Acoel flatworms: earliest extant bilaterian Metazoans, not members of Platyhelminthes.

@article{RuizTrillo1999AcoelFE,
  title={Acoel flatworms: earliest extant bilaterian Metazoans, not members of Platyhelminthes.},
  author={I{\~n}aki Ruiz-Trillo and Marta Riutort and D. Timothy J. Littlewood and Elisabeth Anne Herniou and Jaume Bagu{\~n}{\'a}},
  journal={Science},
  year={1999},
  volume={283 5409},
  pages={
          1919-23
        }
}
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… 

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...

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