Acoelomorpha and Xenoturbellida
Acoelomorpha, comprising Acoela and Nemertodermatida, and Xenoturbellida (with one single hitherto described species, Xenoturbella bocki) are simple, aquatic, acoelomate worms that measure between 100 μm and 1 cm. Acoelomorpha and Xenoturbella are found t
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Acoelomorpha and Xenoturbellida Andreas Hejnol
Chapter vignette artwork by Brigitte Baldrian. © Brigitte Baldrian and Andreas Wanninger. A. Hejnol Sars International Centre for Marine Molecular Biology, University of Bergen, Thormøhlensgate 55, Bergen 5008, Norway e-mail: [email protected] A. Wanninger (ed.), Evolutionary Developmental Biology of Invertebrates 1: Introduction, Non-Bilateria, Acoelomorpha, Xenoturbellida, Chaetognatha DOI 10.1007/978-3-7091-1862-7_9, © Springer-Verlag Wien 2015
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INTRODUCTION Acoelomorpha, comprising Acoela and Nemertodermatida (Ehlers 1985), and Xenoturbellida (with one single hitherto described species, Xenoturbella bocki) are simple, aquatic, acoelomate worms that measure between 100 μm and 1 cm. Acoelomorpha and Xenoturbella are found to cluster together as the monophyletic Xenacoelomorpha in some recent molecular phylogenetic analyses. With only few exceptions all species are marine, with most of them living in the interstitial environment. Xenoturbellids and acoelomorphs possess a simple nervous system that generally is a basiepidermal nerve net; however, in some cases this net is condensed into basiepidermal neurite bundles at different parts of the body or is submerged under the epidermis where condensed brains and submuscular cords are formed (Achatz and Martinez 2012; Hejnol 2015). Some Acoela possess eye spots, while most nemertodermatids, Xenoturbella, and Acoela lack eyes (Rieger et al. 1991). Recent internal phylogenetic analyses suggest that eyes were absent from the ground pattern of Acoelomorpha (Jondelius et al. 2011). A prominent gravitational sensory organ, the statocyst, is present in all xenacoelomorph taxa, albeit with differing ultrastructure (Ferrero 1973; Ehlers 1991). The digestive tract is epithelial in xenoturbellids and nemertodermatids, while in acoels, it forms via a syncytium without a cavity (Smith and Tyler 1985). The digestive system has a single opening that corresponds to the mouth opening of other Bilateria (Hejnol and Martindale 2008a). The mesoderm of acoels consists of a limited number of cell types – which are myocytes, cells associated with gonads, and neoblasts (Chiodin et al. 2013) – whereas nephridia and a blood vascular system are absent. The musculature forms an orthogon composed of ring and longitudinal muscles that surround the whole body. Specialized musculature is present in the copulatory structures of acoels (Ladurner and Rieger 2000; Hooge 2001). Altogether, acoelomorphs and xenoturbellids are rather simply organized animals, and this is also the reason why their phylogenetic position has been of great interest and is still under debate (Fig. 9.1). After the exclusion of Acoelomorpha
from the Platyhelminthes (Carranza et al. 1997; Ruiz-Trillo et al. 1999, 2002; Jondelius et al. 2002; Wallberg et al. 2007; Egger et al. 2009; Paps et al. 2009), molecular phylogenies have produced ambiguous results concerning their definite placement within the tree of life. Phylogenomic studies that include Xenoturbella
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