Evolutionary aspects of pattern formation during clitellate muscle development

As a taxon of the lophotrochozoans, annelids have re-entered scientific investigations focusing on plesiomorphic bilaterian features and the evolutionary changes therein. The view of a clitellate-like plesiomorphic muscle arrangement in annelids has been challenged by recent investigations of polychaete muscle organization. However, there are few investigations of muscle formation in clitellate species that address this problem. Direct comparison of potential homologous muscles between these annelid groups is thus hampered. Somatic muscle formation during embryogenesis of two clitellates-the oligochaete Limnodrilus sp. and the hirudinean Erpobdella octoculata-occurs by distinct processes in each species, even though they share a closed outer layer of circular and an inner layer of longitudinal muscles characteristic of clitellates. In E. octoculata, the first emerging longitudinal muscles are distributed irregularly on the body surface of the embryo whereas the circular muscles appear in an orderly repetitive pattern along the anterioposterior axis. Both primary muscle types consist of fiber-bundles that branch at both their ends. This way the circular muscle bundles divide into a fine muscle-grid. The primary longitudinal muscles are incorporated into a second type of longitudinal muscles, the latter starting to differentiate adjacent to the ventral nerve cord. Those secondary muscles emerge in a ventral to dorsal manner, enclosing the embryo of E. octoculata. In Limnodrilus sp., one dorsal and one ventral bilateral pair of primary longitudinal muscles are established initially, elongating toward posterior. Initial circular muscles are emerging in a segmental pattern. Both muscle layers are completed later in development by the addition of secondary longitudinal and circular muscles. Some features of embryonic longitudinal muscle patterns in Limnodrilus sp. are comparable to structures found in adult polychaete muscle systems. Our findings show that comparative studies of body-wall muscle formation during clitellate embryogenesis are a promising approach to gain further information on annelid muscle arrangements.     

Pattern of body-wall muscle differentiation during embryonic development of Enchytraeus coronatus (Annelida: Oligochaeta; Enchytraeidae)

The plesiomorphic arrangement of body-wall musculature within the annelids is still under discussion. While polychaete groups show a great variety of patterns in their somatic muscles, the musculature of soil-living oligochaetes was thought to represent the characteristic pattern in annelids. Oligochaete body-wall muscles consist of an outer continuous layer of circular and an inner continuous layer of longitudinal muscles, forming a closed tube. Since designs of adult body musculature are influenced by evolutionary changes, additional patterns found during embryogenesis can give further information about possible plesiomorphic features. In oligochaetes, detailed cell-lineage analyses document the origin of the mesoderm and consequently the muscles, but later processes of muscle formation remain unclear. In the present work, body-wall muscle differentiation was monitored during embryogenesis of thesoil-living oligochaete Enchytraeus coronatus (Annelida) by phalloidin staining. Primary circular muscles form in a discrete anterior-to-posterior segmental pattern, whereas emerging longitudinal muscles are restricted to one ventral and one dorsal pair of primary strands, which continuously elongate towards posterior. These primary muscles establish an initial muscle-template. Secondary circular and longitudinal muscles subsequently differentiate in the previous spaces later in development. The prominent ventral primary longitudinal muscle strands on both sides eventually meet at the ventral midline due to neurulation, which moves the ventral nerve cord into a coelomic position, closing the muscle layers into a complete tube. This early embryonic pattern in E. coronatus resembles the adult body-wall muscle arrangements in several polychaete groups as well as muscle differentiation during embryonic development of the polychaete Capitella sp. I.     

Myogenesis in two polyclad platyhelminths with indirect development,Pseudoceros canadensis and Stylostomum sanjuania

SUMMARY Myogenesis of two representatives of Platyhelminthes, Stylostomum sanjuania and Pseudoceros canadensis, was followed from egg deposition until well‐differentiated free‐swimming larval stages, using F‐actin staining and confocal laserscanning microscopy. Zonulae adhaerentes are the only structures to stain before 50% of development between egg deposition and hatching in S. sanjuania, and before 67% of development in P. canadenis. Subsequently, irregular fibers appear in the embryo, followed by a helicoid muscle close to the apical pole. Three longitudinal muscle pairs form, of which the dorsal pair remains more pronounced than the others. Gradually, new muscles form by branching or from double‐stranded muscle zones adjacent to existing muscles. This results in an elaborate muscular bodywall that consists of a single helicoid muscle as well as multiple circular and longitudinal muscles. Diverse retractor muscles insert at the sphincter muscles around the stomodeum. The overall arrangement and formation mode of the larval musculature appears very similar in both species, although only P. canadensis has a primary circular muscle posterior to the helicoid muscle. Muscle formation in the apical region of the embryo precedes that at the abapical pole and the primary longitudinal muscles form slightly later than the primary circular muscles. Myogenesis and larval myoanatomy appears highly conserved among polyclad flatworms, but differs significantly from that of other trochozoan clades. Our data suggest that the larval muscular ground pattern of polyclad larvae comprises a bodywall consisting of a helicoid muscle, circular and longitudinal muscles, several retractor muscles, and sphincter muscles around the stomodeum.     

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.     

F-actin framework in Spirorbis cf. spirorbis (Annelida: Serpulidae): phalloidin staining investigated and reconstructed by cLSM

Abstract. Serpulidae encompasses polychaete species whose members have fused anterior ends bearing a tentacular crown, a heteronomous segmented body with a thorax and abdomen, and “chaetal inversion” between the two tagmata. The sessile filter‐feeding organisms live in self‐built, coiled, calcareous tubes on algae. The F‐actin muscular subset of Spirorbis cf. spirorbis was stained with phalloidin and three‐dimensionally reconstructed by means of cLSM, aiming to investigate (1) how the tentacular crown is organized and moved, (2) whether the internal structures, e.g., musculature, follow the thorax–abdomen inversion, and (3) whether circular muscles are present in serpulids. The third aim is by reason of recent investigations suggesting that lack of circular muscle fibers may be a common situation rather than a rare variation in polychaetes. In this manner, this article is part of a comparative evaluation of polychaete muscle systems. We found that longitudinal muscles of the body wall project into the tentacular crown, and that radioli and pinnulae possess three muscle types each, facilitating their great mobility. Operculum, collar, and a pair of unidentified organs possess distinct F‐actin filaments. The trunk is mainly moved by five longitudinal muscle strands, most obvious in the abdomen: two dorsal, two ventral, and an unpaired ventromedian one, out of which the dorsal ones are the strongest. In anterior regions, the two dorsal strands form a single continuous layer; the separated strands lessen posteriorly. Solitary transverse fibers are located ventrally in the middle of each segment, stretching between longitudinal muscles and coelomic lining laterally, where they end. Peripheral and central dorsoventral muscles, two pairs per segment each, are present. Circular fibers as well as bracing muscles were not detected. The results indicate that the musculature does not follow the thorax–abdomen inversion and Serpulidae represents the 15th polychaete taxon in which circular fibers are totally missing.     

CLSM analysis of the phalloidin-stained muscle system in Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis (Polychaeta; Nerillidae)

The entire muscle system of Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis was three-dimensionally reconstructed from whole mounts. In juvenile and adult specimens the F-actin musculature subset was stained with FITC-conjugated phalloidin and visualized with a confocal laser scanning microscope (cLSM). The muscle system shows the following major organization: 1) circular muscles are totally absent in the body wall; 2) the longitudinal muscles are confined in two ventral and two dorsal thick bundles; 3) additional longitudinal muscles are located in the ventro- and dorsomedian axis; 4) three segmental pairs of ventral oblique muscles elongate into the periphery: the main dorsoventral muscles that run along the body side posterior and dorsally and the anterior and posterior oblique parapodial muscles, which contribute to the ventral chaetal sacs; 5) one segmental pair of dorsal oblique parapodial muscles, contributing to the dorsal chaetal sacs; 6) five to seven small dorsoventral muscles per segment; and 7) complex head and pharyngeal musculature. These results support the belief that absence of circular muscles in the polychaete body wall is much more widely distributed than is currently presumed.     

Segmental differentiation processes in embryonic muscle development of the grasshopper

To analyse segmental differentiation processes in muscle development, we studied the embryogenesis of the ventral body wall muscles in thoracic and abdominal segments of the grasshopper Schistocerca gregaria at the identified cell level. We visualized differentiating muscle pioneer and muscle precursor cells by staining with a muscle-specific monoclonal antibody and with rhodamine-coupled phalloidin. Our results show that a similar pattern of serially reiterated early muscle pioneers is initially established in all segments. Subsequently, two major segmental differentiation processes occur. First, segment-specific sets of additional, later differentiating muscle pioneers are generated de novo. Second, segment-specific sets of existing early muscle precursors are eliminated through atrophy and eventual loss. These events have consequences for matching homonomy of muscles and their innervating motoneurons. Taken together, these processes in the embryo, in concert with postembryonic differentiation events, play critical roles in shaping the highly specialized muscular structures of the mature animal.     

The simplicity of males: Dwarf males of four species of Osedax (Siboglinidae; Annelida) investigated by confocal laser scanning microscopy

Dwarf males of the bone‐eating worms Osedax (Siboglinidae, Annelida) have been proposed to develop from larvae that settle on females rather than on bone. The apparent arrest in somatic development and resemblance of the males to trochophore larvae has been posited as an example of paedomorphosis. Here, we present the first investigation of the entire muscle and nervous system in dwarf males of Osedax frankpressi, O. roseus, O. rubiplumus, and O. “spiral” analyzed by multistaining and confocal laser scanning microscopy. Sperm shape and spermiogenesis, the sperm duct and internal and external ciliary patterns were likewise visualized. The males of all four species possess morphological traits typical of newly settled siboglinid larvae: a prostomium, a peristomium with a prototroch, one elongate segment and a second shorter segment. Each segment has a ring of eight long‐handled hooked chaetae. The longitudinal muscles are distributed as evenly spaced strands forming a grid with the thin outer circular muscles. Oblique protractor and retractor muscles are associated with each of the chaetal sacs. The nervous system comprises a cerebral ganglion, a prototroch nerve ring, paired dorsolateral longitudinal nerves, five ventral longitudinal nerves with paired, posterior ganglia and a terminal commissure, as well as a net of fine peripheral transverse plexuses surrounding the first segment. Internal ciliation occurs as paired ventrolateral bands along the first segment. The bands appear to lead the free mature sperm to a ciliated duct and seminal vesicle lying just behind the prototroch region. A duct then runs from the seminal vesicle into the dorsal part of the prostomium. The similarity of Osedax males to the larvae of Osedax and other siboglinid annelids as well as similarities shown here to the neuromuscular organization seen in other annelid larvae supports the hypothesis of paedomorphosis in males of Osedax. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Phalloidin-rhodamine preparations of Macrostomum hystricinum marinum (Plathelminthes): morphology and postembryonic development of the musculature

Summary A whole-mount fluorescence technique using rhodamine-labeled phalloidin was used to demonstrate for the first time the whole muscle system of a free-living plathelminth, Macrostomum hystricinum marinum. As expected, the body-wall musculature consisted of circular, longitudinal, and diagonal fibers over the trunk. Also distinct were the musculature of the gut and of the mouth and pharynx (circular, longitudinal, and radial). Dorsoventral fibers where restricted in this species to the head and tail regions. Circular muscle fibers in the body wall were often grouped into bands of up to four parallel strands. Surprisingly, diagonal fibers formed two distinct sets, one dorsal and one ventral. Certain diagonal muscle fibers entered the wall of the mouth and were continuous with some longitudinal muscles of the pharynx. Dorsoventral fibers in the rostrum occurred partly in regularly spaced pairs, a fact not known for free-living Plathelminthes. All muscle fibers appeared to be mononucleated. During postembryonic development, the number of circular muscle fibers can be estimated to increase by a factor of 3.5 and that of longitudinal muscles by a factor of 2. Apparently as many as 700–800 circular muscle cells must be added in the region of the gut alone during postembryonic development. Stem cells (neoblasts), identified by TEM in the caudalmost region of the gut, lie along the lateral nerve cords. In the same body region most perikarya of circular muscle cells occurred in a similar position. This suggests that the nucleus-containing part of the cell remains in the position where differentiation starts.     

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.     

Embryonic muscle development in direct and indirect developing marine flatworms (Platyhelminthes, Polycladida)

SUMMARY We compared embryonic myogenesis of the direct-developing acotylean polyclad Melloplana ferruginea with that of Maritigrella crozieri , a cotylean that develops via a larval stage. Fluorescently labeled F-actin was visualized with laser confocal microscopy. Developmental times are reported as percentages of the time from oviposition to hatching: 7 days for M. crozieri and 22  days for M. ferruginea . The epithelium began to form at 30% development in M. crozieri and at 15% development in M. ferruginea . Random myoblasts appeared in peripheral areas of the embryo at 36% and 22–30% development in M. crozeri and M. ferruginea , respectively. Circular and longitudinal muscle bands formed synchronously at 37–44% development in M. crozieri ; yolk obscured observations of early myogenesis in M. ferruginea . An orthogonal muscle grid was established by 45–50% development in both species. Diagonal muscles developed in M. ferruginea at 60–71% development. Hence, juveniles of this species hatch with the same basic body-wall musculature as adults. Larvae of M. crozieri did not hatch with diagonal muscles; these muscles are acquired postmetamorphosis. Additionally, a specialized musculature developed in the larval lobes of M. crozieri . Oral musculature was complex and established by 72% development in both species. Our results are comparable to the muscle differentiation reported for other indirect-developing polyclads and for direct-developing species of macrostomid flatworms. Furthermore, they provide additional support that the orthogonal muscle pattern of circular and longitudinal muscles is a symplesiomorphy of Spiralia.     

Reconstruction of the musculature of <Emphasis Type="Italic">Magelona</Emphasis> cf. <Emphasis Type="Italic">mirabilis</Emphasis> (Magelonidae) and <Emphasis Type="Italic">Prionospio cirrifera</Emphasis> (Spionidae) (Polychaeta,Annelida) by phalloidin labeling and cLSM

Recent investigations have suggested that a lack of circular muscle fibers may be a common situation rather than a rare exception in polychaetes. As part of a comparative survey of polychaete muscle systems, the F-actin musculature subset of Magelona cf. mirabilis and Prionospio cirrifera were labeled with phalloidin and three-dimensionally analyzed and reconstructed by means of cLSM. Obvious similarities are sublongitudinal lateral, circumbuccal, palp retractor, dominating dorsal longitudinal, perpendicular lateral and ventral transverse muscles. Differences between M. cf. mirabilis and P. cirrifera are: (1) two types of prostomial muscles (transversal and longitudinal) in M. cf. mirabilis versus one type (diagonal) in P. cirrifera; (2) one type of palp muscles (longitudinal) in M. cf. mirabilis versus three types (longitudinal, diagonal, circular) in P. cirrifera; (3) five ventral longitudinal muscles (ventromedian, paramedian, ventral) in M. cf. mirabilis versus four (two paramedian, two ventral) in P. cirrifera. Ventral and lateral transverse fibers are present in the thorax, but absent in the abdomen of M. cf. mirabilis. The triangular lumen of the pharynx in M. cf. mirabilis is surrounded by radial muscle fibers; three sets of pharynx diductors attach to its dorsal side. The unique features of P. cirrifera are one pair of brain muscles and segmentally arranged dorsal transverse muscles, the latter located outside the longitudinal muscles. The transverse lateral muscles are restricted to the sides and lie beneath the longitudinal muscles, a pattern described here for the first time. A true, outer layer of circular fibers is absent in both species of Spionida that were investigated.     

Functional morphology of muscles in Tetranchyroderma papii (Gastrotricha)

Movement in gastrotrichs is powered by an interaction of ventral cilia and muscles. In interstitial gastrotrichs, movement among sand grains often requires the additional use of adhesive tubules that allow for behaviors such as escape responses and changes in body position. In this study, we describe orientations and possible mechanical actions of muscles during locomotion in the gastrotrich, Tetranchyroderma papii (Macrodasyida). Fluorescently labeled phalloidin was used to stain F-actin of muscles and visualize muscle patterns. Muscles are arranged in circular, longitudinal, and helicoidal orientations. Circular muscles were in the form of discreet rings around the pharynx and intestine, and contribute to the structure of the oral hood. Longitudinal muscles are largely concentrated on the ventral and ventrolateral sides of the body, where they aid in body flexion, including directional changes during ciliary swimming, body torsion, and escape responses. Helicoidal muscles, present as myocytes in left- and right-hand orientations, lie external of the circular bands and some of the longitudinal bands, and are hypothesized to counteract dilations of the pharynx and intestine during feeding. Extraordinary muscle orientations with undetermined functions include a pair of crossover muscles and a single semicircular muscle band at the caudal end. Accepted: 12 February 2001     

Immunocytochemistry of the nervous system and the musculature of the chordoid larva of Symbion pandora (Cycliophora)

To date, the phylum Cycliophora comprises only one described extant species of acoelomate marine invertebrates, Symbion pandora. Adult specimens live commensally on the mouthparts of the Norwegian lobster, Nephrops norvegicus. Its complicated life cycle includes an asexually produced Pandora larva and a sexually produced chordoid larva. Despite detailed TEM investigations and its inclusion in recent molecular phylogenetic analyses, cycliophoran relationships still remain enigmatic. In order to increase the morphological database, I investigated the anatomy of the nervous system and the musculature of the chordoid larva by applying fluorescence-coupled antibodies against the neurotransmitters serotonin and FMRFamide, as well as FITC-coupled phalloidin to label filamentous F-actin, in combination with confocal laser scanning microscopy. The FMRFamidergic nervous system shows a bilobed anterior ganglion and one pair of ventral nerve cords, while serotonin is distributed in a scattered pattern in the anterior ganglion. In addition, there are two pairs of ventral serotonergic nerves, of which the inner pair fuses with the outer nerve cords in the posterior third of the larva. The musculature comprises an outer layer of six units of circular body wall muscles, several helicoid muscle fibers, a set of paired longitudinal muscles that span the entire anterior-posterior axis of the larva, and a few oblique muscle strands. Furthermore, an anterior muscle complex and one pair of posterior muscles are present. The chordoid organ consists of a number of distinct subunits that are each formed by a dense layer of circular muscle fibers.The overall arrangement of the oblique and longitudinal muscles as well as the body wall musculature in the chordoid larva of Symbion pandora exhibits similarities with the condition found in certain rotifers. This is congruent with some recent phylogenies based on 18S rRNA sequences but additional morphological, developmental, and molecular data are needed to clarify the phylogenetic relationships of Cycliophora.     

Three-dimensional reconstruction of the musculature of Cossura pygodactylata Jones, 1956 (Annelida: Cossuridae)

The musculature of adult specimens of Cossura pygodactylata was studied by means of F-actin labelling and confocal laser scanning microscopy (CLSM). Their body wall is comprised of five longitudinal muscle bands: two dorsal, two ventral and one ventromedial. Complete circular fibres are found only in the abdominal region, and they are developed only on the border of the segments. Thoracic and posterior body regions contain only transverse fibres ending near the ventral longitudinal bands. Almost-complete rings of transverse muscles, with gaps on the dorsal and ventral sides, surround the terminal part of the pygidium. Four longitudinal bands go to the middle of the prostomium and 5–14 paired dorso-ventral muscle fibres arise in its distal part. Each buccal tentacle contains one thick and two thin longitudinal muscle filaments; thick muscle fibres from all tentacles merge, forming left and right tentacle protractors rooted in the dorsal longitudinal bands of the body wall. The circumbuccal complex includes well-developed upper and lower lips. These lips contain an outer layer of transverse fibres, and the lower lip also contains inner oblique muscles going to the dorsal longitudinal bands. The branchial filament contains two longitudinal muscle fibres that do not connect with the body musculature. The parapodial complex includes strong intersegmental and segmental oblique muscles in the thoracic region only; chaetal retractors, protractors and muscles of the body wall are present in all body regions. Muscle fibres are developed in the dorsal and ventral mesenteries. One semi-circular fibre is developed on the border of each segment and is most likely embedded in the dissepiment. The intestine has thin circular fibres along its full length. The dorsal blood vessel has strong muscle fibres that cover its anterior part, which is called the heart. It consists of short longitudinal elements forming regular rings and inner partitions. The musculature of C. pygodactylata includes some elements that are homologous with similar muscular components in other polychaetes (i.e., the body wall and most parapodial muscles) and several unique features, mostly at the anterior end.     

Musculature in polychaetes: comparison of Myrianida prolifera (Syllidae) and Sphaerodoropsis sp. (Sphaerodoridae)

Abstract. The relationship of the polychaete taxa Syllidae and Sphaerodoridae within Phyllodocida is still unresolved: phylogenetic analyses either show them as sister groups or more widely separated. The present article aims to provide information about the structure of the muscular system that could be essential for understanding their relationship. A crucial point is whether the body wall contains circular muscles, which has recently been shown to be absent in more taxa than previously known. The F-actin filaments in members of Myrianida prolifera (Syllidae) and Sphaerodoropsis sp. (Sphaerodoridae) were labeled with phalloidin and their three-dimensional relationships reconstructed by means of confocal laser scanning microscopy. Among the noteworthy differences that emerged between the species are (1) members of M. prolifera possess four, those of Sphaerodoropsis sp. eight, longitudinal muscle strands; (2) the body wall in M. prolifera contains transverse fibers in a typical, supralongitudinal position, while in Sphaerodoropsis sp., corresponding fibers lie beneath the longitudinal strands; (3) pro- and peristomium in M. prolifera have no distinct F-actin fibers, while five longitudinal pairs and three single transverse muscular fibers shape the anterior end in Sphaerodoropsis sp.; (4) the proventricle of M. prolifera comprises primarily radial muscle fibers arranged in distinct rows, while in Sphaerodoropsis sp. the axial proboscis consists of longitudinal and circular fibers and radial fibers are lacking; (5) in M. prolifera, the proximal and distal sections of the two anteriormost pairs of dorsal cirri possess longitudinal myofilaments, which are separate from the body wall musculature; by contrast, all appendages in Sphaerodoropsis sp. do not; (6) both species have bracing muscles: in M. prolifera they are positioned above the longitudinal fibers, whereas in Sphaerodoropsis sp. they are uniquely positioned between longitudinal and sublongitudinal transverse fibers. These results do not support a sister-group relationship of Syllidae and Sphaerodoridae. In addition, Sphaerodoropsis sp. is yet another example in the list of polychaetes lacking typical circular muscles in the body wall.     

Chiton myogenesis: perspectives for the development and evolution of larval and adult muscle systems in molluscs.

We investigated muscle development in two chiton species, Mopalia muscosa and Chiton olivaceus, from embryo hatching until 10 days after metamorphosis. The anlagen of the dorsal longitudinal rectus muscle and a larval prototroch muscle ring are the first detectable muscle structures in the early trochophore-like larva. Slightly later, a ventrolaterally situated pair of longitudinal muscles appears, which persists through metamorphosis. In addition, the anlagen of the putative dorsoventral shell musculature and the first fibers of a muscular grid, which is restricted to the pretrochal region and consists of outer ring and inner diagonal muscle fibers, are generated. Subsequently, transversal muscle fibers form underneath each future shell plate and the ventrolateral enrolling muscle is established. At metamorphic competence, the dorsoventral shell musculature consists of numerous serially repeated, intercrossing muscle fibers. Their concentration into seven (and later eight) functional shell plate muscle bundles starts after the completion of metamorphosis. The larval prototroch ring and the pretrochal muscle grid are lost at metamorphosis. The structure of the apical grid and its atrophy during metamorphosis suggests ontogenetic repetition of (parts of) the original body-wall musculature of a proposed worm-shaped molluscan ancestor. Moreover, our data show that the "segmented" character of the polyplacophoran shell musculature is a secondary condition, thus contradicting earlier theories that regarded the Polyplacophora (and thus the entire phylum Mollusca) as primarily eumetameric (annelid-like). Instead, we propose an unsegmented trochozoan ancestor at the base of molluscan evolution.     

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.     

The adult musculature of two pseudostomid species reveals unique patterns for flatworms (Platyhelminthes,Prolecithophora)

We analyzed the adult musculature of two prolecithophoran species, Cylindrostoma monotrochum (von Graff, 1882) and Monoophorum striatum (von Graff, 1878) using a phalloidin-rhodamine technique. As in all rhabdithophoran flatworms, the body-wall musculature consisted of three muscle layers: on the outer side was a layer of circular muscle fibers and on the inner side was a layer of longitudinal muscle fibers; between them were two different types of diagonally orientated fibers, which is unusual for flatworms. The musculature of the pharynx consisted of a basket-shaped grid of thin longitudinal and circular fibers. Thick anchoring muscle fibers forming a petal-like shape connected the proximal parts of the pharynx with the body-wall musculature. Male genital organs consisted of paired seminal vesicles, a granular vesicle, and an invaginated penis. Peculiar ring-shaped muscles were only found in M. striatum, predominantly in the anterior body part. In the same species, seminal vesicles and penis only had circular musculature, while in C. monotrochum also longitudinal musculature was found in these organs. Female genital organs were only present in M. striatum, where we characterized a vagina interna, and a bursa seminalis. Transverse, crossover, and dorsoventral muscle fibers were lacking in the middle of the body and greatly varied in number and position in both species.     

Dorsal longitudinal stretch receptor of Drosophila melanogaster larva - fine structure and maturation

Insects possess two types of sensory neurons: ciliated type I sensory neurons that innervate external sensory organs and chordotonal organs, and type II sensory neurons that form a subepidermal plexus or innervate stretch receptors. Among stretch receptors, a dorsel longitudinal stretch receptor is highly conserved in insects, being found in all insect orders investigated. Here we describe the topology and anatomical structure of this receptor in the fruit fly embryo and larva using transmission electron microscopy and single cell staining for fluorescence microscopy. The receptor is composed of the dorsal bipolar dendrite neuron, which arises from an archetypal cell lineage, its sister glial cell and the peripheral glial cell accompanying the nerve. The neuron is situated among the muscles in the dorsal body wall on the intersegmental nerve. Its two dendrites stretch the length of the segment to the segmental folds. The neuron is wrapped by both glial cells and surrounded by a common basal lamina, which fans out at the dendritic tips to attach them to the epidermal cells at the segmental borders.