Xenacoelomorpha
| Xenacoelomorpha | |
|---|---|
 | |
| Proporus sp., a xenacoelomorph | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Xenacoelomorpha |
| Subphyla | |
Xenacoelomorpha is a phylum of small and very simple animals composed by the xenoturbellids and the acoelomorphs. It was suggested by molecular data and is also supported by morphological synapomorphies.[1]
Phylogeny
The clade Xenacoelomorpha, grouping Acoelomorpha and the genus Xenoturbella, was revealed by molecular studies. Initially it was considered to be a member of the deuterostomes, more precisely a sister group of the Ambulacraria, but a more recent transcriptome analysis concluded that it is the sister group to the Nephrozoa, which includes the protostomes and the deuterostomes, being therefore the basalmost bilaterian clade.[1][2]
Characteristics
All xenacoelomorphs lack a typical stomatogastric system, i.e., they do not have a true gut. In acoels, the mouth opens directly into a large endodermal syncytium, while in nemertodermatids and xenoturbellids there is a sack-like gut lined by unciliated cells.[4]
Their nervous system is basiepidermal, i.e., located right under the epidermis and do not have a brain. In xenoturbellids it is constituted by a simple nerve net without any special concentration of neurons, while in acoelomorphs it is arranged in a series of longitudinal bundles united in the anterior region by a ring comissure of variable complexity.[5]
The sensory organs include a statocyst and, in some groups, two unicellular ocelli.[4][5]
The epidermis of all xenacoelomorphans is ciliated. The cilia are composed by a set of 9 pairs of peripheral microtubules and one or two central microtubules (patterns 9+1 and 9+2, respectively). The pairs 4–7 terminate before the tip, creating a structure called "shelf".[6]
References
- 1 2 Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J.; Telford, M. J. (10 February 2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature. 470 (7333): 255–258. Bibcode:2011Natur.470..255P. doi:10.1038/nature09676. PMID 21307940.
- ↑ Perseke, M.; Hankeln, T.; Weich, B.; Fritzsch, G.; Stadler, P.F.; Israelsson, O.; Bernhard, D.; Schlegel, M. (August 2007). "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis" (PDF). Theory Biosci. 126 (1): 35–42. doi:10.1007/s12064-007-0007-7. PMID 18087755.
- ↑ Canon, J.T.; Vellutini, B.C.; Smith, J.; Ronquist, F.; Jondelius, U.; Hejnol, A. (4 February 2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. doi:10.1038/nature16520. PMID 26842059.
- 1 2 Achatz, Johannes G.; Chiodin, Marta; Salvenmoser, Willi; Tyler, Seth; Martinez, Pedro (June 2013). "The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)". Organisms Diversity & Evolution. 13 (2): 267–286. doi:10.1007/s13127-012-0112-4. ISSN 1439-6092.
- 1 2 Perea-Atienza, E.; Gavilan, B.; Chiodin, M.; Abril, J. F.; Hoff, K. J.; Poustka, A. J.; Martinez, P. (2015). "The nervous system of Xenacoelomorpha: a genomic perspective". Journal of Experimental Biology. 218 (4): 618–628. doi:10.1242/jeb.110379. ISSN 0022-0949.
- ↑ Franzen, Ake; Afzelius, Bjorn A. (January 1987). "The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations". Zoologica Scripta. 16 (1): 9–17. doi:10.1111/j.1463-6409.1987.tb00046.x. ISSN 0300-3256.