Xenacoelomorpha, Philippe, Brinkmann, Copley, Moroz, Nakano, Poustka, Wallberg, Peterson & Telford, 2011

Martinez, P., Perea-Atienza, E., Gavilán, B., Fernandez, C. & Sprecher, S., 2017, The study of xenacoelomorph nervous systems. Molecular and morphological perspectives, Invertebrate Zoology 14 (1), pp. 32-44 : 34

Xenacoelomorpha : The species selected

The main focus of our research efforts has been the study of neurogenic gene batteries and their connected networks in the phylum Xenacoelomorpha. This is an enigmatic phylum composed of three major taxa: acoel flatworms, nemertodermatids, and xenoturbellids. The earliest branching clade within this phylum, the Xenoturbellida ( Bourlat et al., 2006) was, until recently, only represented by two nominal species: Xenoturbella bocki ( Westblad, 1949) and

X. westbladi ( Israelsson, 1999) , though newly identified specimens have been collected from the Pacific ( Rouse et al., 2016). These worms arenotablylargerthantheacoelomorphs ( Acoela plus Nemertodermatida ) but share a relatively simple morphology with them. The Nemertodermatida consists of several species of marine worms (see: Meyer-Wachsmuth et al., 2014 for a recent assessment); while Acoela contains the majority of the phylum’s species: over 380. Acoels are mostly marine bilateral flatworms with a simple unsegmented body plan. They are triploblastic and acoelomate, with an outer epidermis of multiciliated cells and a single gut opening. They share some morphological features with nemertodermatids, such as epidermal ciliation, intestine organization, certain glandular and sensory structures, and the limited presence of an extracellular matrix ( Raikova et al., 2004; see: Achatz et al., 2013 for a complete bibliography).

To date, all phylogenomic studies demonstrate that Xenacoelomorpha conform a clear monophyletic group, which allows us to study specific genomic and morphological patterns of diversification within it. The monophyly of Xenacoelomorpha is also supported by morphological similarities ( Ehlers, 1985; Smith, Tyler, 1986; Lundin, Hendelberg, 1995; Lundin, 1997, 2000); for instance, the ultrastructure of the ciliary tips and the system of epidermal ciliary rootlets. The phylogenetic relation of Xenacoelomorpha to the rest of metazoan phyla is, however, still controversial. Recent phylogenomic analysis has found that Xenacoelomorpha is the earliest offshoot of the bilaterians ( Hejnol et al., 2009; Cannon et al., 2016). However, a different study suggested that instead, Xenacoelomorpha represent a deuterostomian group sister to the Ambulacraria (the group formed of Echinoderms and Hemichordates) (Philippe et al., 2011). If Xenacoelomorpha represents the earliest offshoot of the bilaterians, understanding the evolution of its NS will be fundamental to explain the origin of bilaterians; and in a good part, the radial–bilateral transition (RBT). In contrast, if Xenacoelomorpha represents a basal deuterostome taxon, the analysis will be valuable to understand the origin and evolution of deuterostomes, when a significant body plan reorganization also occurred. All in all, our interest is the origin of the brain as a complex anatomical structure.