Syndesmis aethopharynx Westervelt & Kozloff, 1990 (Rhabdocoela: Umagillidae): a revisitation supported by scanning electron microscopy and molecular analyses.

Species of Syndesmis Francois, 1886 are rhabdocoel platyhelminths typically found in echinoids. Our knowledge of this group is based on old and insufficient studies, generally representing light microscopy-based species descriptions. Syndesmis aethopharynx Westervelt & Kozloff, 1990 is an understudied endosymbiont of Paracentrotus lividus (Lamarck), which is likely to have been confused with the type-species, i.e. Syndesmis echinorum François, 1886, in the literature. In this work, S. aethopharynx is revisited based on new data on surface morphology and phylogeny and basic ecological data are provided. Scanning electron microscopy analysis revealed that the whole ventral region of the worm is equipped with cilia, which supports the assumption that the unciliated epidermal area reported for some species of umagillids, likewise endosymbiotic in echinoderms, is an apomorphy. Following the results of the molecular phylogenetic analysis, species of Syndesmis are closely-related to symbionts of other echinoderms, i.e. holothurians, and like them, may have evolved from some free-living or symbiotic Provorticidae ancestor. Syndesmis spp. may stand for a key group in studying the evolution of feeding strategies in rhabdocoels, as their phylogenetic position is between intestinal and coelomic symbionts, and since both the digestive tube and perivisceral fluid were recorded as sites of infection. The infection levels were low, likely reflecting the aggregated distribution of the host and the fragile nature of the symbiont.

New insights into the karyotype evolution of the free-living flatworm Macrostomum lignano (Platyhelminthes, Turbellaria).

The free-living flatworm Macrostomum lignano is a model organism for evolutionary and developmental biology studies. Recently, an unusual karyotypic diversity was revealed in this species. Specifically, worms are either 'normal' 2n = 8, or they are aneuploid with one or two additional large chromosome(s) (i.e. 2n = 9 or 2n = 10, respectively). Aneuploid worms did not show visible behavioral or morphological abnormalities and were successful in reproduction. In this study, we generated microdissected DNA probes from chromosome 1 (further called MLI1), chromosome 2 (MLI2), and a pair of similar-sized smaller chromosomes (MLI3, MLI4). FISH using these probes revealed that MLI1 consists of contiguous regions homologous to MLI2-MLI4, suggesting that MLI1 arose due to the whole genome duplication and subsequent fusion of one full chromosome set into one large metacentric chromosome. Therefore, one presumably full haploid genome was packed into MLI1, leading to hidden tetraploidy in the M. lignano genome. The study of Macrostomum sp. 8 - a sibling species of M. lignano - revealed that it usually has one additional pair of large chromosomes (2n = 10) showing a high homology to MLI1, thus suggesting hidden hexaploidy in its genome. Possible evolutionary scenarios for the emergence of the M. lignano and Macrostomum sp. 8 genomes are discussed.

Microturbellarians (Platyhelminthes and Acoelomorpha) in Brazil: invisible organisms? / Microturbelários (Platyhelminthes e Acoelomorpha) no Brasil: organismos invisíveis?

Abstract Microturbellarians typically belong to the benthos and may occur in a wide variety of environments. They are abundant in freshwater and marine ecosystems and may occur in moist terrestrial habitats. However, turbellarians are seldom taken into account in studies of biodiversity. Most studies on Brazilian microturbellarians had taxonomical purposes and were done in the years 1940-1950. Thus, information on their occurrence and ecological aspects are dispersed throughout several papers. We intend here to summarize the biogeographical distribution and ecological aspects of microturbellarians recorded for Brazil, indicating the main gaps in their knowledge and possible actions to enhance studies on this group. There are 239 species of microturbellarians registered for Brazil, with records distributed in 12 states. However, just three states located in southern Brazil have records of 94% of microturbellarian species. Thus, knowledge on the systematics and geographical distribution of Brazilian microturbellarians clearly reflect the scientific activity over many years or decades in two states of southeastern and southern Brazil. Considering the scant information on this group in Brazil, which is also the situation of the Neotropical microturbellarians in general, some actions should be proposed. First, it would be necessary to sample in the diverse biomes, as well as in the various river and sea basins, based on standardized sampling protocols. Second, it would be necessary to encourage diverse research groups to include microturbellarians and/or turbellarians in general into biodiversity inventories and studies on community structure of invertebrates. Third, it is necessary to increase the number of research groups on microturbellarians, in order to augment the studies on their morphology, systematics, and ecology. Considering their abundance, species richness and ecological importance in aquatic environments, despite some peculiarities regarding their sampling, sorting and identification procedures, the challenge to study microturbellarians and enhance knowledge about them in Brazilian ecosystems should be faced.

Resumo Microturbelários são tipicamente bentônicos e podem ocorrer em uma ampla variedade de ambientes. São abundantes em ecossistemas marinhos e de água doce, podendo ocorrer em ambientes terrestres úmidos. Entretanto, turbelários raramente são considerados em estudos de diversidade. A maioria dos estudos sobre microturbelários brasileiros tiveram propósito taxonômico e foram realizados nos anos 1940-1950. Assim, informações sobre ocorrência e aspectos ecológicos estão dispersos em diversos artigos. O objetivo deste trabalho é sumarizar a distribuição biogeográfica e aspectos ecológicos dos microturbelários registrados para o Brasil, indicando as principais lacunas do conhecimento e possíveis ações para ampliar estudos sobre esse grupo. Há 239 espécies de microturbelários registradas no Brasil, com registros distribuídos em 12 estados. No entanto, 94% das espécies de microturbelários foram registradas em apenas três estados localizados no sul e sudeste do Brasil. Assim, o conhecimento sobre a sistemática e distribuição geográfica dos microturbelários claramente reflete as atividades científicas realizadas por muitos anos ou mesmo décadas em dois estados do sudeste e sul do Brasil. Considerando as escassas informações existentes sobre esse grupo no Brasil, assim como a situação dos microturbelários neotropicais em geral, algumas ações devem ser propostas. Primeiramente, é necessário realizar amostragens em diversos biomas, assim como nas várias bacias e regiões hidrográficas marinhas, baseadas em protocolos de amostragem padronizados. Em segundo lugar, faz-se necessário incentivar diversos grupos de pesquisa a incluir microturbelários e/ou turbelários em geral em inventários da biodiversidade e estudos de estrutura de comunidades de invertebrados. Em terceiro lugar, é necessário ampliar o número de grupos de pesquisa em microturbelários, para aumentar os estudos sobre sua morfologia, sistemática e ecologia. Considerando sua abundância, riqueza de espécies e importância ecológica em ambientes aquáticos, apesar de suas peculiaridades de amostragem, triagem e identificação, o desafio de estudar e ampliar o conhecimento sobre microturbelários em ecossistemas brasileiros deve ser enfrentado.

Zoology: War of the Worms.

The phylogenetic affinities of Xenacoelomorpha - the phylum comprising Xenoturbella bocki and acoelomorph worms - are debated. Two recent studies conclude they represent the earliest branching bilaterally symmetrical animals, but additional tests may be needed to confirm this notion.

Microturbellarians (Platyhelminthes and Acoelomorpha) in Brazil: invisible organisms?

Microturbellarians typically belong to the benthos and may occur in a wide variety of environments. They are abundant in freshwater and marine ecosystems and may occur in moist terrestrial habitats. However, turbellarians are seldom taken into account in studies of biodiversity. Most studies on Brazilian microturbellarians had taxonomical purposes and were done in the years 1940-1950. Thus, information on their occurrence and ecological aspects are dispersed throughout several papers. We intend here to summarize the biogeographical distribution and ecological aspects of microturbellarians recorded for Brazil, indicating the main gaps in their knowledge and possible actions to enhance studies on this group. There are 239 species of microturbellarians registered for Brazil, with records distributed in 12 states. However, just three states located in southern Brazil have records of 94% of microturbellarian species. Thus, knowledge on the systematics and geographical distribution of Brazilian microturbellarians clearly reflect the scientific activity over many years or decades in two states of southeastern and southern Brazil. Considering the scant information on this group in Brazil, which is also the situation of the Neotropical microturbellarians in general, some actions should be proposed. First, it would be necessary to sample in the diverse biomes, as well as in the various river and sea basins, based on standardized sampling protocols. Second, it would be necessary to encourage diverse research groups to include microturbellarians and/or turbellarians in general into biodiversity inventories and studies on community structure of invertebrates. Third, it is necessary to increase the number of research groups on microturbellarians, in order to augment the studies on their morphology, systematics, and ecology. Considering their abundance, species richness and ecological importance in aquatic environments, despite some peculiarities regarding their sampling, sorting and identification procedures, the challenge to study microturbellarians and enhance knowledge about them in Brazilian ecosystems should be faced.

Contribution to the knowledge of cotylean flatworms (Turbellaria, Polycladida) from Iranian coasts: Introducing a new species, with remarks on new records.

Very little information is available regarding marine free-living flatworms not only from Iran, but throughout the countries surrounding the Persian Gulf and the Gulf of Oman. The present study first introduces a new euryleptid species, and then reports four pseudocerotid polyclads which inhabit Iranian shallow rocky shores of the Persian Gulf and the Gulf of Oman. Maritigrella makranica sp. nov. is characterized dorsally by a medial cream or white reticulated appearance containing pale orange spots in a honeycomb pattern, a distinct orange submarginal band around the entire body margin and between the marginal tentacles, black spots scattered around mid-dorsal surface, becoming more sparse on raised median region and towards body margin, surrounded by a dark-grey halo around the body midline and orange-black halo towards margin. Three of the four pseudocerotids species belonging to the genera Pseudobiceros Faubel, 1984; Pseudoceros Lang, 1884; and Thysanozoon Grube, 1840, are new records for the studied areas, while the other has been reported in the Persian Gulf previously. Comments on Iranian species are provided and associations of flatworms with ascidians and sponges were observed. 

Metazoan symbionts of the yellow clam, Mesodesma donacium (Bivalvia), in southern Chile: geographical variations.

Mesodesma donacium is a dominant species on sandy beaches along the Chilean coast. However, the only previous parasite records for this species were obtained for the northern Chilean coast (20° S-33° S), which dealt with cestodes, polychaetes, and copepods. In this study, the symbiotic fauna of M. donacium in its southern distributional range is reported, and the geographical variations in the occurrence of this fauna are evaluated. A total of 565 individuals of M. donacium were captured by local fishermen from 5 localities: Mehuín (MEH) (39° 26' S), Carelmapu (CAR) (41° 44' S), Ancud (ANC) (41° 53' S), Cucao (CUC) (42° 35' S), and Quellón (QUE) (43° 24' S), covering a total distance of 450 km. To collect metazoan symbionts, the valves, mantle, gills, gonad, and digestive gland of each specimen of yellow clam were examined, and symbiont identifications were made via morphological and genetic analyses. The prevalence and mean intensity of infestation were calculated for each symbiotic species. Univariate and multivariate analyses were performed to evaluate the differences in symbiotic load between localities. Seven metazoan symbiotic species were recorded. The most abundant species were Paranthessius mesodesmatis, Monorchiidae gen. sp., and Paravortex sp. The copepod P. mesodesmatis and metacercaria Monorchiidae gen. sp. showed a high prevalence in all localities, but their intensity of infection varied among localities. The turbellarian Paravortex sp. was most frequently associated with ANC and CUC. The digenean Sanguinicolidae gen. sp. was recorded only at CAR, and the polychaete Spionidae gen. sp. was recorded only at MEH. In its southern distributional range, M. donacium was characterized by an absence of cestodes. This absence can be explained by the absence of the definitive host. The local environmental conditions in the southern range of the host could explain the differences in symbiotic composition among localities.

A new Imogine species (Turbellaria: Polycladida: Stylochidae) associated with rock oysters (Saccostrea cucullata) from the Persian Gulf, with a review of the genus.

A new species of acotylean polyclad, Imogine qeshmensis sp. nov. is described from Qeshm Island, Persian Gulf, Iran. I. qeshmensis is characterized by the number and arrangement of the tentacular, cerebral and frontal eyes, a body margin with opaque white bands and eyespots and a male genital complex with an highly muscular seminal vesicle equal in size to the prostatic vesicle.

[Symbionts of Mytilus edulis in the littoral and sublittoral zones of the Kandalaksha and Onega Gulfs of the White Sea].

Composition of the fauna of organisms associated with Mytilus edulis in the Kandalaksha and Onega Gulfs of the White Sea has been examined. The following 8 symbiotic species were revealed: Choricystis sp. (Chloro[hyceae), Peniculistoma mytili, Ancistrum mytili (Ciliata, Oligohymenophorea), Urastoma cyprinae, Paravortex sp. (Platyhelminthes, Rhabditophora), and metacercaria of Cercaria parvicaudata, Himasthla sp., and Gymnophallus sp. (Platyhelminthes, Trematoda). Besides, different free-living organisms were found in the mantle cavity; 6 species of nematodes, the planktonic copepod Microsetella norvegica, undetermined copepods, isopod Jaera sp., sea mites of the family Halacaridae, and chironomid larvae. Parameters of infestation and places of localization in the host are given for each group of organisms. Some regularities in the horizontal and vertical distribution of organisms associated with M. edulis are noted.

How the worm got its pharynx: phylogeny, classification and Bayesian assessment of character evolution in Acoela.

Acoela are marine microscopic worms currently thought to be the sister taxon of all other bilaterians. Acoels have long been used as models in evolutionary scenarios, and generalized conclusions about acoel and bilaterian ancestral features are frequently drawn from studies of single acoel species. There is no extensive phylogenetic study of Acoela and the taxonomy of the 380 species is chaotic. Here we use two nuclear ribosomal genes and one mitochondrial gene in combination with 37 morphological characters in an analysis of 126 acoel terminals (about one-third of the described species) to estimate the phylogeny and character evolution of Acoela. We present an estimate of posterior probabilities for ancestral character states at 31 control nodes in the phylogeny. The overall reconstruction signal based on the shape of the posterior distribution of character states was computed for all morphological characters and control nodes to assess how well these were reconstructed. The body-wall musculature appears more clearly reconstructed than the reproductive organs. Posterior similarity to the root was calculated by averaging the divergence between the posterior distributions at the nodes and the root over all morphological characters. Diopisthoporidae is the sister group to all other acoels and has the highest posterior similarity to the root. Convolutidae, including several "model" acoels, is most divergent. Finally, we present a phylogenetic classification of Acoela down to the family level where six previous family level taxa are synonymized.

Bryoplana xerophila n. g. n. sp., a new limnoterrestrial microturbellarian (Platyhelminthes, Typhloplanidae, Protoplanellinae) from epilithic mosses, with notes on its ecology.

Bryoplana xerophila, a new genus and species of limnoterrestrial protoplanelline platyhelminth, was found in moss and soil covering a concrete wall in northern Alabama, USA. Bryoplana xerophila is the first taxon of limnoterrestrial Protoplanellinae recorded from North America and is one of the few rhabdocoels known from dry habitats. It is unique within Protoplanellinae in lacking rhabdites, having a pharynx rosulatus in the frontal half of the body, and lacking sclerotized parts in the male system. Notes on encystment, reproduction and feeding behavior are given. An updated identification key to all known genera of Protoplanellinae is presented.

Embryonic origins of hull cells in the flatworm Macrostomum lignano through cell lineage analysis: developmental and phylogenetic implications.

The development of macrostomid flatworms is of interest for evolutionary developmental biology research because these taxa combine characteristics of the canonical spiral cleavage pattern with significant deviations from this pattern. One such deviation is the formation of hull cells, which surround the remaining embryonic primordium during early development. Using live observations with a 4D microscope system, histology, and 3D reconstructions, we analyzed the ontogeny of these hull cells in the macrostomid model organism Macrostomum lignano. Our cell lineage analysis allowed us to find the precursors of the hull cells in this species. We discuss the relation between macrostomid development and the development of other spiralians and the question of whether hull cells are homologous within rhabditophoran flatworms.

Assessing the root of bilaterian animals with scalable phylogenomic methods.

A clear picture of animal relationships is a prerequisite to understand how the morphological and ecological diversity of animals evolved over time. Among others, the placement of the acoelomorph flatworms, Acoela and Nemertodermatida, has fundamental implications for the origin and evolution of various animal organ systems. Their position, however, has been inconsistent in phylogenetic studies using one or several genes. Furthermore, Acoela has been among the least stable taxa in recent animal phylogenomic analyses, which simultaneously examine many genes from many species, while Nemertodermatida has not been sampled in any phylogenomic study. New sequence data are presented here from organisms targeted for their instability or lack of representation in prior analyses, and are analysed in combination with other publicly available data. We also designed new automated explicit methods for identifying and selecting common genes across different species, and developed highly optimized supercomputing tools to reconstruct relationships from gene sequences. The results of the work corroborate several recently established findings about animal relationships and provide new support for the placement of other groups. These new data and methods strongly uphold previous suggestions that Acoelomorpha is sister clade to all other bilaterian animals, find diminishing evidence for the placement of the enigmatic Xenoturbella within Deuterostomia, and place Cycliophora with Entoprocta and Ectoprocta. The work highlights the implications that these arrangements have for metazoan evolution and permits a clearer picture of ancestral morphologies and life histories in the deep past.

First report of the introduction of an Australian temnocephalidan into the New World.

One of the preferred freshwater crayfish species in astaciculture is the large parastacid Cherax quadricarinatus, a native to Australia. This crayfish yields good production in warm water, and, therefore, it has been translocated from Australia to other countries. One such country is Uruguay, where the initial attempt in astaciculture is being conducted. In a Uruguayan crayfish farm, a large number of Diceratocephala boschmai (Platyhelminthes, Temnocephalida) were found on the body surface and branchial chamber of C. quadricarinatus. This is the first report of an introduced temnocephalidan in the New World, and of D. boschmai in a locality outside Australia/New Guinea. The original description of D. boschmai was based on a few characters that separated this species from the other Temnocephalida; therefore, a detailed description of the Uruguayan population of D. boschmai is also provided. The conditions in which D. boschmai is found in Uruguay are suitable for its development and reproduction. This raises concern about the effect that its release into the environment might have.

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela).

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades and even the basic question of whether a pharynx is a primitive trait within the Acoela and homologous to the pharynx of platyhelminth turbellarians is open. By using fluorescence staining of musculature, as well as conventional histological techniques and transmission electron microscopy, the present study sets focus on the mouth and pharynx (where present) of seven species of Acoela within Paratomellidae, Solenofilomorphidae, Hofsteniidae, Proporidae, and Convolutidae, as well as one species of Nemertodermatida and Catenulida, respectively. It is shown that among the investigated families of acoels there is a great variability in muscle systems associated with the mouth and pharynx and that pharynx histology and ultrastructural characters are widely diverse. There are no close similarities between the acoel pharynges and the catenulid pharynx but there is a general resemblance of the musculature associated with the mouth in the representatives of Paratomellidae and Nemertodermatida. On the basis of the profound differences in pharynx morphology, three major conclusions are drawn: 1) the pharynges as present in Recent acoels are not homologous to the pharynx simplex characteristic for Catenulida and Macrostomida within the Platyhelminthes; 2) the different muscular pharynx types of acoels are not homologous between higher taxa and thus a single acoel-type pharynx simplex cannot be defined; 3) the presence of a muscular pharynx most likely does not represent the ancestral state.

Redescription and designation of a neotype of Temnocephala talicei Dioni, 1967 (Platyhelminthes: Temnocephalida).

Temnocephala talicei was described from Uruguayan material found mainly on Aegla prado. Its original description was based principally on its distinctive penial stylet, and, therefore, many aspects of the species anatomy that are now important in the taxonomy of the genus were not contemplated; type specimens were never designated; and the material used to describe the species has been lost. The present work provides a redescription of the species based upon material collected from the type host and in the type locality. Given the possibility of confusion in its identification, and in order to preserve its taxonomic stability, a neotype of T. talicei is designated. The closest species to T. talicei is Temnocephala mertoni, from which it can be separated by the presence of conspicuous intestinal septa, a markedly asymmetrical sphincter in the vagina, markedly lobed testes, a straight penial stylet with a non-sinuous distal portion of the shaft, a characteristic large, 'mouthpiece'-shaped introvert without discrete thickenings, and approximately 10 crowns of well-developed spines of decreasing length placed throughout the whole extension of the introvert. A comparison of material from Uruguay, Argentina, and Paraguay shows that there is little variation in the morphometry of the species. Finally, a comparison of the post-tentacular syncytia of T. talicei and T. mertoni shows that this is not always a species-specific character.

A new Paravortex (Platyhelminthes, Dalyellioida) endoparasite of Mesodesma mactroides (Bivalvia, Mesodesmatidae) from Uruguay.

Many species of turbellarians (Platyhelminthes) are known to live associated with other organisms, especially invertebrates, as commensals or parasites. The family Graffillidae (Rhabdocoela) includes two genera that parasitize mollusks, Graffílla and Paravortex. Within the latter genus, six species were described as associated with mollusks. In other instances, unnamed Paravortex species were mentioned as parasites of other bivalves and of the body surface of fishes. In the present work, a new Paravortex species that was found in the intestine of Mesodesma mactroides from the Atlantic coast of Uruguay is described. In addition, a bibliographical revision of the known Paravortex species with their respective hosts, location, and distribution is made. Paravortex nicolli, described by Szidat for the Argentinean coast, is mentioned for the first time after the original description, and the authorship and date of description of Paravortex tapetis Noury-Sraïri 1989 are elucidated.

Morphology and ultrastructure of the pharynx in Solenofilomorphidae (Acoela).

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades. Here, we analyze the pharynges of three members of the Solenofilomorphidae, Myopea sp. and two species of the genus Solenofilomorpha, by means of light and transmission electron microscopy. Special focus is placed on the ultrastructure of the pharyngeal musculature, epidermis surrounding the mouth, pharyngeal epithelium, and junction with the digestive parenchyma. The main goal of this study was to evaluate the usefulness of certain characters for broader comparisons within the Acoela. Among the three species, characters relating to position of the mouth, presence and elaboration of sphincter muscles, presence of pharyngeal glands, and ultrastructure of epitheliosomes proved to be variously species- and genus-specific. The arrangement of pharyngeal muscles and their connection with body wall musculature, ultrastructure of receptor cells, and morphology of a nonciliated glandular region in the posterior pharynx, in contrast, appear to be characteristic of the family Solenofilomorphidae and thus of predominant interest for comparisons with other acoel families.

Comparative morphology of the bursal nozzles in acoels (Acoela, Acoelomorpha).

Systematics of the Acoela is particularly difficult because of the paucity of readily discernible morphological features. In other soft-bodied worms, sclerotized structures, such as copulatory stylets, provide important characters that can be seen in whole mounts, but acoels generally lack such features. Among the few sclerotized structures in acoels are bursal nozzles-tubiform outlets on the seminal bursae that are believed to be conduits (spermatic ducts) through which allosperm are transported to the oocytes. Early classifications of the Acoela used features of the female reproductive system, including bursal nozzles, for distinguishing major groups, but the current system essentially ignores them as too plastic to provide higher-level distinctions. We used confocal and electron microscopy to further characterize bursal nozzles in five acoel species, and found all composed of actin-reinforced extensions of stacked, flat mesenchymal cells. In Notocelis gullmarensis, Aphanostoma bruscai, and Daku woorimensis, the nozzle is a stiffened region of the same cells forming the wall of the bursa. By contrast, in Wulguru cuspidata cells forming the nozzle are distinct from those of the bursa. The so-called bursal cap of A. bruscai and D. woorimensis has small sclerotized disjunct units within it, also composed of stacked, flat, actin-reinforced cells. The nozzle of W. cuspidata, prominent like that of other convolutid acoels, is relatively complex, its actin-reinforced cells sandwiched with secretory cells and its base bearing a "sorting apparatus" of egg-shaped cells that send narrow processes inside the spermatic duct. Cases of sperm inside the nozzle corroborate its assumed role in reproduction. Whereas most nozzles sit at the end of the bursa facing the ovary, in species of Pseudmecynostomum and purportedly in a few other acoels, they sit between the female pore and the bursa, constituting what we call a vaginal nozzle. All bursal nozzles of acoels show a common ground pattern indicating common ancestry, but certain features discerned through electron and confocal microscopy show promise of providing synapomorphies for grouping some species.

Mitochondrial genome data support the basal position of Acoelomorpha and the polyphyly of the Platyhelminthes.

We determined 9.7, 5.2, and 6.8 kb, respectively, of the mitochondrial genomes of the acoel Paratomella rubra, the nemertodermatid Nemertoderma westbladi, and the free-living rhabditophoran platyhelminth Microstomum lineare. The identified gene arrangements are unique among metazoans, including each other, sharing no more than one or two single gene boundaries with a few distantly related taxa. Phylogenetic analysis of the amino acid sequences inferred from the sequenced genes confirms that the acoelomorph flatworms (acoels+nemertodermatids) do not belong to the Platyhelminthes, but are, instead, the most basal extant bilaterian group. Therefore, the Platyhelminthes, as traditionally constituted, is a polyphyletic phylum.