Xenacoelomorpha

The clade (groupings of organisms based on their most recent shared/common ancestors) Xenacoelomorpha groups the Acoelomorpha and the genus Xenoturbella, due to molecular studies.[4] Initially this phylum was considered to be a member of the deuterostomes,[2] (meanining during development, as an embryo, the anus develops first and then the mouth), but because of recent transcriptome analysis, it was concluded that phylum Xenacoelomorpha is the sister group (two closest relatives in a phylogenetic tree) to the Nephrozoa, which includes both the protostomes (where in development of the embryo, the mouth forms first, then the anus) and the deuterostomes, therefore phylum Xenacoelomorpha is the basalmost bilaterian clade.[5][6] This means they are neither a deuterostome nor protostome.

However, this argument is difficult to conjure because these animals have high rates of mutation and therefore their basal placement in phylogeny trees may be due to the long branch attraction (LBA) where animals that share morphological features are grouped together as morphological features are often passed from ancestor to descendants. These analyses suggest that the xenacoelomorphs are the sister group of Ambulacraria forming the clade Xenambulacraria and that therefore its simplicity evolved from a complex ancestor.[7][8] Having a larger number of species within this group would allow for better conclusions and analysis to be made within the phylum and in groups closely related to the phylum.

It is also important to remember that this is a new phylum which means that there is new information still being discovered and considered which can change their place on the phylogeny tree. At this current time, this is how phylum Xenacoelomorpha is represented.

All species within phylum xenacoelomorphs are bilateral, meaning they have a mirror image on their right and left axis, like humans. While they are triploblasts (meaning they have the three germ layers: ectoderm, endoderm, and mesoderm) they do not have a true gut (they lack a coelom) because they have an acoelomate body plan. While animals that are diploblastic (only have two germ layers: ectoderm and endoderm) also lack a coelom, they do not have an acoelomate body plan because they lack the mesoderm germ layer. 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.[9]

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.[10]

The sensory organs include a statocyst (for balance) and some groups have two unicellular ocelli (simple eyes).[9][10]

The epidermis of all species within the phylum is ciliated. The cilia are composed of 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 a "shelf".[11]

This phylum consists of species that are free-living, parasitic, and symbiotic. They are small flat like worms found in marine and sometimes brackish water environments, on the sediments. They can be found at depths of almost 4 km and near hydrothermal vents. The phylum is hermaphrodite (male and female sex organs) and reproduces sexually with direct development, meaning they skip the vulnerable larval stage.

• List of bilaterial animal orders