Xenoturbella is an enigmatic animal that is merely a ciliated bag with epithelial epidermis and gastrodermis, a subepidermal nerve plexus and a ventral mouth, but without an anus or any distinct organs. It is marine, free living, and up to 4 cm long. Its simplicity in organization has led to diverse interpretations during the last 50 years: as an acoelomorph £atworm, a paedomorphic holothurian or enteropneust, or a unique representative of a plesiomorphic phylum. I report here the previously unknown embryology of Xenoturbella that unequivocally corroborates a bivalve relationship and thus once and for all dismisses the potential new phylum. The simplicity of the adult Xenoturbella is due to neither plesiomorphy nor paedomorphy. It is caused by metamorphosis from a trochophore larva of molluscan type with a de¢ned organ system, including a concentrated nervous system with ganglia, to an adult without any de¢ned organs.

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The phylogenetic position of Xenoturbella bocki has been a matter of controversy since its description in 1949. We sequenced a second complete mitochondrial genome of this species and performed phylogenetic analyses based on the amino-acid sequences of all 13 mitochondrial proteincoding genes and on its gene order. Our results confirm the deuterostome relationship of Xenoturbella. However, in contrast to a recently published study [Bourlat et al., Nature 444: 85-88 (2006)], our data analysis suggests a more basal branching of Xenoturbella within the deuterostomes, rather than a sister-group relationship to the Ambulacraria (Hemichordata and Echinodermata).

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