The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis).

Acoels are among the simplest worms and therefore have often been pivotal in discussions of the origin of the Bilateria. Initially thought primitive because of their "planula-like" morphology, including their lumenless digestive system, they were subsequently dismissed by many morphologists as a specialized clade of the Platyhelminthes. However, since molecular phylogenies placed them outside the Platyhelminthes and outside all other phyla at the base of the Bilateria, they became the focus of renewed debate and research. We review what is currently known of acoels, including information regarding their morphology, development, systematics, and phylogenetic relationships, and put some of these topics in a historical perspective to show how the application of new methods contributed to the progress in understanding these animals. Taking all available data into consideration, clear-cut conclusions cannot be made; however, in our view it becomes successively clearer that acoelomorphs are a "basal" but "divergent" branch of the Bilateria.

Molecular architecture of muscles in an acoel and its evolutionary implications.

We have characterized the homologs of an actin, a troponin I, and a tropomyosin gene in the acoel Symsagittifera roscoffensis. These genes are expressed in muscles and most likely coexpressed in at least a subset of them. In addition, and for the first time for Acoela, we have produced a species-specific muscular marker, an antibody against the tropomyosin protein. We have followed tropomyosin gene and protein expression during postembryonic development and during the posterior regeneration of amputated adults, showing that preexisting muscle fibers contribute to the wound closure. The three genes characterized in this study interact in the striated muscles of vertebrates and invertebrates, where troponin I and tropomyosin are key regulators of the contraction of the sarcomere. S. roscoffensis and all other acoels so far described have only smooth muscles, but the molecular architecture of these is the same as that of striated fibers of other bilaterians. Given the proposed basal position of acoels within the Bilateria, we suggest that sarcomeric muscles arose from a smooth muscle type, which had the molecular repertoire of striated musculature already in place. We discuss this model in a broad comparative perspective.

New Austrognathiidae (Gnathostomulida: Conophoralia) from Hong Kong and Japan: microscopic anatomy, ultrastructure and evolutionary implications.

We describe two new species, Austrognathia glandifera and Austrognatharia orientis using observations on squeezed, live specimens as well as histological sections and transmission electron microscopy. The protonephridia of Austrognatharia orientis are composed of a terminal cell, a canal cell, and a nephroporus cell. The monociliated terminal cell constitutes the so-called filtration area. The canal cell harbors the lacunar system and the protonephridial duct, which is surrounded by six filamentous rods, which originate external to and in between the microvilli of the terminal cell and stretch along the entire length of the canal cell. The female copulatory organs of the investigated species are very different. Austrognathia glandifera has a bursa and a vagina whereas A. orientis only has a weakly defined bursal tissue and no detectable vagina. The bursa is divided into an anterior and a posterior part; at the anterior end a special area is formed by interdigitations of the cells of the bursal wall. The male copulatory organs in the Conophoralia are uniform, composed of an anterior, glandular portion consisting of a proximal part with medium-grained and a distal part with coarse-grained appearance and a penis that is delineated by a basal lamina and has an ejaculatory duct as well as a gonopore. Parenchymal cells are present and serve to embed the bursa and the male copulatory organ dorsolaterally. Our data on the fine structure of various tissues indicate that the Conophoralia are the "less derived" sister taxon of the Scleroperalia.

Electron microscopy of flatworms standard and cryo-preparation methods.

Electron microscopy (EM) has long been indispensable for flatworm research, as most of these worms are microscopic in dimension and provide only a handful of characters recognizable by eye or light microscopy. Therefore, major progress in understanding the histology, systematics, and evolution of this animal group relied on methods capable of visualizing ultrastructure. The rise of molecular and cellular biology renewed interest in such ultrastructural research. In the light of recent developments, we offer a best-practice guide for users of transmission EM and provide a comparison of well-established chemical fixation protocols with cryo-processing methods (high-pressure freezing/freeze-substitution, HPF/FS). The organisms used in this study include the rhabditophorans Macrostomum lignano, Polycelis nigra and Dugesia gonocephala, as well as the acoel species Isodiametra pulchra.