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Recent phylogenetic analyses of ribosomal and protein coding nuclear genes place the marine worms within the Nemertodermatida as one of the oldest lineages among the bilaterian animals. We studied the early embryonic cleavage in Nemertoderma westbladi to provide the first account of nemertodermatid early development. Live embryos were studied with interference microscopy and fixed embryos were either sectioned or studied with confocal laser scanning microscopy. Initially the divisions in the embryo are radial, but then micromeres are shifted clockwise generating a spiral pattern. The four-cell stage is characterized by duets of macromeres and micromeres and thus resembles the duet cleavage reported from members of the Acoela. However, subsequent stages differ from the acoel duet pattern and also from quartet spiral cleavage. The optimization of the cleavage pattern on current phylogenetic hypotheses with Nemertodermatida and Acoela as early bilaterian branches is discussed.

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This contribution is one of the very first reports on Swedish species of freshwater Catenulida, a group of free-living, small flatworms. A total of 13 species were collected from 33 localities all over the country. Four species, all belonging to the taxon Stenostomum, are new to science. S. gotlandense n. sp., found on Gotland, has small ciliated pits located more posteriorly than other species of Stenostomum, large anterior brain lobes and a large mouth opening with peculiar wrinkles of the proximal rim of the pharynx. S. handoelense n. sp., from Jämtland, has long ciliated pits and a mobile prostomium with a lined appearance due to “metamerical” anterior brain lobes. S. heebuktense n. sp., from Bohuslän, has small ciliated pits and an extremely large and muscular pharynx which, in living animals, is bluish. S. steveoi n. sp., found in the provinces Bohuslän and Småland, has small ciliated pits and a large rectangular mouth opening. The remaining 9 species are illustrated and commented upon where necessary. For the sake of completeness three undescribed species are mentioned. Some general remarks on identification of catenulid species are given.