Category:Xenacoelomorpha

Xenacoelomorpha[2] is a basal bilaterian phylum of small and very simple animals, grouping the xenoturbellids with the acoelomorphs. This grouping was suggested by morphological synapomorphies,[3] and confirmed by phylogenomic analyses of molecular data.[4][2] Xenacoelomorphs emerged with the Nephrozoa as sister clade.

Phylogenetics
The clade Xenacoelomorpha, grouping Acoelomorpha and the genus Xenoturbella, was revealed by molecular studies.[4] Initially it was considered to be a member of the deuterostomes[2], 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.[5][6]

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.[7]

The nervous system is basiepidermal, i.e., located right under the epidermis, and a brain is absent. 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.[8]