New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny

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 flatworm, 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 defined organ system, including a concentrated nervous system with ganglia, to an adult without any defined organs.