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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 Urochordata

Phylum Urochordata

(p.365) 64 Phylum Urochordata
Animal Evolution

Claus Nielsen

Oxford University Press

The Urochordata is a phylum of very distinct, marine animals comprising roughly 3,000 known species. The urochordates, or tunicates, have traditionally been arranged in three classes: Ascidiacea, Thaliacea, and Appendicularia (also called Larvacea). The sorberaceans and Octacnemus, which lack the branchial basket, are considered specialised ascidians. However, molecular phylogeny, along with combined morphological and molecular data, indicate that the planktonic thaliaceans are an ingroup of the ascidians. Some studies place the appendicularians, which include the model organism Oikopleura, as the sister group of the ascidians, but others place them in an ingroup position. Based on the ‘new’ chordate phylogeny, which places the urochordates and vertebrates as sister groups, the urochordates are presumed to have evolved from free-swimming chordates.

Keywords:   urochordates, Urochordata, tunicates, Ascidiacea, Thaliacea, Appendicularia, ascidians, thaliaceans, appendicularians, chordates

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