Xenacoelomorpha
| Xenacoelomorpha | |
|---|---|
 |
|
| Proporus sp., a xenacoelomorph | |
| Scientific classification | |
| Kingdom: | |
| Phylum: |
Xenacoelomorpha
|
| Subphyla | |
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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 problematic 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][3]
Characteristics
All xenacoelomorphs lack a typical stomatogastric system, i.e., they do not have a true gut. In acoels, the mouth opens directly into the mesoderm or into a large 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
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- ↑ 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. Available on-line at [1]
- ↑ Cannon JT, Vellutini BC, Smith J, Ronquist F, Jondelius U, Hejnol A. (2016) "Xenacoelomorpha is the sister group to Nephrozoa." Nature 530(7588):89-93. doi: 10.1038/nature16520
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