in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to Ambulacraria within deuterostomes (i.e., the Xenambulacraria hypothesis) on the one hand, and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other. However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference. Anatomy Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, not even having a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They lack body cavities (acoelomate structure), a hindgut or an anus. The epidermal cells of acoelomorphs are unable to proliferate, a feature that is only shared with rhabditophoran flatworms and was for some time considered a strong evidence for the position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells. The nervous system of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was the precursor of the cephalization of the nerve system in more derived bilaterians. After decapitation, such a "brain" (rather, a cerebroid ganglion) regenerates in a few weeks. The sensory organs include a statocyst – which presumably helps them orient to gravity –, and, in some cases, ancestral pigment-spot ocelli capable of detecting light. Acoelomorphs are simultaneous hermaphrodites, but have no gonads and no ducts associated with the female reproductive system. Instead, gametes are produced from the mesenchymal cells that fill the body between the epidermis and the digestive vacuole. References External links Phylogeny of Lower Worms of the Meiofauna (Acoelomorpha) Acoelomorpha at the Turbellarian taxonomic database Acoelomorpha: A Phylogenetic Headache. Earthling Nature Animal subphyla