both morphologically well delimited. The position of the tardigrades has been uncertain, but new studies on their embryology strongly support a relationship with the arthropods. The anomalocaridids have been interpreted as stem-group arthropods, although the structure of the mouth implies a position as stem-group ecdysozoans. The palaeoscolecids have been regarded as stem-group ecdysozoans, but they may also be stem-group priapulids.

Keywords:   ecdysis, Ecdysozoa, arthropods, Panarthropoda, Cycloneuralia, tardigrades, anomalocaridids, ecdysozoans, palaeoscolecids, priapulids

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