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).
- Bourlat, S. J., T. Juliusdottir, C. J. Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S. Lander, M. Thorndyke, H. Nakano, A. B. Kohn, A. Heyland, L. L. Moroz, R. R. Copley, M. J. Telford (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature 444 (7115): 85–88. Bibcode 2006Natur.444...85B. doi:10.1038/nature05241. PMID 17051155
- Bourlat, S. J., C. Nielsen, A. E. Lockyer, D. Timothy, J. Littlewood, M. J. Telford (2003). "Xenoturbella is a deuterostome that eats molluscs". Nature 424 (6951): 925–928. doi:10.1038/nature01851. PMID 12931184.
- Edgecombe, G. D.; Giribet, G.; Dunn, C. W.; Hejnol, A.; Kristensen, R. M.; Neves, R. C.; Rouse, G. W.; Worsaae, K. et al. (2011). "Higher-level metazoan relationships: Recent progress and remaining questions". Organisms Diversity & Evolution 11 (2): 151. doi:10.1007/s13127-011-0044-4
- Haszprunar, G., R.M. Rieger, P. Schuchert (1991). "Extant 'Problematica' within or near the Metazoa." In: Simonetta, A.M. & Conway Morris, S. (eds.): The Early Evolution of Metazoa and the Significance of Problematic Taxa. Oxford Univ. Press, Cambridge. pp. 99–105
- Hejnol, A., Obst, M., Stamatakis, A., Ott, M., Rouse, G. W., Edgecombe, G. D., et al. (2009). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society, Series B, 276, 4261–4270.
- Israelsson, Olle and Graham E Budd (2005). "Eggs and embryos in Xenoturbella (phylum uncertain) are not ingested prey". Development Genes and Evolution 215: 358-63.
- Lundin, K. (1998). The epidermal ciliary rootlets of Xenoturbella bocki (Xenoturbellida) revisited: new support for a possible kinship with the Acoelomorpha (Platyhelminthes). Zoologica Scripta, 27, 263–270.
- Noren, M. and U. Jondelius (1997). "Xenoturbella's molluscan relatives..". Nature 390 (6655): 31–32. Bibcode 1997Natur.390...31N. doi:10.1038/36242
- Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis". Theory Biosci. 126(1):35-42.
- Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J. et al. (2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature 470 (7333): 255-258. Bibcode 2011Natur.470..255P. doi:10.1038/nature09676.
- Raikova, O. I., Reuter, M., Jondelius, U., & Gustafsson, M. K. S. (2000). An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.). Zoomorphology, 120, 107–118.
- Telford, M. J. (2008). "Xenoturbellida: the fourth deuterostome phylum and the diet of worms". Genesis 46 (11): 580–586. doi:10.1002/dvg.20414. PMID 18821586.
- Wellcome Trust Press Release. Enigmatic worm identified as mankind's long lost relative – Accessed January 3, 2008
- Westblad, E (1949) Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type. Arkiv för Zoologi 1:3-29.
- Xenoturbella – Back to the Basics – Accessed January 3, 2008