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

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Abstract
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 18 S 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.