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Science 19 March 1999:
Vol. 283. no. 5409, pp. 1919 - 1923
DOI: 10.1126/science.283.5409.1919
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Reports

Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes

Iñaki Ruiz-Trillo, 1 Marta Riutort, 1 D. Timothy J. Littlewood, 2 Elisabeth A. Herniou, 2 Jaume Baguñà 1*

Because of their simple organization the Acoela have been considered to be either primitive bilaterians or descendants of coelomates through secondary loss of derived features. Sequence data of 18S ribosomal DNA genes from non-fast evolving species of acoels and other metazoans reveal that this group does not belong to the Platyhelminthes but represents the extant members of the earliest divergent Bilateria, an interpretation that is supported by recent studies on the embryonic cleavage pattern and nervous system of acoels. This study has implications for understanding the evolution of major body plans, and for perceptions of the Cambrian evolutionary explosion.

1 Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain.
2 Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.
*   To whom correspondence should be addressed. E-mail: bagunya{at}porthos.bio.ub.es

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