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Nemertodermatida is the sister group of the Acoela, which together form the Acoelomorpha, a taxon that comprises bilaterally symmetric, small aquatic worms. While there are several descriptions of the embryology of acoel species, descriptions of nemertodermatid development are scarce. To be able to reconstruct the ground pattern of the Acoelomorpha it is crucial to gain more information about the development of several nemertodermatid species. Here we describe the development of the nemertodermatid  Meara stichopi  using light and fluorescent microscopic methods.    Results  We have collected  Meara stichopi  during several seasons and reconstruct the complex annual reproductive cycle dependent on the sea cucumber  Parastichopus tremulus . Using common fluorescent markers for musculature (BODIPY FL-phallacidin) and neurons (antibodies against FMRFamide, serotonin, tyrosinated-tubulin) and live imaging techniques, we followed embryogenesis which takes approximately 9–10 weeks. The cleavage pattern is stereotypic up to the 16-cell stage. Ring- and longitudinal musculature start to develop during week 6, followed by the formation of the basiepidermal nervous system. The juvenile is hatching without mouth opening and has a basiepidermal nerve net with two dorsal neurite bundles and an anterior condensation.    Conclusions  The development of  Meara stichopi  differs from the development of Acoela in that it is less stereotypic and does not follow the typical acoel duet cleavage program. During late development  Meara stichopi  does not show a temporal anterior to posterior gradient during muscle and nervous system formation.

Development and juvenile anatomy of the nemertodermatid Meara stichopi (Bock) Westblad 1949 (Acoelomorpha)