Stem cells (neoblasts) in Platyhelminthes are pluripotent, and likely totipotent, undifferentiated cells which retain throughout adult life the capacity to proliferate and from which all somatic cells as well as the germ cells derive. However, basic data on the pool and heterogeneity of neoblasts, their rates of differentiation into sets and subsets of differentiated cells, and their migration to different body regions are still lacking. To fill this gap, S-phase cells in the macrostomid Macrostomum sp. were labeled with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU). S-phase cells were found to be neoblasts and to be distributed in two bands along the lateral sides of the body leaving unlabeled the median axis of the body and the region anterior to the eyes. This distribution is parallel to that of mitotic cells demonstrated using an antibody to phosphorylated histone H3. At different chase times, clusters of BrdU-labeled cells appear, labeled cells migrate to formerly unlabeled areas, and they differentiate into several somatic cell types and into germ cells. Finally, continuous exposure to BrdU shows an extensive renewal of the epithelial cells. Altogether, these results strengthen the idea of platyhelminth neoblasts as an unparalleled stem-cell system within the Animal Kingdom calling for further investigation.

A new model organism among the lower Bilateria and the use of digital microscopy in taxonomy of meiobenthic Platyhelminthes: Macrostomum lignano, n. sp. (Rhabditophora, Macrostomorpha) Abstract Macrostomum lignano n. sp. is a member of the Macrostomorpha, the basal-most subtaxon of the Platyhelminthes-Rhabditophora. This new species can be easily cultured in the laboratory and has been already the subject of several developmental/evolutionary studies. The small size, with only about 25 000 cells constituting the major bilaterian organ systems, makes this simultaneous hermaphrodite a possible candidate for a new model organism that is phylogenetically more basal than any of the model organisms currently used in such studies within the Bilateria. M. lignano belongs to the largest genus of the Macrostomorpha. Over 100 marine, fresh water and brackish water species are contained in the genus Macrostomum, some of them with worldwide distribution pattern. Within it, M. lignano is a member of the M. tuba-species group, which we have summarized here. In the species description, we have used a novel approach to document such small soft-bodied meiobenthic organisms: we provide extensive digital micrographical documentation, which are deposited as a CD together with the type material.

The lion's share of studies on regeneration in Plathelminthes (flatworms) has been so far carried out on a derived taxon of rhabditophorans, the freshwater planarians (Tricladida), and has shown this group's outstanding regeneration capabilities in detail. Sharing a likely totipotent stem cell system, many other flatworm taxa are capable of regeneration as well. In this paper, we present the regeneration capacity of Macrostomum lignano, a representative of the Macrostomorpha, the basal-most taxon of rhabditophoran flatworms and one of the most basal extant bilaterian protostomes. Amputated or incised transversally, obliquely, and longitudinally at various cutting levels, M. lignano is able to regenerate the anterior-most body part (the rostrum) and any part posterior of the pharynx, but cannot regenerate a head. Repeated regeneration was observed for 29 successive amputations over a period of almost 12 months. Besides adults, also first-day hatchlings and older juveniles were shown to regenerate after transversal cutting. The minimum number of cells required for regeneration in adults (with a total of 25,000 cells) is 4,000, including 160 neoblasts. In hatchlings only 1,500 cells, including 50 neoblasts, are needed for regeneration. The life span of untreated M. lignano was determined to be about 10 months.

The concept of Gösta Jägersten of a primary biphasic metazoan life‐cycle, consisting of a planktotrophic larva and a benthic adult, forms the basis for several theories on metazoan phylogeny. In this paper the assumed planktotrophic life‐style of the larva is critically analyzed and reconsidered. It is shown, in particular for the Mollusca, that a biphasic life‐cycle with a lecithotrophic larva is probably the plesiomorphic condition. Character distribution and structural data suggest a parallel evolution of the downstream collecting system used in planktotrophic larvae or filter‐feeding adults of gastropods, bivalves and other spiralian or aschelminth taxa. In the basic metazoans (Parazoa, Placozoa, coelenterates) direct or lecithotrophic development dominates by far. For the acoelomate (Platyhelminthes, Gnathostomulida) and pseudocoelomate taxa direct development is probably the plesiomorphic condition. The structural similarities of the upstream collecting system in tentaculate and deuterostome phyla may also be explained by parallel events of heterochrony out of an ancestor with adult filter‐feeding. The main conclusion of this survey is that larval planktotrophy is likely to be secondary and not a plesiomorphic condition among the Bilateria. Accordingly, theories which are based on the assumed plesiomorphy of larval planktotrophy of the Bilateria, need careful reevaluation.

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Since first described, acoels were considered members of the flatworms (Platyhelminthes). However, no clear synapomorphies among the three large flatworm taxa - the Catenulida, the Acoelomorpha and the Rhabditophora - have been characterized to date. Molecular phylogenies, on the other hand, commonly positioned acoels separate from other flatworms. Accordingly, our own multi-locus phylogenetic analysis using 43 genes and 23 animal species places the acoel flatworm Isodiametra pulchra at the base of all Bilateria, distant from other flatworms. By contrast, novel data on the distribution and proliferation of stem cells and the specific mode of epidermal replacement constitute a strong synapomorphy for the Acoela plus the major group of flatworms, the Rhabditophora. The expression of a piwi-like gene not only in gonadal, but also in adult somatic stem cells is another unique feature among bilaterians. These two independent stem-cell-related characters put the Acoela into the Platyhelminthes-Lophotrochozoa clade and account for the most parsimonious evolutionary explanation of epidermal cell renewal in the Bilateria. Most available multigene analyses produce conflicting results regarding the position of the acoels in the tree of life. Given these phylogenomic conflicts and the contradiction of developmental and morphological data with phylogenomic results, the monophyly of the phylum Platyhelminthes and the position of the Acoela remain unresolved. By these data, both the inclusion of Acoela within Platyhelminthes, and their separation from flatworms as basal bilaterians are well-supported alternatives.

We studied the embryonic development of body-wall musculature in the acoel turbellarian Convoluta pulchra by fluorescence microscopy using phalloidin-bound stains for F-actin. During stage 1, which we define as development prior to 50% of the time between egg-laying and hatching, actin was visible only in zonulae adhaerentes of epidermal cells. Subsequent development of muscle occurred in two distinct phases: first, formation of an orthogonal grid of early muscles and, second, differentiation of other myoblasts upon this grid. The first elements of the primary orthogonal muscle grid appeared as short, isolated, circular muscle fibers (stage 2; 50% developmental time), which eventually elongated to completely encircle the embryo (stage 3; at 60% of total developmental time). The first primary longitudinal fibers appeared later, along with some new primary circular fibers, by 60-63% of total developmental time (stage 4). From 65 to 100% of total developmental time (stages 5 to 7), secondary fibers, using primary fibers as templates, arose; the number of circular and longitudinal muscles thus increased, and at the same time parenchymal muscles began appearing. Hatchlings (stage 8) possessed about 25 circular and 30 longitudinal muscles as well as strong parenchymal muscles. The remarkable feature of the body wall of many adult acoel flatworms is that longitudinal muscles bend medially and cross each other behind the level of the mouth. We found that this development starts shortly after the appearance of the ventral mouth opening within the body wall muscle grid. The adult organization of the body-wall musculature consists of a grid of several hundred longitudinal and circular fibers and a few diagonal muscles. Musculature of the reproductive organs developed after hatching. Thus, extensive myogenesis must occur also during postembryonic development. Comparison between the turbellarians and the annelids suggests that formation of a primary orthogonal muscle grid and its subsequent use as a template for myoblast differentiation are the two basic developmental phases in vermiform Spiralia if not in the Bilateria as a whole. Finally, our new data suggest that for the Acoela the orthogonal primary patterning of longitudinal and circular muscles in the body wall is achieved without using originally positional information of the nervous system.

Traditionally, regeneration research has been closely tied to flatworm research, as flatworms (Plathelminthes) were among the first animals where the phenomenon of regeneration was discovered. Since then, the main focus of flatworm regeneration research was on triclads, for which various phenomena were observed and a number of theories developed. However, free-living flatworms encompass a number of other taxa where regeneration was found to be possible. This review aims to display and to compare regeneration in all major free-living flatworm taxa, with special focus on a new player in the field of regeneration, Macrostomum lignano (Macrostomorpha). Findings on the regeneration capacity of this organism provide clues for links between regeneration and (post-)embryonic development, starvation, and asexual reproduction. The role of the nervous system and especially the brain for regeneration is discussed, and similarities as well as particularities in regeneration among free-living flatworms are pointed out.