Xenoturbellida: the fourth deuterostome phylum and the diet of worms.

Since the discovery of the marine worm Xenoturbella bocki in 1915 by Sixten Bock and its first published description by Einar Westblad (Westblad,1949, Arkiv Zoologi 1:3-29), Xenoturbella was generally allied to the turbellarian flatworms, perhaps most closely to acoelomorphs. In 1997, however, analyses of ribosomal DNA (Norén and Jondelius, 1997, Nature 390:31-32) and developing oocytes (Israelsson, 1997, Nature 390:32) [and, subsequently, embryos (Israelsson, 1999, Proc R Soc Lond B 266:835-841)] recovered from Xenoturbella specimens led to the surprising conclusion that it was in fact a highly degenerate bivalve mollusc. Bourlat et al. showed in 2003 that this result was due to contamination from bivalves in its diet (Bourlat et al.,2003, Nature 424:925-928). Our analyses showed Xenoturbella is a deuterostome, related to the Ambulacraria (echinoderms and hemichordates). Subsequent work has shown that Xenoturbellida is a separate lineage from the Ambulacraria and therefore constitutes the fourth deuterostome phylum (Bourlat et al.,2006, Nature 444:85-88). I consider this phylogenetic position in the light of what is known of its genetics, morphology, and ontogeny. I examine what this phylogenetic position for Xenoturbella can tell us about its own evolution and what light this might shine on the common ancestor of the deuterostomes and hence on the origins of the chordates. Copyright 2008 Wiley-Liss, Inc.