Xenacoelomorpha[2] (/ˌzɛnəˌsɛloʊˈmɔːrfə/) is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs.

[8] However, some studies point out that their basal placement may be caused by high mutation rates leading to long branch attraction (LBA).

These analyses suggest that the xenacoelomorphs are instead the sister group of Ambulacraria forming the clade Xenambulacraria and that despite their simple body plans, they actually derive from a more complex ancestor.

Ambulacraria Xenacoelomorpha Chordata Ecdysozoa Spiralia For multiple decades, the genus Xenoturbella contained only one species, X. bocki.

A 2016 study analyzed three of the four Nemertodermatid genera and found that Sterreria and Meara are closer to each other than to Nemertoderma, while Nemertinoides was left unplaced.

X. japonica X. bocki X. hollandorum X. monstrosa X. churro X. profunda Actinoposthiidae Antigonariidae Antroposthiidae Diopisthoporidae Nadinidae Paratomellidae Taurididae Paratomellidae Hallangiidae Hofsteniidae Solenofilomorphidae Proporidae Isodiametridae Dakuidae Convolutidae Mecynostomidae Ascoparia Flagellophora Nemertoderma Nemertinoides Meara Sterreria The phylum consists of small, flat and worm-like creatures found in marine and sometimes brackish water environments, on the sediments.

The phylum is hermaphroditic (all individuals have both male and female sex organs) and reproduces sexually with direct development, meaning they skip a potentially vulnerable larval stage.

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.

In acoelomorphs the nervous system is arranged in a series of longitudinal bundles, united in the anterior region by a ring comissure of variable complexity.