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Ultrastructure of obliquely striated annelid muscle fibers. (A) Lumbricillus bülowi (Enchytraeidae, Clitellata). Cross section of ribbon-shaped longitudinal fiber showing typical arrangement of sarcoplasmic reticulum (sr), z-rods (z), and myofilaments. (B) Stygocapitella subterranea (Parergodrilidae). Enlargement showing thick (arrows) and thin (arrowheads) myofilaments between sarcoplasmic reticulum (sr) and z-rods (z). (C) Microphthalmus carolinensis (Hesionidae, Phyllodocida). Longitudinal section of two fibers: on the left typical double oblique striation pattern (x–y plane), on the right seemingly cross-striation pattern (x–z plane). sr, sarcoplasmic reticulum; z, z-rod. (D and E) Nerilla antennata (Nerillidae, Phyllodocida). Hirudinean type of muscle fibers with central sarcoplasm containing nucleus (n) and mitochondria (m). Longitudinal fibers of body wall. (D) Cross section. (E) Longitudinal section (x–z plane); arrowheads mark plasma membrane of single fiber. C courtesy of Prof. W. Westheide, Osnabrück.
Source publication
A body wall musculature comprising an outer layer of circular fibers and an inner layer of longitudinal fibers is generally
seen as the basic plan in Annelida. Additional muscles may be present such as oblique, parapodial, chaetal, and dorsoventral
muscles. The longitudinal muscle fibers do not form a continuous layer but are arranged in distinct b...
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... of annelid muscle fibers is compara- tively well known and has been reviewed by Lanzavecchia and others (1988) and Gardiner (1992), so only a brief summary will be given here. As a rule annelid muscle fibers are of the double obli- quely striated type ( Fig. 6A-E). These fibers can be viewed as cross-striated fibers having an extremely reg- ular structure. The striation angle depends on the degree of contraction and usually is between 2 and 15 ( Fig. 6C; Lanzavecchia and others 1988). Each fiber has only one nucleus. The obliquity occurs because filaments at corresponding positions in the ...
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... (1988) and Gardiner (1992), so only a brief summary will be given here. As a rule annelid muscle fibers are of the double obli- quely striated type ( Fig. 6A-E). These fibers can be viewed as cross-striated fibers having an extremely reg- ular structure. The striation angle depends on the degree of contraction and usually is between 2 and 15 ( Fig. 6C; Lanzavecchia and others 1988). Each fiber has only one nucleus. The obliquity occurs because filaments at corresponding positions in the fibers are longitudinally staggered by a fraction of their length. This pattern is only visible in longitudinal sections following the longer axis of the fiber, commonly called the x-z plane. If a ...
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... filaments at corresponding positions in the fibers are longitudinally staggered by a fraction of their length. This pattern is only visible in longitudinal sections following the longer axis of the fiber, commonly called the x-z plane. If a section is slightly tilted with respect to this axis the double oblique striation pattern becomes obvious (Fig. 6C). This pattern is due to a helical arrangement of the functional units, the so- called sarcomeres, which extend only to about half of the width of a fiber. In cross sections (so-called x-y plane) each fiber appears to be divided into radial fields consisting of groups of myofilaments separated by z-rods and sarcoplasmic reticulum ( Fig. ...
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... (Fig. 6C). This pattern is due to a helical arrangement of the functional units, the so- called sarcomeres, which extend only to about half of the width of a fiber. In cross sections (so-called x-y plane) each fiber appears to be divided into radial fields consisting of groups of myofilaments separated by z-rods and sarcoplasmic reticulum ( Fig. 6A and B). Each field consists of an area with only actin filaments, followed by a zone containing actin and myosin fila- ments and a central area with only myosin filaments (Fig. 6B). Depending on contraction of the fiber, the width of each zone varies. Due to the helical arrange- ment of the contractile elements, there is one plane in ...
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... sections (so-called x-y plane) each fiber appears to be divided into radial fields consisting of groups of myofilaments separated by z-rods and sarcoplasmic reticulum ( Fig. 6A and B). Each field consists of an area with only actin filaments, followed by a zone containing actin and myosin fila- ments and a central area with only myosin filaments (Fig. 6B). Depending on contraction of the fiber, the width of each zone varies. Due to the helical arrange- ment of the contractile elements, there is one plane in longitudinal sections in which the fibers appear as cross- striated fibers ( Fig. 6C and E). This plane, called the y-z plane, usually represents the narrow axis of a muscle cell. ...
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... followed by a zone containing actin and myosin fila- ments and a central area with only myosin filaments (Fig. 6B). Depending on contraction of the fiber, the width of each zone varies. Due to the helical arrange- ment of the contractile elements, there is one plane in longitudinal sections in which the fibers appear as cross- striated fibers ( Fig. 6C and E). This plane, called the y-z plane, usually represents the narrow axis of a muscle cell. Mostly the muscle fibers are of this so-called flat- tened or ribbon-shaped type. It is further characterized by separation of the contractile and non-contractile part in that the sarcoplasm containing nucleus, mitochon- dria, and glycogen is found ...
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... cross-striated muscles ( Lanzavecchia and others 1988;Gardiner 1992). Among the helical fibers two types can be dis- tinguished: those having no central cytoplasmic axis and those with a central cytoplasmic axis. In the latter, the nucleus and mitochondria are positioned centrally and the whole contractile apparatus is located periph- erally ( Fig. 6D and E). Such fibers are thought to repre- sent an autapomorphy of Hirudinea but are present in certain taxa of polychaetes as well. For instance, in Nerillidae the musculature of the body wall is also composed of such fibers, which in all probability evolved convergently in this taxon of meiofaunal poly- chaetes with uncertain affinities. ...
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Citations
... diagonal ones in platyhelminthes [32]. There may be exceptions to this generalization; for example, nematodes [33], nematomorphs [34], tardigrades [35,36], arthropods [2,37] and some polychaete annelids [38] lack body-wall circular muscles, most likely due to secondary loss. The Early Cambrian lobopodian Tritonychus has an outer layer of longitudinal, a middle layer of oblique and an inner layer of circular muscles [21]. ...
Cycloneuralians are ecdysozoans with a fossil record extending to the Early Cambrian Fortunian Age and represented mostly by cuticular integuments. However, internal anatomies of Fortunian cycloneuralians are virtually unknown, hampering our understanding of their functional morphology and phylogenetic relationships. Here we report the exceptional preservation of cycloneuralian introvert musculature in Fortunian rocks of South China. The musculature consists of an introvert body-wall muscular grid of four circular and 36 radially arranged longitudinal muscle bundles, as well as an introvert circular muscle associated with 19 roughly radially arranged, short retractors. Collectively, these features support at least a scalidophoran affinity, and the absence of muscles associated with a mouth cone and scalids further indicates a priapulan affinity. As in modern scalidophorans, the fossil musculature, and particularly the introvert circular muscle retractors, may have controlled introvert inversion and facilitated locomotion and feeding. This work supports the evolution of scalidophoran-like or priapulan-like introvert musculature in cycloneuralians at the beginning of the Cambrian Period.
... connectome. These communities correspond to anatomical territories in the larval body 234 ( 2014; Purschke and Müller, 2006). The other modules include parapodial muscles and 241 chaetal sac cells, in the segmental parapodia. ...
Cells form networks in animal tissues through synaptic, chemical and adhesive links. Invertebrate muscle cells often connect to other cells through desmosomes, adhesive junctions anchored by intermediate filaments. To study desmosomal networks, we skeletonised 853 muscle cells and their desmosomal partners in volume electron microscopy data covering an entire larva of the annelid Platynereis . Muscle cells adhere to each other, to epithelial, glial, ciliated, and bristle-producing cells and to the basal lamina, forming a desmosomal connectome of over 2,000 cells. The aciculae - chitin rods that form an endoskeleton in the segmental appendages - are highly connected hubs in this network. This agrees with the many degrees of freedom of their movement, as revealed by video microscopy. Mapping motoneuron synapses to the desmosomal connectome allowed us to infer the extent of tissue influenced by motoneurons. Our work shows how cellular-level maps of synaptic and adherent force networks can elucidate body mechanics.
... Vertebrate skeletal muscles consist of multiple fibres of multinucleate striated myotubes, full of actin-myosin cytoskeletal fibrils that constitute the primary protein reservoir in the body. While it has been found that some arthropods also form multinucleate cells [1, 25,122], many other invertebrates have been found with only mononucleate muscle cells, including cnidarians [109], nematodes [82], annelids [100,130], molluscs [44,59,89], and the invertebrate chordates amphioxus [31]. Interestingly, members of the other invertebrate chordate lineage, tunicates, have been found to have multinucleate muscle cells in the adult body wall musculature, which arise via myoblast fusion [102]. ...
Background
The formation and functioning of muscles are fundamental aspects of animal biology, and the evolution of ‘muscle genes’ is central to our understanding of this tissue. Feeding-fasting-refeeding experiments have been widely used to assess muscle cellular and metabolic responses to nutrition. Though these studies have focused on vertebrate models and only a few invertebrate systems, they have found similar processes are involved in muscle degradation and maintenance. Motivation for these studies stems from interest in diseases whose pathologies involve muscle atrophy, a symptom also triggered by fasting, as well as commercial interest in the muscle mass of animals kept for consumption. Experimentally modelling atrophy by manipulating nutritional state causes muscle mass to be depleted during starvation and replenished with refeeding so that the genetic mechanisms controlling muscle growth and degradation can be understood.
Results
Using amphioxus, the earliest branching chordate lineage, we address the gap in previous work stemming from comparisons between distantly related vertebrate and invertebrate models. Our amphioxus feeding-fasting-refeeding muscle transcriptomes reveal a highly conserved myogenic program and that the pro-orthologues of many vertebrate myoblast fusion genes were present in the ancestral chordate, despite these invertebrate chordates having unfused mononucleate myocytes. We found that genes differentially expressed between fed and fasted amphioxus were orthologous to the genes that respond to nutritional state in vertebrates. This response is driven in a large part by the highly conserved IGF/Akt/FOXO pathway, where depleted nutrient levels result in activation of FOXO, a transcription factor with many autophagy-related gene targets.
Conclusion
Reconstruction of these gene networks and pathways in amphioxus muscle provides a key point of comparison between the distantly related groups assessed thus far, significantly refining the reconstruction of the ancestral state for chordate myoblast fusion genes and identifying the extensive role of duplicated genes in the IGF/Akt/FOXO pathway across animals. Our study elucidates the evolutionary trajectory of muscle genes as they relate to the increased complexity of vertebrate muscles and muscle development.
... In addition to reduced weight, the loss of some muscle groups (e.g. the interacicular muscles as documented here) [10,42] may increase parapodial flexibility. Increased parapodial flexibility may be further supported by the development of other muscle groups such as elongated parapodial musculature [5,9,12,59]. Notably, these features are repeated in each segment and thus provide an overall reduction of weight and density of the entire animal. ...
Annelids are predominantly found along with the seafloor, but over time have colonized a vast diversity of habitats, such as the water column, where different modes of locomotion are necessary. Yet, little is known about their potential muscular adaptation to the continuous swimming behaviour required in the water column. The musculature and motility were examined for five scale worm species of Polynoidae (Aphroditiformia, Annelida) found in shallow waters, deep sea or caves and which exhibit crawling, occasional swimming or continuous swimming, respectively. Their parapodial musculature was reconstructed using microCT and computational three-dimensional analyses, and the muscular functions were interpreted from video recordings of their locomotion. Since most benthic scale worms are able to swim for short distances using body and parapodial muscle movements, suitable musculature for swimming is already present. Our results indicate that rather than rearrangements or addition of muscles, a shift to a pelagic lifestyle is mainly accompanied by structural loss of muscle bundles and density, as well as elongation of extrinsic dorsal and ventral parapodial muscles. Our study documents clear differences in locomotion and musculature among closely related annelids with different lifestyles as well as points to myoanatomical adaptations for accessing the water column.
... As for the musculature, it has been repeatedly shown that many annelid groups (including several Phyllodocida) do not possess circular muscles. In these cases, their bracing function is carried out by the socalled bracing muscles that are associated with the parapodial muscle complex (Purschke & Müller, 2006;Tzetlin et al., 2002;Tzetlin & Filippova, 2005). In syllids, supralongitudinal transverse muscle fibers have been found in specimens of Myrianida prolifera (Müller, 1778) and Typosyllis antoni Filippova et al., 2010). ...
The sponge‐dwelling Syllidae Ramisyllis multicaudata and Syllis ramosa are the only annelid species for which a branched body with one head and multiple posterior ends is known. In these species, the head is located deep within the sponge, and the branches extend through the canal system of their host. The morphology of these creatures has captivated annelid biologists since they were first discovered in the late XIXth century, and their external characteristics have been well documented. However, how their branched bodies fit within their symbiotic host sponges and how branches translate into internal anatomy has not been documented before. These features are crucially relevant for understanding the body of these animals and, therefore, the aim of this study was to investigate these aspects. In order to assess these questions, live observation, as wells as histology, immunohistochemistry, micro‐computed tomography, and transmission electron microscopy techniques were used on specimens of R. multicaudata. By using these techniques, we show that the complex body of R. multicaudata specimens extends greatly through the canal system of their host sponges. We demonstrate that iterative external bifurcation of the body is accompanied by the bifurcation of the longitudinal organ systems that are characteristic of annelids. Additionally, we also highlight that the bifurcation process leaves an unmistakable fingerprint in the form of newly‐described “muscles bridges”. These structures theoretically allow one to distinguish original and derived branches at each bifurcation. Last, we characterize some of the internal anatomical features of the stolons (reproductive units) of R. multicaudata, particularly their nervous system. Here, we provide the first study of the internal anatomy of a branched annelid. This information is not only crucial to deepen our understanding of these animals and their biology, but it will also be key to inform future studies that try to explain how this morphology evolved.
... The most important and conspicuous muscles in the general annelid body plan are those of the body wall (Tzetlin and Filippova 2005;Purschke and Mü ller 2006). Body wall musculature comprises an outer layer of circular muscles and an inner layer of longitudinal muscle (Figure 10.2C). ...
... Traditionally, most descriptions of annelid muscular systems resulted from dissections, histological sectioning, and transmission electron microscopy; more recently, the combination of labeling F-actin with fluorescently labeled phalloidin, laser scanning confocal microscopy, and three-dimensional reconstruction has allowed more sensitive detection of finer muscle fibers and a better understanding of annelid muscular structure (Purschke and Mü ller 2006). ...
Axial regeneration, the ability to regrow structures along the main body axis, is a widespread, likely ancestral trait of metazoans. Despite its assumed adaptive value, regenerative ability has decreased, disappeared and regained many times during the evolution animal lineages. In most animals capable of regeneration, the process comprises three main stages: wound healing, cell reorganization and morphogenesis of new replacement structures through redeployment of embryonic developmental pathways. Annelids are unique among segmented animals in that many species can regenerate both anterior and posterior ends, rebuilding both terminal, non-segmental tissues and segmental units. Annelid regeneration goes through five main stages: wound healing, blastema formation, blastema differentiation, resegmentation and growth. These five stages involve processes of wound repair, cell migration and proliferation, regeneration of neural and muscular tissues, and generation of segmental structures, along with reorganization of existing tissues to relieve size and position mismatches between the regenerated structures and the stump. During axial regeneration, tissues from non-segmental caps (anterior prostomium or posterior pygidium) form first, and then segmental units are intercalated between the new cap and the stump. Segment formation during anterior regeneration likely involves formation of a transient segment addition zone. In contrast, during posterior regeneration, a new and persistent posterior segment addition zone forms between the pygidium and the stump. While current knowledge of the developmental mechanics and genetics involved in annelid regeneration is still scant, existing data suggest that the process involves redeployment of developmental pathways related to stem and germline cells maintenance, embryogenesis and juvenile/adult posterior growth.
... For similar reasons, the ventromedian longitudinal muscle and ventrolateral longitudinal muscle are structurally important for flexible movement, so the ventromedian longitudinal muscle is located at the center, and several ventrolateral longitudinal muscles are located on each side for support ( Figure 2C-F). Unlike polychaete, which has distinct longitudinal muscles and weakly developed transverse muscles (circular muscles) or even absent [14,15], it was shown that the longitudinal and transverse muscles of Urechis unicinctus were developed simultaneously from anterior to posterior body axis. This may be correlated with the lifestyle of U. unicinctus living in U-shape burrows of soft sediments, whereas polychaeta taxa use their parapodia for walking and swimming, thus selecting for longitudinal musculature rather than circular muscle. ...
Echiura is one of the most intriguing major subgroups of phylum Annelida because, unlike most other annelids, echiuran adults lack metameric body segmentation. Urechis unicinctus lives in U-shape burrows of soft sediments. Little is known about the molecular mechanisms underlying the development of U. unicinctus. Herein, we overviewed the developmental process from zygote to juvenile U. unicinctus using immunohistochemistry and F-actin staining for the nervous and muscular systems, respectively. Through F-actin staining, we found that muscle fibers began to form in the trochophore phase and that muscles for feeding were produced first. Subsequently, in the segmentation larval stage, the transversal muscle was formed in the shape of a ring in an anterior-to-posterior direction with segment formation, as well as a ventromedian muscle for the formation of a ventral nerve cord. After that, many muscle fibers were produced along the entire body and formed the worm-shaped larva. Finally, we investigated the spatiotemporal expression of Uun_st-mhc, Uun_troponin I, Uun_calponin, and Uun_twist genes found in U. unicinctus. During embryonic development, the striated and smooth muscle genes were co-expressed in the same region. However, the adult body wall muscles showed differential gene expression of each muscle layer. The results of this study will provide the basis for the understanding of muscle differentiation in Echiura.
... The poorly developed circular layer seen in these studies agrees with previous works on other annelid families. However, some areas of the body wall of S. gracilis appear to have a thin layer of this kind of muscular fibres, which is more evident in HIS images than in micro-CT ones, thus confirming once again what has been previously reported for other syllid species (Mettam, 1967;Mettam, 1971;Tzetlin & Filippova, 2005;Purschke & Müller, 2006;Filippova et al., 2010;Helm et al., 2013;Helm & Capa, 2015). Muscular fibres of the parapodial muscle complex are always well developed in vagile parapodial bearing species, as is the case of S. gracilis, and consists of numerous muscles associated with the parapodial wall, intrinsic muscles located inside the parapodium, diagonal and transversal (circular) muscles and muscles attached to aciculae and compound chaetae (Storch, 1968;Tzetlin & Filippova, 2005;Helm et al., 2013;Helm & Capa, 2015). ...
Background. The overall anatomy of the genus Syllis (Annelida: Syllidae) has been largely studied; however, an integrative approach considering different anatomical techniques has never been considered. Here, we use micro-computed X-ray tomography (micro-CT) to examine the internal anatomy of Syllis gracilis Grube, 1840, along with other widely available techniques.
Methods. We studied the anatomy of the marine annelid S. gracilis through an integrative approach, including micro-CT along with stereo and light compound microscopy (STM, LCM), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and histological sectioning (HIS). In this manner, we evaluated the applicability of micro-CT for the examination of annelid anatomy by testing whether the images obtained make it possible to visualize the main body structures, in comparison with other current techniques, of the various elements of its internal anatomy.
Results. Overall external and internal body elements are clearly shown by the integrative
use of all techniques, thus overcoming the limitations of each when studied separately.
Any given method shows disparate results, depending on the body part considered.
For instance, micro-CT provided good images of the external anatomy, including relevant characters such as the shape, length and number of articles of dorsal parapodial cirri. However, it is especially useful for the examination of internal anatomy, thus allowing for 3D visualization of the natural spatial arrangement of the different organs. The features best visualized are those of higher tissue density (i.e., body musculature, anterior parts of the digestive tract), particularly in 3D images of unstained specimens, whereas less electrodense tissues (i.e., the peritoneal lining of septa and nervous system) are less clearly visualized. The use of iodine stain with micro-CT has shown advantages against non-staining for the adequate observation of delicate elements of low density, such as the segmental organs, the connective between the ganglia, the ventral nerve cord and segmental nerves.
Discussion. Main external anatomical elements of S. gracilis are well shown with micro-CT, but images show lesser optical resolution and contrast when compared to micrographs provided by SEM and CLSM, especially for fine structural features of chaetae. Comparison of micro-CT and HIS images revealed the utility and reliability of the former to show the presence, shape and spatial disposition of most internal body organs; the resolution of micro-CT images at a cellular level is, however, much lower than that of HIS, which makes both techniques complementary.
... Rieger and Ladurner (2001) proposed that the body wall organization in the vermiform bilaterian stem species had an outer circular, a diagonal, and an inner longitudinal muscle layer (OCM, DM, ILM) (Fig. 7b). This arrangement of body wall muscle layers has been described in different worm-shaped invertebrates including Acoelomorpha (Hooge 2001), Platyhelminthes (Hooge 2001;Tyler and Hooge 2004;Bolanõs and Litvaitis 2009), Nemertea (Chernyshev 2010), Annelida (Tzetlin and Filippova 2005;Purschke and Müller 2006), Mollusca (Scherholz et al. 2015), and Phoronida (Chernyshev and Temereva 2010). This arrangement is likely to be plesiomorphic for nemerteans as well as other vermiform bilaterians (Hooge 2001;Chernyshev 2010). ...
Knowledge on the detailed body plan in Palaeonemertea (a group putatively considered to retain ancestral character states within Nemertea) is essential in understanding the morphological evolution among not only nemerteans but also the rest of bilaterians in general; however, such information has been scanty in previous literature. In this study, we examined the body wall musculature, the position of the nervous system, and the musculature of the proboscis apparatus in 14 species of Archinemertea (a palaeonemertean clade comprised of Cephalotrichidae and Cephalotrichellidae) using confocal laser scanning microscopy with phalloidin labeling as well as classical Mallory’s trichrome staining. Unexpectedly, all the archine-merteans possessed a diagonal muscle layer lying directly ‘above’—instead of ‘below’, as usually found in most vermiform animals—the body wall outer circular muscle layer, a character state that has never been reported among Bilateria except for some acoels and proseriates. We speculate that the aberrant position of the outer diagonal muscle layer might have been actualized as a result of a flip-flop in a genetic regulatory circuit that controls the body wall muscular morphogenesis, which must have happened independently among the bilaterian evolution at least once in Acoela (Xenacoelomorpha), Proseriata (Platyhelminthes), and Archinemertea (Nemertea), respectively. Based on available phylogenetic data and the results from the present observation, we reconstructed the evolution of morphological characters within Archinemertea and its subclades. The outer diagonal muscle layer is likely a synapomorphy for Archinemertea (Cephalotrichidae + Cephalotrichellidae). Members of Balionemertes and Cephalotrichella possess some unique states for palaeonemerteans in muscular morphology (muscular strand associated with lateral nerve cord; longitudinal muscles between brain and extraganglionic tissue; proboscis with outer longitudinal muscle strands). We propose that muscle fibers embedded in lateral nerve cords, termed myofibrillae, in hoplo- and heteronemerteans are homologous with the body wall longitudinal muscles near the lateral nerves surrounded by circular muscles.
... The recent comparative studies on the segmental musculature in annelids revealed that the absence of segmental circular musculature is much more widely distributed than previously mentioned (Filippova et al. , 2006(Filippova et al. , 2010Purschke and Müller 2006). Among the annelid families studied in this research, the absence of the circular muscle elements in the body segments was demonstrated for Phyllodocidae and Polynoidae (Ivanov and Tzetlin 1997;Storch 1968). ...
The annelid body can be subdivided into three main regions: the prostomium, the body segments, and the pygidium. The prostomium and pygidium, originating from the episphere and the posterior part of the hyposphere of a trochophore larva, are usually mentioned as non-homologous to body segments. However, recent studies revealed that the pygidium of several annelid species is far more complex than previously mentioned and possesses some segment-like features. To assess the diversity of a pygidial organization, I describe the innervation and muscular system of the pygidium in 19 annelid species belonging to the order Phyllodocida using phalloidin labeling, immunohistochemistry and confocal scanning microscopy. The musculature of the pygidium varies between families and usually consists of the circular and/or horseshoe-shaped hindgut sphincter muscles. In several families it can be accompanied by small additional transversal or dorso-ventral muscles. In contrast to the variable musculature, the pygidial innervation is far more uniform and in general comprises two huge main longitudinal nerves, a terminal commissure between them, and paired circumpigidial nerves. The pygidial epithelium bears numerous receptor cell endings, suggesting that the pygidium may act as an important sensory organ. The obtained results are in accordance with the recent data and indicate that such muscular and nervous organization may be characteristic of the whole order Phyllodocida.
