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Xenoturbellida is a phylum of ciliated marine worms.  It contains a single genus, Xenoturbella, and two species, Xenoturbella bocki and Xenoturbella westbladi.

The first known species (Xenoturbella bocki) was discovered in 1915 by Sixten Bock but the first published description was only in 1949 by Einar Westblad (Westblad 1949). Its taxonomic position has been considered enigmatic since its discovery. Earlier it was suspected to be closely related to molluscs (Noren & Jondelius 1997), but it turned out that the DNA analysis of this study was contaminated with DNA from molluscs which Xenoturbella may have eaten (Bourlat et al. 2003; Israelsson & Budd 2006). The genus is now the sole member of its own phylum Xenoturbellida (Haszprunar et al. 1991; Bourlat et al. 2006), and there is strong support from both morphological and molecular studies for a close relationship with Acoelomorpha (Lundin 1998, Raikova et al. 2000, Hejnol et al. 2009).

A 2003 DNA study positioned Xenoturbella as a primitive deuterostome outside the established phyla (Bourlat et al. 2003). The deuterostome affiliations were recently corroborated by studies that indicate a basal position of this phylum within the deuterostomes (Perseke et al 2007, Telford 2008) or a sister group relationship with the echinoderms and hemichordates (Philippe et al. 2011). However, some consider the evidence for a position within deuterostomes weak and favor the placement of Xenoturbella + Acoelomorpha more basally among Metazoa (Edgecombe et al. 2011).

Xenoturbella has a very simple body plan: it has no brain, no through gut, no excretory system, no organized gonads (but does have gametes; eggs and embryos occur in follicles [Israelsson and Budd 2005]), or any other defined organs except for a statocyst containing flagellated cells; it has cilia and a diffuse nervous system. The animal is up to 4 centimetres (1.6 in) long, and has been found off the coasts of Sweden, Scotland and Iceland (Wellcome Trust Press Release). 

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