Xenoturbella is a deuterostome that eats molluscs

Xenoturbella bocki, first described in 1949 (ref. 1), is a delicate, ciliated, marine worm with a simple body plan: it lacks a through gut, organized gonads, excretory structures and

coelomic cavities. Its nervous system is a diffuse nerve net with no brain. Xenoturbella's affinities have long been obscure and it was initially linked to turbellarian flatworms1.

Subsequent authors considered it variously as related to hemichordates and echinoderms owing to similarities of nerve net and epidermal ultrastructure2,3, to acoelomorph flatworms based on

body plan and ciliary ultrastructure4,5,6 (also shared by hemichordates7), or as among the most primitive of Bilateria8. In 1997 two papers seemed to solve this uncertainty: molecular

phylogenetic analyses9 placed Xenoturbella within the bivalve molluscs, and eggs and larvae resembling those of bivalves were found within specimens of Xenoturbella10,11. This molluscan

origin implies that all bivalve characters are lost during a radical metamorphosis into the adult Xenoturbella. Here, using data from three genes, we show that the samples in these studies

were contaminated by bivalve embryos eaten by Xenoturbella and that Xenoturbella is in fact a deuterostome related to hemichordates and echinoderms.

We thank M. Akam and R. Jenner for comments on the manuscript, I. Ruiz Trillo for sharing unpublished results, and the scientists of Kristineberg Marine Station for help in sample

collection. We are grateful for support from the Wellcome Trust to M.J.T. and to D.T.J.L.

University Museum of Zoology, Department of Zoology, Downing Street, CB2 3EJ, Cambridge, UK

Zoological Museum (University of Copenhagen), Universitetsparken 15, DK-2100, Copenhagen, Denmark

Department of Zoology, The Natural History Museum, Cromwell Road, SW7 5BD, London, UK

The authors declare that they have no competing financial interests.

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