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. J. Morphol, 2009. © 2008 Wiley‐Liss, Inc.     

Patterns of musculature as taxonomic characters for the Turbellaria Acoela

While turbellarians are generally assumed to have body-wall musculature consisting routinely of longitudinal, circular, and diagonal fibers, members of the Acoela examined by a fluorescence-microscopy technique specific for actin showed more complicated and distinctive arrangements of muscles, giving promise for better delimiting taxa within this taxonomically difficult order. Certain globose or tear-drop-shaped worms such as Convoluta pulchra and species of Pseudaphanostoma, Mecynostomum, and Otocelis, showed a complex pattern in which muscles longitudinal in the anterior half of the body arc diagonally across the posterior half; complex brushes of parenchymal muscles that cross at the level of the statocyst and arc postero-laterally also characterize these groups. The more elongate acoel Paratomella sp. was found to have musculature dominated by strictly longitudinal fibers and with relatively weak circular fibers and few fibers running diagonally to the body axis, yet the elongate mecynostomid Paedomecynostomum bruneum showed a crossing of antero-longitudinal fibers similar to that seen in the more globose Mecynostomum sp. A distinctive looping of muscles around the mouth is seen in P. bruneum and the Anaperidae. Such similarities and differences in pattern of musculature promise to provide easily recognizable characters for taxonomy of the Acoela at levels ranging from species to family. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unique patterns of longitudinal body-wall musculature in the Acoela (Plathelminthes): the ventral musculature of Convolutriloba longifissura

The ventral musculature of Convolutriloba longifissura (Acoela) has been studied using electron microscopy and fluorescently labeled whole mounts to demonstrate filamentous actin. Attention was directed to the reorganization and renewal of musculature during asexual reproduction and the adaptation of muscle sets for special predatory behavior. Three ventral subepidermal muscle layers could be distinguished in adult C. longifissura: (1) outer circular muscles that encircle the body, (2) intermediate modified longitudinal muscles with concentric pattern around the mouth and V-shaped orientation in the posterior part of the animal, and (3) inner special pore muscles with radial alignment fanning out from the mouth. Additionally, a few very fragile muscles were found at the anterior margin of the animal. The anterior ventral muscle system built a funnel with the mouth opening as organizing center. The special radial muscles and the antagonistically concentric muscles are perfectly adapted to catch prey in such a way that the funnel is put over the prey to press it through the mouth into the digestive syncytium. Convolutriloba longifissura shows a unique way of asexual reproduction by a two-step fission which results in three individuals. Immediately after separation from the mother animal, daughter individuals are missing the concentric and the radial muscle sets around the mouth completely, but within 30 h these sets are renewed for the most part. Two to three days after separation, the mouth opening is visible and the animals move for capturing prey. The peculiar course of longitudinal muscles in C. longifissura with concentric rings anteriorly and a V-shape muscle layer posteriorly shows that the pattern of body-wall musculature in such basal Plathelminthes as the Acoela may be highly modified from the original pattern of longitudinal and circular muscles.     

The triradiate sucking pharynx in animal phylogeny

Independent sucking/swallowing pharynges, as found in members of at least ten animal phyla, are pharynges that are opened by muscles that are not attached to the outer body wall. Their musculature is derived from either mesoderm or ectoderm, the latter taking the form of a myoepithelium. I review results of previous work on the morphology of independent sucking/swallowing pharynges among invertebrates and provide new information on the ultrastructure of the pharynx of the cyclostome bryozoan Crisia eburnea. The various morphologies of this type of pharynx have been used in some phylogenetic considerations, but only the myoepithelial sucking pharynx with a triradiate lumen was considered significant in analyses of relationships between phyla. However, I argue that this shape is the only one that makes an efficient suction pump, and this, together with different orientations of the myoepithelial pharyngeal pump and its phylogenetic distribution, indicates that the triradiate myoepithelial pharynx has evolved convergently in a number of lineages.     

Ultrastructural investigations on the differentiation of genital hard structures in free-living platyhelminths and their phylogenetic significance

Ultrastructure and differentiation of penis stylets and stylet needles have been investigated in representatives of various groups of free-living platyhelminths, viz. the Acoela, Macrostomida, Typhloplanoida, Kalyptorhynchia, and Dalyellioida. In all these groups, the differentiation of such hard parts occurs intracellularly but in different ways in the different groups. The ultrastructure of the bursal mouth piece in an acoel platyhelminth is not comparable to the hard structures in male copulatory organs. The presence of penial copulatory organs having intracellular hard structures appears to be an autapomorphy of the Euplatyhelminthes. Several characters in the ultrastructure and development of these structures can be used as autapomorphies for various platyhelminth groups.     

Comparative ultrastructure of the pharynx simplex in turbellaria

Summary The simple pharynges in thirteen species of Turbellaria in the orders Macrostomida, Haplopharyngida, Catenulida, and Acoela have been studied by electron microscopy. After consideration of the functional aspects of the pharynx simplex, the relationship of the pharynx simplex ultrastructure to the phylogeny of the above mentioned groups is analyzed.The Haplopharyngida and Macrostomida are united as a group by the following characters: a pharynx transition zone of 1–5 circles of insunk cells with modified ciliary rootlets or no cilia, pharynx sensory cells without stereocilia collars and with a variable number of cilia, a prominent nerve ring with more than 30 axons circling the pharynx at the level of the beginning of the pharynx proper distal to the gland ring, 2 or more gland cell types in the pharynx, with at least two layers of muscle present and the longitudinal muscles derived from regular and special body wall circular muscles and a prominent post-oral nerve commissure. This specific arrangement can be distinguished from the other pharynx simplex types and is called the pharynx simplex coronatus.The catenulid pharynx simplex is characterized by the lack of a prominent nerve ring, no prominent post-oral commissure, a transition zone with epidermal type ciliary rootlets, recessed monociliated sensory cells, and one or no type of pharynx gland cell. The Acoela are specialized because of the epidermal type rootlets in the pharynx proper. They also lack a transition zone and a prominent nerve ring and have monociliated sensory cells different from the catenulid type.Ultrastructural characters of the pharynx simplex support the view that the Haplopharyngida-Macrostomida are monophyletic. The more primitive catenulid pharynx probably arose from a common ancestral pool with the Haplopharyngida and Macrostomida, although it does not appear possible presently to establish a clear monophyletic line for these forms. The various pharynx types within the Acoela appear to indicate independent origins with no clear link to the basic pharynx simplex type in the three other orders.Abbreviations Used in Figures a nerve axon - ar accessory rootlet - bb basal body - bn brain-nerve ring commissure - c caudal rootlet - ce centriole - ci cilium - cm circular muscle - cp ciliary pit - cu cuticle - cw cell web - d dictyosome - dp proximal pharynx proper cell - e epidermis - er rough endoplasmic reticulum - f fibrous rod - g gastrodermis - gc gastrodermal gland cell - he heterochromatin - i intercellular matrix - lc lateral nerve cord - lm longitudinal muscle - m mitochondria - mo mouth - mt microtubules - mv microvilli - n nucleus - nr nerve ring - ns neurosecretory granules - p pharynx proper - ph pharynx - po post-oral commissure - r rostral rootlet - rm radial muscle - s sphincter - sc sensory cell - sj septate junction - sr sensory rootlet - t transition zone - u ultrarhabdite - v vertical rootlet - va food vacuole - za zonula adhaerens - 1 type I gland cell - 2 type II gland cell - 3 type III gland cell - 4 type IV gland cell - 5 type V gland cell - 6 type VI gland cell - 7 type VII gland cell     

Comparative ultrastructure of the gastrotrich pharynx and the evolution of myoepithelial foreguts in aschelminthes

Summary Structural and ultrastructural data are presented for the myoepithelial pharynges of 20 species of Gastrotricha representing the marine Macrodasyida and marine and freshwater Chaetonotida. A comparative analysis reveals that pharynges with several plesiomorphic characters occur in Chordodasys among the Macrodasyida and Neodasys among the Chaetonotida. The Gastrotricha are systematized based on pharyngeal characters and the system is shown to be concordant with all recent classifications of the group. The plesiomorphic design of the Gastrotrich pharynx is given as: a cylindrical pharyngeal pump composed of monociliated myoglanduloepithelial cells surrounding a circular or oval lumen; radial myofibrils organized into several cross-striated sarcomeres with Z-discs composed of planar aggregations of dense bodies; excitation-contraction coupling is achieved by peripheral couplings of SR with the sarcolemma; apical cell surface with microvilli protruding through the two-layered cuticle; pharynx with at least 3 longitudinal tracts of monociliated sensory cells; nerves as at least 4 basal intraepithelial, longitudinal tracts of neurites. As a model for muscle cell evolution, the investigation postulates a monociliated, cross-striated myoglanduloepithelial cell as the original muscle cell design within the Gastrotricha. Triradiate myoepithelial foreguts occur only in Bryozoa (Ectoprocta), Gastrotricha, Nematoda and Tardigrada. The potential homology of pharyngeal organization of the latter three phyla is discussed. Based on pharyngeal structure, it is concluded that Gastrotricha (Chaetonotida-Paucitubulatina) and Nematoda share several apomorphic characters and share, therefore, a most recent common ancestor. Affinities of Tardigrada with Aschelminthes are considered feasible but currently inconclusive for lack of sufficient comparative ultrastructural data for the Tardigrada.     

New tools for resolving phylogenies: a systematic revision of the Convolutidae (Acoelomorpha, Acoela)

Molecular sequence data, morphological characters of spermatozoa, and newly obtained morphological characters of penis musculature are used here to revise the systematics of the family Convolutidae (Acoela). Species having isodiametric penes with non-anastomosing longitudinal muscles are transferred to the family Isodiametridae fam. nov. Species with longitudinal penis muscle fibres that anastomose or cross-over each other remain in the Convolutidae. Some species of the genera Convoluta and Conaperta (Convolutidae) are transferred to the genus Isodiametra gen. nov. (Isodiametridae fam. nov.). The genus Stomatricha (Otocelididae) is transferred to the family Convolutidae. Convoluta opisthandropora (Convolutidae) is transferred to the genus Pseudohaplogonaria (Haploposthiidae). Aphanostoma sanguineum (Convolutidae) is transferred to the genus Pseudactinoposthia (Actinoposthiidae).     

Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study

The ultrastructure and distribution of receptor cells near the mouth and (where present) the pharynx of Hofstenia miamia, Proporus bermudensis, Conaperta thela, and Convoluta convoluta (Acoela) were investigated by transmission electron microscopy and confocal laser scanning microscopy of specimens stained with a fluorescence marker for actin. Five types of monociliary receptors were identified: (1) non‐collared receptors with a single long and narrow ciliary rootlet; (2) non‐collared receptors with a wide main ciliary rootlet and a smaller posterior rootlet; (3) non‐collared receptors with a single wide and hollow ciliary rootlet with a granulated core; (4) Collar (?) receptors with obliquely radial filament bundles in the cell apex and with a single hollow ciliary rootlet composed of numerous strand‐like elements; and (5) Collar receptors lacking a striated rootlet but with a granular body (swallow's nest rootlet). While H. miamia bears the first two receptor types, P. bermudensis has receptors of type 1, 3 and 5, and Cona. thela and Conv. convoluta have receptors of type 3, 4 and 5. The density of receptors is generally highest at the anterior body tip, regardless of where the mouth is located. Most receptor types occur scattered over the whole body but type 2 receptors of H. miamia are restricted to the pharynx and mouth region. The lack of a common receptor type specific for the mouth and pharynx of the investigated species points to an independent origin of the pharynges in Hofsteniidae and in Proporidae and of the mouth tube in Convolutidae. Moreover, the homology of the so‐called collar receptors in Acoela with typical collar receptors in other invertebrates is questioned.     

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.     

Anatomy and ultrastructure of the ventral pharyngeal organs of Saccocirrus(Saccocirridae) and Protodriloides(Protodriloidae fam. n.) with remarks on the phylogenetic relationships within the Protodrilida (Annelida: Polychaeta)

General anatomy and ultrastructure of the ventral pharyngeal organs were investigated in Succocirrus krusudensis, Protodriloides chaetifer and P. symbioticus. Succocirrus papillocercus, a species without a ventral pharyngeal organ, was included for comparison. The two genera show homologous similarities in their pharynges: bulbus muscle composed of transverse muscle fibres and interstitial cells, those cells with small cell bodies and processes containing prominent tonofilaments which are orientated dorsoventrally and connect the bulbus epithelium with the investing muscle; bulbus muscle fibres circomyarian with nuclei and mitochondria located basally, investing muscle also with interstitial cells, which lack tonofilaments; oesophagus surrounded by gland cells opening into the pharynx. Most likely, a ventral pharynx with these characters was already present in the stem species of Saccocirridae, Protodriloidae fam. n. and Protodrilidae and evolutionary processes led to several changes: a tongue-like organ with a prominent tip and supporting elements is a synapomorphic character of Protodrilus and Succocirrus; the pharyngeal organ is reduced to stomodeal pouches and salivary glands in S. papillocercus and completely lost in Asromus raenioides. These results and data from previous studies are summarized in a tentative phylogenetic dendrogram and allow the introduction of a new family, Protodriloidae.     

Frontal glands and frontal sensory structures in the Macrostomida (Turbellaria)

The ultrastructure of the frontal gland complex of six species of Macrostomida is investigated. In all species it comprises an array of discretely emerging gland necks of at least two gland types, including one with rhammite secretion granules and one with rhahdite granules. Moreover, mucous glands and glands containing other secretion granules are found in Microstomum sp. No intermeditate form which would allow bridging of the present lack of ultrastructural, histochemical and positional similarities between the Macrostomida and the Acoela is found in the examined species. Therefore, the probability of homology between the frontal organs of the Acoela and the frontal glands of the Macrostomida remains low. Even though two or three tyes of sensory receptors are found distributed over the anterior end of all examined species, the frontal gland complex does not appear to be sensory. Because of the uniformity in frontal gland ultrastructure. relationships within the Macrostomida based on this character alone cannot be detected.     

On the nature of pharyngeal muscle cells in the Tardigrada

As part of a transmission electron microscopic study of the embryological development in tardigrades, the ultrastructure of the pharynx was examined. The intent was to establish whether the pharyngeal muscle cells constitute an eclodermal myoepithelium (as in many aschelminth pharynges) or whether they are mesodermal (as in certain other aschelminth groups and among articulates). In the latter case the cuticle would be produced solely by specialized epithelial cells. The eutardigrade species Halobiotus crispae Kristensen, 1982, was investigated in four embryological stages, as a newly hatched juvenile, in the active adult stage, and in the hibernation stage pseudosimplex I. A comparison was made with the arthrolardigrade Actinarctus doryphorus Schulz, 1935, in the active adult stage and in the simplex stage. The results indicate that the tardigrade pharynx is an ectodermal myoepithelium. The muscles appear to be truly cross-striated and monosarcomerial. The phylogenetic implications of these findings are discussed briefly.     

Revision of the Childiidae (Acoela), a total evidence approach in reconstructing the phylogeny of acoels with reversed muscle layers

The Childiidae sensu Dörjes 1968 comprises the acoel worms characterized by a cone‐shaped penis with muscular or sclerotized elements. Based on differences in body‐wall musculature arrangement, Hooge (2001) recently restricted the family to the genus Childia Graff, 1910 and placed the remaining genera to his new family Actinoposthiidae Hooge 2001 . This rearrangement has been questioned ( Raikova et al. 2004 ). We reconstructed the phylogeny of the Childiidae sensu Dörjes 1968 by means of a total evidence analysis including Histone H3, 28S rDNA and new 18S rDNA sequences, as well as 50 morphological characters. New characters of the muscular system and copulatory organs discovered through confocal laser scanning microscopy of phalloidin‐stained specimens are included in the phylogenetic analysis. A total of 12 taxa (nine ingroup and three outgroup) were used in the parsimony analysis of the 18S data set, which was aligned with different parameters for a sensitivity analysis, and the combined data set (18S + 28S + H3 + morphology). Incongruence in the node support of the groups among the four partitions was very low in the total evidence tree; except for the H3 partition. The conflict observed in the H3 partition is likely due to large homoplasy observed in the synonymous alternatives at both first and third codon positions. All data partitions demonstrated that Actinoposthia beklemischevi Mamkaev 1965 , and the newly defined taxon Childiidae (comprising Childia and Paraphanostoma Westblad 1942 ) are not close relatives. The monophyly of Childia and Paraphanostoma is strongly supported by both the 18S and 28S data partitions. Our study also reveals additional apomorphies uniting Childia with Paraphanostoma from body‐wall musculature, statocyst muscles and male copulatory organ. Muscular system, statocyst muscles, male copulatory organ and nervous system characters proved to be the best characters for taxonomic delimitations of subtaxa within the Childiidae, whereas the seminal bursa (a frequently used character in the taxonomy of Acoela) was highly homoplastic. We also described the body‐wall musculature of six Paraphanostoma species, which is characterized by the reversed arrangement of the longitudinal and circular muscle layers, and by the absence of diagonal muscles on the ventral side of the body and the presence of two types of diagonal muscles on the dorsal side. Childia groenlandica (Levinsen, 1879) is nested among the Paraphanostoma species in our total evidence tree, so we synonymize Paraphanostoma with Childia; all former members of Paraphanostoma are transferred to Childia.     

Ultrastructure of the pharynx of Prorhynchus (Platyhelminthes, Lecithoepitheliata)

The pharynx variabilis of Prorhynchus is strongly muscular, with a small pharyngeal fold and a thin surrounding sheath. There is one row of inner longitudinal musclcs, up to six rows of inner circular muscles, many radial muscles, one row of outer circular and one row of outer longitudinal muscles, with no sphincter muscle groups. Three kinds of secretion, produced in a cluster of gland cell bodies posterior to the pharynx, enter the pharynx wall. They travel anteriorly in ducts and two kinds unite in a common duct just prior to discharging into the anterior region of the pharynx lumen. The perikarya of lumen epithelial cells lie within the pharynx musculature and, at the anterior and posterior margins of the pharynx, external to the pharynx. Bundles of ciliated receptors are numerous at the anterior and posterior constrictions. Similarities in the ultrastructure of flame bulbs of Rhabdocoela and Lecithoepitheliata suggest a relationship between these groups. However, the usefulness of pharynx ultrastructure for platyhelminth phylogeny cannot be assessed until complete ultrastructural studies of various groups of Rhabdocoela have been made.     

Monograph of the solenofilomorphidae (Turbellaria: Acoela)

Morphological studies of eleven new species in the family permitted reconstruction of progressive evolutionary lines, and this strongly implies progressive evolutionary lines for other acoela. Solenofilomorpha funilis n. sp., Myopea crassula n. g. n. sp., M. latafaucium n. sp., Fusantrum rhammiphorum n. g. n. sp. and Endocincta punctata n. g. n. sp. are described from the coast of South Carolina, U.S.A., and S. guaymensis n. sp. is described from the Gulf of California. Five additional unnamed species from the U. S. Pacific Coast, Sweden and Tunisia are briefly described. Two species groupings were based on correlation between two different arrangements of pharynx muscles and trio arrangements of antrum muscles and further correlations with lesser characters. The membership of the two groupings in a single family make it clear that evolution proceeded by addition of structures to a less complex common ancestor. Functional and behavioral considerations support the evolutionary path drawn from morphology. Distinctive statocyst structure, ciliary interconnections and sperm morphology shared by acoels show them to have a common ancestry, but clearly different pharynges and male structures cannot be consistent with evolution by reductions. A common ancestor of low complexity has probably given rise to several partly parallel evolutionary lines which together form the structurally diverse Acoela. The history of acoel systematics is also briefly summarized.     

Embryonic muscle development of Convoluta pulchra (Turbellaria-acoelomorpha, platyhelminthes)

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.     

The intestinal musculature of Derocheilocaris typica (Crustacea,Mystacocarida) – A different and unique pattern

An ultrastructural study of the intestine of Derocheilocaris typica revealed an organization of the midgut musculature, which is unique in the Crustacea. This species unusual anal skeletomusculature has also not been seen before.The intestinal musculature of D. typica displays different patterns in the fore-, mid-, and hindgut. Around the foregut, eight pairs of dilator muscles complement a contiguous carpet of circular muscles around the foregut. Their coordinated action serves to suck in food and pass it to the midgut. A pair of large glands, each consisting of three cells, opens into the foregut above the mouth. The midgut musculature differs from any previously described. Circular muscles give rise to thin, longitudinal protrusions and short longitudinal muscles. The distribution of all of them is irregular. Thus the short longitudinal muscles, which have a length of approximately one segment, vary from none to five within a segment. The last abdominal segment is exceptional, by having 15–20 short longitudinal muscles. The hindgut has three longitudinal muscle groups each consisting of three muscles, one dorsally and one on each side. The posterior end of the midgut and the hindgut suggests that they act together to achieve defecation. The importance of the peri-intestinal cells as part of the nutritional process is emphasized.     

Mandibular arch musculature of anuran tadpoles, with comments on homologies of amphibian jaw muscles

This study analyzes the structure of the mandibular arch musculature in larval, metamorphic, and postmetamorphic anurans of 26 species and makes comparisons with larvae of three caudate and one gymnophione species. Major transformations in early evolution of anuran larvae comprise, for example, the powering of the larval upper jaw cartilages by relocating insertion sites of mandibular arch levators; splitting of some larval muscles into two muscles or muscle heads (m. intermandibularis, m. lev. mand. externus, m. lev. mand. longus); evolution of a muscle invading the lower lip of the oral disk (m. mandibulolabialis), and shift of origin of the internus and longus muscles from dorsal on the cranium to sites on the ventral otic capsule and palatoquadrate, respectively. In all these characters, Ascaphus truei shares the plesiomorphic conditions with caudates. The larva of Xenopus laevis is remarkable because the insertion pattern of three larval mandibular muscles anticipates the postmetamorphic condition of frogs in general and also resembles the caudate condition. Discoglossids, bombinatorids, pelobatids, and neobatrachians are largely similar in their muscle arrangements. The filter-feeding microhylids, however, have most clearly modified the general neobatrachian pattern. Past conflicts in the interpretation and naming of muscles can be attributed to the implicit or explicit homology assumptions used. In particular, the muscles' relations to the branches of the trigeminal nerve have been the dominant criteria for inferring homology and has led to inconsistencies. This concept is questioned herein. It is observed that the relative position of the ramus mandibularis (V(3)) is more variable interspecifically in anuran larvae than previously thought. The relations of the nerve branches and muscles in larvae are maintained during metamorphosis. Considering the muscle pattern to be more conserved in interspecific comparisons than the position of the nerve branches results in a new interpretation of muscle homologies and a hypothesis of jaw muscle evolution in amphibians that is more parsimonious than earlier views. A new, simplified terminology for the jaw musculature is proposed that is applicable for larvae and adults. It maximizes information content and reflects the hypothesized homologies of amphibian jaw muscles.     

Walking on inclines: how do desert ants monitor slope and step length

Background

In order to increase the weak database concerning the organogenesis of Acoela – a clade regarded by many as the earliest extant offshoot of Bilateria and thus of particular interest for studies concerning the evolution of animal bodyplans – we analyzed the development of the musculature of Symsagittifera roscoffensis using F-actin labelling, confocal laserscanning microscopy, and 3D reconstruction software.

Results

At 40% of development between egg deposition and hatching short subepidermal fibres form. Muscle fibre development in the anterior body half precedes myogenesis in the posterior half. At 42% of development a grid of outer circular and inner longitudinal muscles is present in the bodywall. New circular muscles either branch off from present fibres or form adjacent to existing ones. The number of circular muscles is higher than that of the longitudinal muscles throughout all life cycle stages. Diagonal, circular and longitudinal muscles are initially rare but their number increases with time. The ventral side bears U-shaped muscles around the mouth, which in addition is surrounded by a sphincter muscle. With the exception of the region of the statocyst, dorsoventral muscles are present along the entire body of juveniles and adults, while adults additionally exhibit radially oriented internal muscles in the anterior tip. Outer diagonal muscles are present at the dorsal anterior tip of the adult. In adult animals, the male gonopore with its associated sexual organs expresses distinct muscles. No specific statocyst muscles were found. The muscle mantles of the needle-shaped sagittocysts are situated along the lateral edges of the animal and in the posterior end close to the male gonopore. In both juveniles and adults, non-muscular filaments, which stain positively for F-actin, are associated with certain sensory cells outside the bodywall musculature.

Conclusion

Compared to the myoanatomy of other acoel taxa, Symsagittifera roscoffensis shows a very complex musculature. Although data on presumably basal acoel clades are still scarce, the information currently available suggests an elaborated musculature with longitudinal, circular and U-shaped muscles as being part of the ancestral acoel bodyplan, thus increasing the possibility that Urbilateria likewise had a relatively complicated muscular ground pattern.