Reconstruction of the muscle system in Antygomonas sp. (Kinorhyncha,Cyclorhagida) by means of phalloidin labeling and cLSM

In the present investigation the entire muscle system of the cyclorhagid kinorhynch Antygomonas sp. was three-dimensionally reconstructed from whole mounts by means of FITC-phalloidin labeling and confocal scanning microscopy. With this technique, which proved to be especially useful for microscopically small species, we wanted to reinvestigate and supplement the findings obtained by histological and electron microscopical methods. The organization of the major muscle systems can be summarized as follows: 1) All muscle fibers, apart from the intestinal ones, the spine, and the mouth cone muscles, show a cross-striated pattern; 2) Dorsal longitudinal muscle fibers as well as segmentally arranged dorsoventral fibers occur from segment III to XIII; 3) Diagonal muscle fibers are located laterally in segments III to X; 4) Two rings of circular fibers are present in segment II, forming the closing apparatus in Cyclorhagida. Further circular muscles are present in segment I, forming the mouth cone and the eversible introvert, and in the pharyngeal bulb.     

Myoanatomy of Myzostoma cirriferum (Annelida,Myzostomida): Implications for the evolution of the myzostomid body plan

Studies of rare genomic marker systems suggest that Myzostomida are a subgroup of Annelida and phylogenomic analyses indicate an early divergence of this taxon within annelids. However, adult myzostomids show a highly specialized body plan, which lacks typical annelid features, such as external body annulation, coelomic cavities with metanephridia, and segmental ganglia of the nervous system. The putative loss of these features might be due to the parasitic/symbiotic lifestyle of myzostomids associated with echinoderms. In contrast, the larval anatomy and adult locomotory system resemble those of annelids. To clarify whether the myoanatomy of myzostomids reflects their relationship to annelids, we analyzed the distribution of f‐actin, a common component of muscle fibers, in specimens of Myzostoma cirriferum using phalloidin‐rhodamine labeling in conjunction with confocal laser‐scanning microscopy. Our data reveal that the musculature of the myzostomid body comprises an outer circular layer, an inner longitudinal layer, numerous dorsoventral muscles, and prominent muscles of the parapodial complex. These features correspond well with the common organization of the muscular system in Annelida. In contrast to other annelids, however, several elements of the muscular system in M. cirriferum, including the musculature of the body wall, and the parapodial flexor muscles, exhibit radial symmetry overlaying a bilateral body plan. These findings are in line with the annelid affinity of myzostomids and suggest that the apparent partial radial symmetry of M. cirriferum arose secondarily in this species. Based on our data, we provide a scenario on the rearrangements of muscle fibers that might have taken place in the lineage leading to this species. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Nephridial and gonoduct distribution patterns in Nerillidae (Annelida: Polychaeta) examined by tubulin staining and cLSM

The distribution and configuration of nephridia and gonoducts are described for seven species from seven genera of the interstitial polychaete family Nerillidae. The ciliated nephridia and gonoducts were identified by tubulin staining and examined with a confocal laser scanning microscope. The following species of the seven to nine-segmented nerillids were examined: Leptonerilla prospera, Nerilla antennata (nine segments); Nerillidium mediterraneum, Trochonerilla mobilis, Gen. sp. A (eight segments); and Aristonerilla brevis, Paranerilla limicola (seven segments). Two of the examined species are hermaphroditic (N. mediterraneum and Gen. sp. A). Segmented nephridia can be found from the first to the last segment, with a total of two to five pairs. One to three pairs of segmented spermioducts are present in all species. One pair of gonoducts is found in all species, except for P. limicola, where they are absent. Nephridia vary in length from half to almost twice the length of a segment and may be curled up in loops. In A. brevis and P. limicola the nephridia are discontinuously ciliated. The distribution and configuration of spermioducts and gonoducts are also variable, although to a lesser extent. The spermioduct distribution is generally consistent within genera and therefore of systematic significance. Nephridia and gonoducts are never found together in the same segments, and the results indicate that gonoducts and nephridia have developed from the same anlagen. The distribution patterns of nephridia and gonoducts are discussed with respect to segmentation, systematics, and development.     

Morphology of the nervous system of Polychaeta (Annelida)

The article summarizes our up to date knowledge about the morphology of the annelid, especially the polychaete, central and peripheral nervous system. Since the cephalic nervous system was in the focus of controversial discussions for decades, the structure of its neuropile, associated ganglia and nerves is reviewed in detail. The enormous variation of the ventral nerve cord and peripheral nerves is presented as well as a theory how this might have evolved. A ground pattern of the polychaete nervous system is suggested, based on developmental and regeneration studies.     

The musculature of horsehair worm larvae (<Emphasis Type="Italic">Gordius aquaticus</Emphasis>,<Emphasis Type="Italic"> Paragordius varius</Emphasis>, Nematomorpha): F-actin staining and reconstruction by cLSM and TEM

The musculature of larvae of Gordius aquaticus was investigated by laser-scanning microscopy and compared to transmission electron microscopic data for the larva of Paragordius varius. In the anterior portion of the body, the preseptum, four different muscle groups can be distinguished: (1) 12 anterior parietal muscles in the body wall, (2) six oblique muscles that function as retractors of the introvert, (3) six proboscideal muscles, which function as retractors for the proboscis, and (4) six muscles associated with spines of the outermost of the three rings of spines. The posterior portion of the body, the postseptum, possesses four pairs of longitudinal muscle strands in G. aquaticus, the postseptal parietal muscles, that are located dorsolaterally and ventrolaterally. These are not clearly visible in P. varius, where instead three pairs of parietal muscles are present. Additional small muscles are associated with the terminal spines and with the duct running from the pseudointestine to the body wall. All fibers show a cross-striated pattern although this striation is less obvious at the ends of the fibers.     

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.     

The formation of the nervous system during larval development in Triops cancriformis (Bosc) (crustacea,Branchiopoda): An immunohistochemical survey

We provide data of the development of thenervous system during the first five larval stages of Triops cancriformis. We use immunohistochemical labeling (against acetylated α‐tubulin, serotonin, histamine, and FMRFamide), confocal laser scanning microscopy analysis, and 3D‐reconstruction. The development of the nervous system corresponds with the general anamorphic development in T. cancriformis. In larval stage I (L I), all brain parts (proto‐, deuto‐, and tritocerebrum), the circumoral connectives, and the mandibular neuromere are already present. Also, the frontal filaments and the developing nauplius eye are already present. However, until stage L III, the nauplius eye only consists of three cups. Throughout larval development, the protocerebral network differentiates into distinct subdivisions. In the postnaupliar region, additional neuromeres and their commissures emerge in an anteroposterior gradient. The larval nervous system in L V consists of a differentiated protocerebrum including a central body, a nauplius eye comprising four cups, a circumoral nerve ring, mandibular‐ and postnaupliar neuromeres up to the seventh thoracic segment, each featuring an anterior and a posterior commissure, and two parallel connectives. The presence of a protocerebral bridge is questionable. The distribution of neurotransmitters in L I is restricted to the naupliar nervous system. Over the course of the five stages of development, neurotransmitter distribution also follows an anteroposterior gradient. Each neuromere is equipped with two ganglia innervating the locomotional appendages and possesses a specific neurotransmitter distribution pattern. We suggest a correlation between neurotransmitter expression and locomotion. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Homology of prostomial and pharyngeal structures in eunicida (Annelida) based on innervation and morphological similarities

Eunicidan bristle worm families are commonly identified by the shape of their prostomia and pharyngeal structures. However, current hypotheses of homology among these structures are conflicting, making it difficult to assess morphological evolution, reconstruct phylogeny, and produce a stable classification. To generate more consistent hypotheses of homology among eunicidan anterior structures, the author examined the anterior morphology and the nervous system stained with anti‐α‐tubulin and serotonin antibodies in representative species of Eunicidae, Onuphidae, Oenonidae, Dorvilleidae and Lumbrineridae. The shape of the brain varied conspicuously among families; however, it has mostly the same commissures (usually two of the dorsal and five of the ventral roots of the circumoesophageal connective). The stomatogastric system is also conservative in composition, having two main pairs of stomatogastric nerves which vary in their relative position among the different families. Innervation similarities combined with correspondence and topological morphological similarities made it possible to present explicit hypotheses of primary homology of features, such as buccal lips, pharyngeal fold, and dorsolateral fold anterior extension. Buccal lips are present in all families; however, ventral pads on the prostomium of the Dorvillea line of Dorvilleidae are anterior prolongations of the pharyngeal fold and not buccal lips. All examined taxa, except dorvilleid species, have conspicuous dorsolateral fold anterior extension. In Eunicidae, this anterior extension is a transverse band, while in other families it is a pair of folds. Observed similarities also gave insights on the homology of maxillary elements of Dorvilleidae, providing background knowledge for future studies. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

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.