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Xenoturbella is a genus of very simple up to a few centimeters long. It contains a small number of marine benthos worm-like species.
The first known species ( Xenoturbella bocki) was collected in 1878 and 1879 in the Gullmar fiord on the Swedish west coast by August Malm and is stored in the collection of the Gothenburg Natural History Museum. A specimen is on display in the exhibition. It was collected again in the Gullmar fiord in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad. The type specimens are kept at the Swedish Museum of Natural History in Stockholm.
The nervous system is composed by a net of interconnected beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.
Species of Xenoturbella also lack a respiratory, circulatory and excretory system. In fact, there are no defined organs, except for an anterior statocyst containing flagellated cells and a frontal pore organ. There are no organized , but are produced. Adults producing sperm are very rarely observed, but eggs and are known to occur in follicles.
Research on the species Xenoturbella bocki has shown it to have external fertilization, with eggs and sperm being released from new openings in the body wall. Gametes released into the water through ruptures also occurs in Xenoturbella
Eggs of Xenoturbella are wide, pale orange and opaque. Newly hatched embryos are free-swimming (tending to stay close to water surface) and ciliated. They feature no mouth and they do not apparently feed. They are similar to the juveniles of acoelomate Neochildia fusca.
The two smaller species, X. bocki and X. hollandorum, which are up to long, are found in shallower waters less than deep. They form a clade together with a third species, X. japonica, which is slightly over long and was found in waters less than deep. Three larger species, X. monstrosa, X. churro, and X. profunda, which were or greater long and lived in deeper waters , form another clade.
A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria. The deuterostome affiliations were then recovered by studies that indicate a basal position of this phylum within the deuterostomes or in a sister group relationship with the Ambulacraria.
However, morphological characters, such as the structure of epidermal cilium, suggested a close relationship with Acoelomorpha, another problematic group. The study of the embryonic stages of Xenoturbella also showed that it is a direct developer without a feeding larval stage, and this developmental mode is similar to that of acoelomorphs. Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha.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. This clade was named Xenacoelomorpha.
The monophyly of Xenacoelomorpha soon became established, but its position as either a basal bilaterian clade or a deuterostome remained unresolved until 2016 when two new studies, with increased gene and taxon sampling, again placed Xenoturbella as the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as sister to Nephrozoa (Protostomia plus Deuterostomia), and therefore the basalmost bilaterian phylum.
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