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     

The musculature of Draculiciteria tessalata (Chaetonotida, Paucitubulatina): implications for the evolution of dorsoventral muscles in Gastrotricha

The muscular system of the marine interstitial gastrotrich Draculiciteria tessalata (Chaetonotida, Paucitubulatina) was analyzed with fluorescent phalloidin. Muscles in circular, longitudinal, helicoidal and dorsoventral orientations were found. Circular muscles were present as discreet rings on the pharynx only. Five pairs of longitudinal muscles were found in dorsal, lateral and ventral positions. One of the two pairs of lateral muscles is newly described for the species. Helicoidal muscles, external to the circular muscles and some longitudinal bands, spiraled around the pharynx and anterior portion of the intestine. Two pairs of segmentally-arranged dorsoventral muscles were also present. Lateral dorsoventral muscles extended from the base of the pharynx to the anterior part of the caudal furca. Medial dorsoventral muscles extended from the pharyngeal-intestinal junction into each ramus of the caudal furca. A hypothesis on the evolution of dorsoventral muscles in D. tessalata is proposed which includes a splitting of circular muscles into separate somatic and splanchnic components with a further displacement of both muscle sets into a dorsoventral orientation.     

The musculature of three species of gastrotrichs surveyed with confocal laser scanning microscopy (CLSM)

The muscular system of gastrotrichs consists of circular, longitudinal and helicoidal bands that when analysed with confocal laser scanning microscopy, provide new insights into their functional organization and phylogenetic importance. We therefore undertook a comparative study of the muscle organization in three species of Gastrotricha from the orders Macrodasyida (Paradasys sp., Lepidodasyidae; Turbanella sp., Turbanellidae) and Chaetonotida (Polymerurus nodicaudus, Chaetonotidae). The general muscle organization of the marine interstitial macrodasyidans, Paradasys and Turbanella, not only confirms earlier observation on other species but also adds new details concerning the organization and number of helicoidal, longitudinal and other muscle bands (e.g. semicircular band). The freshwater, epibenthic–epiphytic chaetonotid, Polymerurus nodicaudus, has a similar muscular organization to other species of Chaetonotidae, especially species of Chaetonotus, Halichaetonotus and Lepidodermella. Perhaps unique to Polymerurus is the combined presence of an unbranched Rückenhautmuskel (also in Halichaetonotus and Lepidodermella) and a specialized dorsoventral caudal muscle, which flank the splanchnic component of the longitudinal muscles (only in Chaetonotus and Lepidodermella). This combination, together with the presence of splanchnic dorsoventral muscles, known only in Xenotrichulidae, implies a unique phylogenetic position for Polymerurus, and indicates a potential basal position of this taxon among the Chaetonotidae studied so far (i.e. Aspidiophorus, Chaetonotus, Halichaetonotus and Lepidodermella).     

Phylogeny of Gastrotricha: a morphology-based framework of gastrotrich relationships

Currently, the phylum Gastrotricha is divided into the orders Macrodasyida and Chaetonotida, with the structure of the myoepithelial pharynx being an important distinguishing feature. Macrodasyida currently has six recognized families, and Chaetonotida comprises seven families. However, within-group relationships are poorly understood. To arrive at a better understanding of gastrotrich systematics and phylogeny, we performed the first cladistic analysis of nearly all known gastrotrich genera using 71 morphological characters. Results suggest that the Gastrotricha is a monophyletic group (supported by 82% of bootstrap replications) with its most primitive taxa distributed among the families Dactylopodolidae and Neodasyidae. Monophyly of Macrodasyida and Chaetonotida was supported by 90% and 52% bootstrap replications, respectively. Within the Macrodasyida, the families Dactylopodolidae, Turbanellidae, Macrodasyidae, and Thaumastodermatidae all formed monophyletic clades. The families Planodasyidae and Lepidodasyidae were paraphyletic. Among the Chaetonotida, the marine family Xenotrichulidae was monophyletic, supported by 51% of bootstrap replications. A second clade containing all freshwater families was supported by 62% bootstrap values. However, Chaetonotidae were paraphyletic. Using this analysis as a framework, we now can explore possible patterns of evolution within it, and arrive at a consensus of the gastrotrich ground pattern. Moreover, in future molecular studies of metazoan phylogeny, we will be able to select gastrotrich species that are more appropriate representatives of the phylum.     

Musculature of the penial complex: A new criterion in unravelling the phylogeny of Hygrophila (Gastropoda: Pulmonata)

The taxonomy of freshwater pulmonates (Hygrophila) has been in a fluid state warranting the search for new morphological criteria that may show congruence with molecular phylogenetic data. We examined the muscle arrangement in the penial complex (penis and penis sheath) of most major groups of freshwater pulmonates to explore to which extent the copulatory musculature can serve as a source of phylogenetic information for Hygrophila. The penises of Acroloxus lacustris (Acroloxidae), Radix auricularia (Lymnaeidae), and Physella acuta (Physidae) posses inner and outer layers of circular muscles and an intermediate layer of longitudinal muscles. The inner and outer muscle layers in the penis of Biomphalaria glabrata consist of circular muscles, but this species has two intermediate longitudinal layers separated by a lacunar space, which is crossed by radial and transverse fibers. The muscular wall of the penis of Planorbella duryi is composed of transverse and longitudinal fibers, with circular muscles as the outer layer. In Planorbidae, the penial musculature consists of inner and outer layers of longitudinal muscles and an intermediate layer of radial muscles. The penis sheath shows more variation in muscle patterns: its muscular wall has two layers in A. lacustris, P. acuta, and P. duryi, three layers in R. auricularia and Planorbinae and four layers in B. glabrata. To trace the evolution of the penial musculature, we mapped the muscle characters on a molecular phylogeny constructed from the concatenated 18S and mtCOI data set. The most convincing synapomorphies were found for Planorbinae (inner and outer penis layers of longitudinal muscles, three-layered wall of the penis sheath). A larger clade coinciding with Planorbidae is defined by the presence of radial muscles and two longitudinal layers in the penis. The comparative analysis of the penial musculature appears to be a promising tool in unraveling the phylogeny of Hygrophila.     

Evolution of body wall musculature

A body wall musculature comprising an outer layer of circularfibers and an inner layer of longitudinal fibers is generallyseen as the basic plan in Annelida. Additional muscles may bepresent such as oblique, parapodial, chaetal, and dorsoventralmuscles. The longitudinal muscle fibers do not form a continuouslayer but are arranged in distinct bands in polychaetes. Mostlythere are four to six bands, usually including prominent ventraland dorsal bands. However, other patterns of muscle band arrangementalso exist. The ventral nerve cord lies between the two ventralbands in certain polychaetes, and is covered by an additionallongitudinal muscle band of comparatively small size. In manypolychaetes with reduced parapodia and in Clitellata a moreor less continuous layer of longitudinal fibers is formed. Clitellatais the only group with a complete layer of longitudinal musculature.Circular fibers are usually less developed than the longitudinalmuscles. However, recent investigations employing phalloidinstaining in combination with confocal laser scanning microscopyrevealed that absence of circular muscles is much more widelydistributed within the polychaetes than was previously known.This necessitates thorough reinvestigations of polychaete musclesystems, and this feature has to be taken into account in furtherdiscussions of the phylogeny and evolution of Annelida.     

The muscular system of Musellifer delamarei (Renaud-Mornant, 1968) and other chaetonotidans with implications for the phylogeny and systematization of the Paucitubulatina (Gastrotricha)

We studied comparatively the muscle organization of several gastrotrich species, aiming at shedding some light on the evolutionary relationships among the taxa of the suborder Paucitubulatina. Under confocal laser scanning microscope, the circular muscles were present in the splanchnic position as incomplete circular rings in Musellifer delamarei (Chaetonotidae) and Xenotrichula intermedia (Xenotrichulidae) and as dorsoventral bands in Xenotrichula punctata, Heteroxenotrichula squamosa and Draculiciteria tesselata (Xenotrichulidae); in the somatic position, M. delamarei shares the presence of dorsoventral muscles with all the Xenotrichulidae, in contrast with the remaining Chaetonotidae that lack these muscles. Maximum parsimony analysis of the muscular characters confirmed monophyly of Paucitubulatina and Xenotrichulidae, while the Chaetonotidae was paraphyletic, with the exclusion of Musellifer , which is the most basal genus within the Paucitubulatina. Xenotrichulidae is the sister taxon to Chaetonotidae, which in turn has Polymerurus as the most basal taxon. In general, the results agree with recent phylogenetic inferences based on molecular characters and support the hypothesis that, within Paucitubulatina, dorsoventral muscles are plesiomorphies retained in marine, interstitial, hermaphroditic gastrotrichs. Dorsoventral muscles were subsequently lost during changes in lifestyle and reproduction modality that took place with the invasion of the freshwater environment. This new information prompted us to reconsider the systematization of Chaetonotidae, proposing the establishment of Muselliferidae fam. nov. to include the genera Musellifer and Diuronotus .  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 379–398.     

Musculature of Seison nebaliae Grube, 1861 and Paraseison annulatus (Claus, 1876) revealed with CLSM: a comparative study of the gnathiferan key taxon Seisonacea (Rotifera)

The somatic muscular systems of two species of Seisonacea (Rotifera), Seison nebaliae and Paraseison annulatus, are described using fluorescently labelled phalloidin in combination with confocal laser scanning microscopy. Their overall muscular arrangement is similar and consists of segmentally organised longitudinal fibres that extend the length of the body and are surrounded by semi-circular (= incomplete) bands. However, differences in the musculature between the two species are present and possibly reflect specific adaptations in feeding strategy and locomotion related to the occupation of individual niches on their host, the leptostracan crustacean N. bipes. For example, S. nebaliae has semi-circular muscles in the head region only, while P. annulatus possesses incomplete circular muscles also in the trunk region; furthermore, there are also differences in the arrangements and number of longitudinal muscles. The muscular systems of all rotifer species examined so far are compared in order to establish the ground pattern of the last putative ancestor as well as to seek for traits of systematic importance. Results from both species corroborate earlier hypotheses on the arrangement of muscles in the putative common ancestor of Rotifera, which suggested an orthogonal arrangement consisting of a series of probably continuous (not segmental) inner longitudinal muscles, surrounded by semi-circular fibres, ventrally opened. However, significant morphological and ecological variations among taxa investigated so far show that a consistent correlation between muscular traits and specific ecological features and/or phylogeny is still far from being clear. Hence, musculature of additional taxa, representing the systematic width and occupying a diverse range of habitats, should be investigated.     

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.     

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.     

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.     

Histology and Ultrastructure of the Body Wall in the Phoronid Phoronopsis harmeri

The histology and ultrastructure of the body wall in Phoronopsis harmeriwere studied using light microscopy and TEM. The ectoderm epithelium of tentacles, anterior body region, and ampulla consists of monociliary cells. Gram-negative bacteria were found between microvilli, in the protocuticle of the anterior region, and in the ampulla. The epithelium of the posterior body region lacks both monociliary cells and bacteria. The bundles of nerve fibers run between the layer of epithelial cells and basal membrane. The musculature of the body wall comprises circular and longitudinal muscles. The circular muscle fibers are applied to the basal membrane and constitute a solid layer extending almost throughout the length of the body. This pattern is broken in the posterior body region, where there is no solid layer of circular musculature, and the latter is arranged in isolated muscle bands. In the ampullar (terminal) body region, the inversion of circular and longitudinal muscle layers takes place, so that the latter appears to be pressed against the basal membrane. The apical surfaces of longitudinal muscle cells bear cytoplasmic processes; some of the cells have a flagellum. The basal portion of the longitudinal muscle cells forms a cytoplasmic process containing bundles of tonofilaments. The processes of all cells making up the muscle bands are interwoven and anchored to the basal membrane.     

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.     

Development and embryonic pattern of body wall musculature in the crassiclitellate Eisenia andrei (Annelida,Clitellata)

During early development of Eisenia andrei (Crassiclitellata), a loose arrangement of primary circular and longitudinal muscles encloses the whole embryo. Circular muscles differentiate in an anterior–posterior progression creating a segmental pattern. Primary circular muscles emerge at the segmental borders while later in development the central part of each segment is filled with circular strands. Longitudinal muscles develop in an anterio‐posterior manner as well, but by continuous lengthening. Muscle growth is not restricted by segmental boundaries. The development begins with one pair of prominent longitudinal muscles differentiating ventrally along the right and the left germ band. These first muscles provide a guiding structure for the parallel organization of the afterwards differentiating longitudinal musculature. Additional primary longitudinal muscles emerge and form, together with the initial circular muscles, the primary muscle grid of the embryo. During the following development, secondary longitudinal muscle strands develop and integrate themselves into the primary grid. Meanwhile the primary circular muscles split into thin strands in a ventral to dorsal progression. Thus, a fine structured mesh of circular and longitudinal muscles is generated. Compared to other “Oligochaeta”, embryonic muscle patterns in E. andrei are adapted to the development of a lecithotrophic embryo. Nevertheless, two general characteristics of annelid muscle development become evident. The first is the segmental development of the circular muscles from a set of initial muscles situated at the segment borders. Second, there is a continuous development of primary longitudinal muscles starting at the anterior pole. At least one pair of main primary longitudinal strands is characteristic in Annelida. The space between all primary strands is filled with secondary longitudinal strands during further development. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Muscular system in polychaetes (Annelida)

The structure of the polychaete muscular system is reviewed. The muscular system comprises the muscles of the body wall, the musculature of the parapodial complex and the muscle system of the dissepiments and mesenteries. Various types of organisation of the longitudinal and circular components of the muscular body wall are distinguished. In Opheliidae, Polygordiidae, Protodrilidae, Spionidae, Oweniidae, Aphroditidae, Acoetidae (=Polyodontidae), Polynoidae, Sigalonidae, Phyllodocidae, Nephtyidae, Pisionidae, and Nerillidae circular muscles are lacking. It is hypothesised that the absence of circular muscles represents the plesiomorphic state in Annelida. This view contradicts the widely accepted idea of an earthworm-like musculature of the body wall comprising an outer layer of circular and an inner layer of longitudinal fibres. A classification of the various types of parapodial muscle construction has been developed. Massive and less manoeuvrable parapodia composed of many components like those of Aphrodita are regarded to represent the plesiomorphic state in recent polychaetes. An analysis of the diversity of the muscular structure supports the hypothesis that the primary mode of life in polychaetes was epibenthic and the parapodial chaetae had a protective function.     

Comparative morphology of the somatic musculature in species of Hexarthra and Polyarthra (Rotifera, Monogononta): Its function in appendage movement and escape behavior

Species of Hexarthra and Polyarthra are freshwater rotifers with well-known escape behaviors that result from interactions with planktonic predators. Both rotifers bear a suite of mobile appendages that function in evasive maneuvers and saltatory jumps through the water column, but the anatomical and functional bases of these actions are poorly understood. Here, we use a combination of phalloidin staining, confocal laser scanning microscopy, and video analysis to describe the morphology of the somatic muscles that supply the mobile appendages in order to understand how they function in escape behavior. Results show that species of Hexarthra, which bear six radially distributed limbs, possess a highly complex trunk musculature that supplies the inside of each limb with its own abductor and adductor muscles, i.e., a direct muscle supply. The singular dorsal and ventral limbs each receive a pair of large abductor and adductor muscles (four muscles total per limb), while the paired dorsolateral and ventrolateral limbs each receives three muscles (two abductors, one adductor per limb). Contraction of the abductor muscles creates a power stroke in the form of an anterior sweep of the limbs, which leads to a three-dimensional tumbling of the rotifer through the water column. Alternatively, species of Polyarthra possess 12 blade-like appendages that are arranged into four equal bundles; each bundle receives an indirect muscle supply that attaches to the shoulder of the paddles. A single longitudinal paddle muscle supplies each dorsolateral bundle, while a pair of longitudinal paddle muscles supplies each ventrolateral bundle. Contraction of these muscles, whether singly or in concert, functions to abduct the paddles in a power stroke, leading to rotation of the body and movement of the rotifer. The recovery stroke is hypothesized to be a multi-step process that begins with reorientation of the appendages prior to adduction, followed by contraction of various muscles to antagonize the paddle muscles. In total, these observations reveal novel complexities in the rotifer muscular system that aids our understanding of the biophysics of predator avoidance in appendage-bearing rotifers.     

Non-oblique myofilament arrangement in the dorsal muscle of Sabellastarte magnifica

Summary As described in other invertebrate muscles, most thick myofilaments of the dorsal longitudinal muscle of Sabellastarte magnifica appear to be obliquely arranged with respect to the longitudinal axis of the myofibrils. Some sections, however, show long bundles of myofilaments parallel to the longitudinal axis of the myofibrils. Since the oblique striation concept cannot account for such images, a different (longitudinal) arrangement is proposed for Sabellastarte which can account for all observed images. A wax and threads model built according to this arrangement has been used to demonstrate that by oblique sectioning the longitudinal model can generate a false appearance of filament obliquity.Part of this study has been presented at the Eighty Third Annual Session of the American Association of Anatomists. Supported by Grant # NS-07464 from the NINDS. The author wishes to thank Mrs. Graciela G. Aguilar for her multiple and valuable assistance, and Mr. Faustino McKenzie for collecting the specimens.     

Architecture of the nervous system in two Dactylopodola species (Gastrotricha, Macrodasyida)

Immunohistochemical stainings have become standard tools to describe the nervous system, but usually only singular or few markers are used and consequently show only subsets of neurons within the nervous system. We investigated two species of Dactylopodola (Gastrotricha, Macrodasyida) with a broad set and combination of markers, to represent the nervous system in a more holistic approach. We suggest that markers for both neurotubuli (tubulin) and neurotransmitters (e.g. serotonin, FMRF-amides, histamine) should be used. Combinations with markers for the musculature (phalloidin) and nuclei (propidiumiodide or other markers) help to reveal spatial patterns and when used with TEM can provide a more precise picture of the spatial relationships of particular nerves. Species of Dactylopodola have a brain consisting of a solid dorsal commissure and a fine ventral commissure. Cell somata of brain cells are arranged lateral to the dorsal commissure and form a dumbbell-like brain. Additionally, projections into the head region, head sensory organs, one pair of lateroventral nerve cords with three commissures and stomatogastric nerves are described. Obviously, some longitudinal transmitter-specific fibres run in parallel to the main longitudinal nerve and represent additional longitudinal fibres. In comparison with the nervous system architecture of other gastrotrich species and that of different bilaterian animals it is speculated that the gastrotrich nervous system retains several ancestral features, such as being commissural and not a compact brain.     

Functional morphology of the muscles in Philodina sp. (Rotifera: Bdelloidea)

Whole-mounts of Philodina sp., a bdelloid rotifer, were stained with fluorescent-labeled phalloidin to visualize the musculature. Several different muscle types were identified including incomplete circular bands, coronal retractors and foot retractors. Based on the position of the larger muscle bands in the body wall, their function during creeping locomotion and tun formation was inferred. Bdelloid creeping begins with the contraction of incomplete circular muscle bands against the hydrostatic pseudocoel, resulting in an anterior elongation of the body. One or more sets of ventral longitudinal muscles then contract bringing the rostrum into contact with the substrate, where it presumably attaches via adhesive glands. Different sets of ventral longitudinal muscles, foot and trunk retractors, function to pull the body forward. These same longitudinal muscle sets are also used in `tun' formation, in which the head and foot are withdrawn into the body. Three sets of longitudinal muscles supply the head region (anterior head segments) and function in withdrawal of the corona and rostrum. Two additional pairs of longitudinal muscles function to retract the anterior trunk segments immediately behind the head, and approximately five sets of longitudinal retractors are involved in the withdrawal of the foot and posterior toes. To achieve a greater understanding of rotifer behavior, it is important to elucidate the structural complexity of body wall muscles in rotifers. The utility of fluorescently-labeled phalloidin for the visualization of these muscles is discussed and placed in the context of rotifer functional morphology.     

Musculature of two bdelloid rotifers, Adineta ricciae and Macrotrachela quadricornifera: organization in a functional and evolutionary perspective

Organization of muscles in microinvertebrates has often been studied to answer functional questions and understand phylogenetic relationships among taxa. In this study, the musculature of two bdelloid species, Adineta ricciae and Macrotrachela quadricornifera , was illustrated, and their organization was compared with other rotifer taxa to generate possible hypotheses of evolutionary relationships among Rotifera. The two species share a common habitat but differ from each other in feeding and locomotion. A. ricciae feeds on the biofilm by scraping it, is unable to swim, and slides on the head cilia using the foot to propel over the substratum. M. quadricornifera feeds by filtration, can swim, and advances by looping in a leech-like motion. Their musculature, stained with TRITC-phalloidin, was observed using confocal laser scanning microscopy. Major differences between the two species were observed in the muscles of head and foot, possibly reflecting differences in their life style. Muscles of the trunk were similarly arranged: circular muscles surrounded longitudinal bands, which were inserted at different points on the body wall. In both bdelloids, circular muscles of the trunk were incomplete ventrally, a condition also present in Seison and in soft-bodied monogononts from benthos. Within rotifers, circular muscles in the form of complete rings are present in acanthocephalans and in soft-bodied planktonic monogononts but are absent in loricate monogononts, which generally possess dorsoventral bands. The diversity of muscle organization among rotifers was interpreted and discussed.