A new genus and species of monostiliferous hoplonemertean (Nemertea: Enopla: Monostilifera) from Japan

A new genus and species of monostiliferous hoplonemertean, Diopsonemertes acanthocephala gen. et sp. nov., is described from Otsuchi Bay, Japan. Significant anatomical features of the new form include a body wall longitudinal musculature anteriorly divided into inner and outer layers by connective tissue, no pre-cerebral septum, the presence of a thin coat of diagonal muscle fibres between the body wall longitudinal and circular muscle layers in the foregut body region, cephalic retractor muscles derived only from the inner portion of the divided longitudinal muscles and a rhynchocoel more than half the body length.     

Pericarp anatomy and evolution inCoriaria (Coriariaceae)

The first overall study of pericarp anatomy ofCoriaria is presented to discuss its evolution and relationships within a genus. All 14 species investigated (including 11 narrowly defined species) have somewhat bilaterally flattened mature fruits with five to seven (or more) longitudinal costae. They share a usually nine-(or more-)cell-layered (at intercostal region), stratified mature pericarp, which is basically constructed by an exocarp, an outer, a middle and an inner zone of mesocarp, and an endocarp. While a multi-layered endocarp is composed of circumferentially elongate fibres, a multi-layered inner zone of the mesocarp comprises longitudinally elongate fibres. Despite its uncertain systematic value, the presence of those fibres arranged crisscross is a characteristic feature of the genus. Comparisons among species indicate thatCoriaria terminalis, a species of the Eastern Hemisphere, retains a basic or archaic, well-stratified pericarp structure similar to the one found in all the species investigated of the Southern and Western Hemisphere, and that four species of Asia,Coriaria napalensis, C. sinica, C. intermedia andC. japonica, share a specialized structure (lacking the outer zone of the mesocarp) indicative of their mutual close affinity. Comparisons further suggest distinctness ofCoriaria intermedia, as well as variously derived position ofC. myrtifolia andC. japonica.     

The neuroactive substances and associated muscle system in Rhipidocotyle campanula (Digenea,Bucephalidae) from the intestine of the pike Esox lucius

We report about the muscular system and the serotonergic and FMRFamidergic components of the nervous system of the Bucephalidae trematode, Rhipidocotyle campanula, an intestinal parasite of the pike. We use immunocytochemical methods and confocal scanning laser microscopy (CLSM). The musculature is identified by histochemical staining with fluorescently labeled phalloidin. The body wall musculature of R. campanula contains three layers of muscle fibres – the outer thin circular, intermediate longitudinal and inner diagonal muscle fibres running in two opposite directions. The digestive system of R. campanula possess of a well-developed musculature: radial, longitudinal and circular muscle elements are detected in the pharynx, circular and longitudinal muscle filaments seen in the oesophagus, and longitudinal and the circular muscle fibres were found in the intestinal wall. Specific staining indicating the presence of actin muscle filaments occurs in the cirrus sac localized in the posterior body region. The frontal region of anterior attachment organ, the rhynchus, in R. campanula is represented by radial muscle fibres. The posterior part of the rhynchus comprise of radial muscles forming the organ's wall, and several strong longitudinal muscle bundles. Serotonergic and FMRFamidergic structures are detected in the central and peripheral compartments of the nervous system of R. campanula, that is, in the paired brain ganglia, the brain commissure, the longitudinal nerve cords, and connective nerve commissures. The innervations of the rhynchus, pharynx, oesophagus and distal regions of the reproductive system by the serotonergic and FMRFamidergic nervous elements are revealed. We compare our findings obtained on R. campanula with related data for other trematodes.     

Body wall muscles in the haplotaxidsHaplotaxis gordioides andPelodrilus leruthi (Annelida,Oligochaeta)

Summary The muscle organization of the body wall in two species of Haplotaxida is described.Haplotaxis gordioides andPelodrilus leruthi show significant differences in the longitudinal muscle layer. The observations suggest that inH. gordioides, the muscle fibres could foreshadow the flat circomyarian organization of microdrile muscles, while theP. leruthi organization recalls the pinnate arrangement of megadrile body wall.     

Myoanatomy and serotonergic nervous system of plumatellid and fredericellid phylactolaemata (lophotrochozoa,ectoprocta)

The phylogenetic position of the Ectoprocta within the Lophotrochozoa is discussed controversially. For gaining more insight into ectoproct relationships and comparing it with other potentially related phyla, we analysed the myoanatomy and serotonergic nervous system of adult representatives of the Phylactolaemata (Plumatella emarginata, Plumatellavaihiriae, Plumatella fungosa, Fredericella sultana). The bodywall contains a mesh of circular and longitudinal muscles. On its distal end, the orifice possesses a prominent sphincter and continues into the vestibular wall, which has longitudinal and circular musculature. The tentacle sheath carries mostly longitudinal muscle fibres in Plumatella sp., whereas F. sultana also possesses regular circular muscle fibres. Three groups of muscles are associated with the lophophore: 1) Lophophoral arm muscles (missing in Fredericella), 2) epistome musculature and 3) tentacle musculature. The epistome flap is encompassed by smooth muscle fibres. A few fibres extend medially over the ganglion to its proximal floor. Abfrontal tentacle muscles have diagonally arranged muscle fibres in their proximal region, whereas the distal region is formed by a stack of muscles that resemble an inverted ‘V’. Frontal tentacle muscles show more variation and either possess one or two bases. The digestive tract possesses circular musculature which is striated except at the intestine where it is composed of smooth muscle fibres. The serotonergic nervous system is concentrated in the cerebral ganglion. From the latter a serotonergic nerve extends to each tentacle base. In Plumatella the inner row of tentacles at the lophophoral concavity lacks serotonergic nerves. Bodywall musculature is a common feature in many lophotrochozoan phyla, but among other filter feeders like the Ectoprocta is only present in the ‘lophophorate’ Phoronida. The longitudinal tentacle musculature is reminiscent of the condition found in phoronids and brachiopods, but differs to entoproct tentacles. Although this study shows some support for the ‘Lophophorata’, more comparative analyses of possibly related phyla are required. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

Muscle differentiation in blackspot seabream (Pagellus bogaraveo, Brunnich): histochemical and immunohistochemical study of the fibre types

The aim of this work was to gain insights into the mechanism of muscle differentiation and growth in Pagellus bogaraveo, by studying muscle fibre phenotypes identified by immunohistochemistry. At hatching, several layers of deep fast-white fibres were covered by a superficial fibre monolayer. At 5 days, slow-red fibres appeared near the lateral line nerve. At 40 days, the intermediate-pink muscle became visible, and in the slow-red and fast-white muscle layers transitions from larval myosin isoforms to the isoforms typical of adult muscle occurred. Between 70 and 100 days, small fibres with a distinct ATPase profile appeared throughout the fast-white muscle, marking the onset of “mosaic” hyperplasia. The myosin of the original superficial monolayer fibres underwent two myosin transformations, before being slowly replaced by an adult slow-red isoform. In juveniles and adults, the slow-red muscle layer could be resolved into two distinct types. The analysis of fibre phenotypes indicated that post-larval muscle growth occurred by two distinct stages of hyperplasia. This study offers a basis for further comparative and experimental studies with this economically relevant species, namely for identifying factors influencing its muscle growth dynamics and disclosing underlying mechanisms.     

The pregenital abdominal musculature in phasmids and its implications for the basal phylogeny of Phasmatodea (Insecta: Polyneoptera)

Recently several conflicting hypotheses concerning the basal phylogenetic relationships within the Phasmatodea (stick and leaf insects) have emerged. In previous studies, musculature of the abdomen proved to be quite informative for identifying basal taxa among Phasmatodea and led to conclusions regarding the basal splitting events within the group. However, this character complex was not studied thoroughly for a representative number of species, and usually muscle innervation was omitted. In the present study the musculature and nerve topography of mid-abdominal segments in both sexes of seven phasmid species are described and compared in detail for the first time including all putative basal taxa, e.g. members of Timema, Agathemera, Phylliinae, Aschiphasmatinae and Heteropteryginae. The ground pattern of the muscle and nerve arrangement of mid-abdominal segments, i.e. of those not modified due to association with the thorax or genitalia, is reconstructed. In Timema, the inner ventral longitudinal muscles are present, whereas they are lost in all remaining Phasmatodea (Euphasmatodea). The ventral longitudinal muscles in the abdomen of Agathemera, which span the whole length of each segment, do not represent the plesiomorphic condition as previously assumed, but might be a result of secondary elongation of the external ventral longitudinal muscles. Sexual dimorphism, common within the Phasmatodea, also applies to the muscle arrangement in the abdomen of some species. Only in the females of Haaniella dehaanii (Heteropteryginae) and Phyllium celebicum (Phylliinae) the ventral external longitudinal muscles are elongated and span the length of the whole segment, possibly as a result of convergent evolution.     

Thermal dependence of contractile properties of single skinned muscle fibres from Antarctic and various warm water marine fishes including Skipjack Tuna (Katsuwonus pelamis) and Kawakawa (Euthynnus affinis)

Summary Single fast fibres and small bundles of slow fibres were isolated from the trunk muscles of an Antarctic (Notothenia neglecta) and various warm water marine fishes (Blue Crevally,Carangus melampygus; Grey Mullet,Mugil cephalus; Dolphin Fish,Coryphaena hippurus; Skipjack-tuna,Katsuwonus pelamis and Kawakawa,Euthynuus affinis). Fibres were chemically skinned with the nonionic detergent Brij 58.For warm water species, maximum Ca²⁺-activated tension (P ₀) almost doubled between 5–20°C with little further increase up to 30°C. However, when measured at their normal body temperatures,P ₀ values for fast fibres were similar for all species examined, 15.7–22.5 N · cm^–2. Ca²⁺-regulation of contraction was disrupted at temperatures above 15°C in the Antarctic species, but was maintained at up to 30°C for warm water fish.Unloaded (maximum) contraction speeds (V _max) of fibres were determined by the slacktest method. In general,V _max was approximately two times higher in white than red muscles for all species studied, except Skipjack tuna. For Skipjack tuna,V _max of superficial red and white fibres was similar (15.7 muscle lengths · s^–1 (L ₀ · s^–1)) but were 6.5 times faster than theV _max of internal red muscle fibres (2.4±0.2L ₀ · s^–1) (25°C). V _max forN. neglecta fast fibres at 0–5°C (2–3L ₀ · s^–1) were similar to that of warm water species measured at 10–20°C. However, when measured at their normal muscle temperatures, theV _max for the fast muscle fibres of the warm water species were 2–3 times higher than that forN. neglecta.In general,Q _{10(15–30°C)} values forV _max were in the range 1.8–2.0 for all warm water species studied except Skipjack tuna.V _max for the internal red muscle fibres of Skipjack tuna were much more temperature dependent (Q _{10(15–30°C)}=3.1) (P<0.01) than for superficial red or white muscle fibres. The proportion of slower red muscle fibres in tuna (28% for 1 kg Skipjack) is 3–10 times higher than for most teleosts and is related to the tuna's need to sustain high cruising speeds. We suggest that the 8–10°C temperature gradient that can exist in Skipjack tuna between internal red and white muscles allows both fibre types to contract at the same speed. Therefore, in tuna, both red and white muscle may contribute to power generation during high speed swimming.     

Anatomy and ultrastructure of ventral pharyngeal organs and their phylogenetic importance in Polychaeta (Annelida)

Summary Transmission electron microscopic studies were carried out on the ventral pharyngeal organs in Ctenodrilus serratus and Scoloplos armiger. The pharyngeal organs are composed of a muscle bulbus and a tongue-like organ. In both species the muscle bulbus consists of transverse muscle fibres and interstitial cells with voluminous cell bodies and dorsoventral tonofilaments; the investing muscle runs into the tongue-like organ; the nuclei of the investing muscle fibres are located in caudal bulges; salivary glands are not present, but numerous gland cells occur in the bulbus epithelium. The tongue-like organ, however, is formed by lateral folds (C. serratus) or a bridge-like structure (S. armiger). The specific structure of the bulbus muscle is probably a homologous characteristic also occurring in several other polychaete families. The phylogenetic importance of this ventral pharynx is discussed and a hypothesis is suggested to explain the differentiation of certain other ventral pharyngeal organs from this probably primitive type.     

Praealbonemertes whangateaunienses n. gen. and n. sp., a new heteronemertean from New Zealand

Abstract

A new genus and species of heteronemertean, Praealbonemertes whangateaunienses n. gen. and n. sp., is described and illustrated. The species is characterised by inter alia a cephalic lacuna with strands of longitudinal muscle fibres, a proboscis with three muscle layers, and a well-developed muscle plate dorsal to the foregut and anterior intestine. The material was collected in New Zealand.     

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.     

On the absence of circular muscle elements in the body wall of Dysponetus pygmaeus (Chrysopetalidae, 'Polychaeta', Annelida)

The annelid body wall generally comprises an outer layer of circular muscle fibres and an inner layer of longitudinal muscle fibres as well as parapodial and chaetal muscles. An investigation of Dysponetuspygmaeus (Chrysopetalidae) with confocal laser scanning microscopy showed that circular muscles are entirely absent. Further studies indicate that this feature is characteristic for all Chrysopetalidae. A scrutiny of the literature showed a similar situation in many other polychaetes. This lack of circular muscle fibres may either be due to convergence or represent a plesiomorphic character. Since circular muscles are very likely important for burrowing forms but not necessary for animals which proceed by movements of their parapodial appendages or cilia, this problem is also related to the question of whether the ancestral polychaete was epi‐ or endobenthic.     

Comparative body wall musculature and muscle fibre ultrastructure in branchiobdellidans (Annelida: Clitellata), and their phylogenetic significance

The body wall muscles in five species of branchiobdellidans are all arranged in the oligochaete pattern and the muscle fibres are obliquely striated. The structure of the circular muscle fibres do vary to some degree. The longitudinal muscle fibres in Ankyrodrilus legaeus, Branchiobdella kozarovi, and Xironogiton instabilis all are round circomyarian and thus double-obliquely striated. These species represent three of the four genera composing the family Branchiobdellidae. Although Bdellodrilus illuminatus and Cambarincola fallax, from the families Bdellodrilidae and Cambarincolidae, respectively, also possess a few round circomyarian fibres, most are polyplatymyarian comparable to single-obliquely striated fibres. A similar division of branchiobdellidan families is obtained based on the number of anterior nephridial pores. The muscular structure in the branchiobdellidans shows both similarities and differences with the leeches and the lumbriculid oligochaetes. One phylogenetic explanation for this is that the branchiobdellidans separated from the common clitellate ancestor before the oligochaetes and leeches became recognizable taxa.     

Fibre types in the locomotory muscles of an antarctic teleost,Notothenia rossii

Summary The metabolic and structural differentiation of locomotory muscles of Notothenia rossii has been investigated. In this species sustained locomotion is achieved by sculling with enlarged pectoral fins (labriform locomotion), whilst the segmental myotomal muscle is reserved for burst activity. Red, white and subepidermal fibres can be distinguished in the trunk by histochemical and ultrastructural criteria. The main pectoral muscle (m. adductor profundus) consists entirely of red fibres. These three main fibres types show differences in histochemical staining profiles, capillarization, myofibril shape and packing, and lipid and mitochondrial content. The fractional volume of mitochondria amounts to 38% for pectoral, 30% for red myotomal and 1.9% for white myotomal fibres. Enzyme activities of red pectoral muscle are consistent with a higher potential for aerobic glucose and fatty acid oxidation than for the red myotomal fibres. Mg²⁺ Ca²⁺ -myofibrillar ATPase activities are similar for red pectoral and myotomal muscles and approximately half of those white fibres. Specialisations of N. rossii muscles associated with labriform swimming and locomotion at Antarctic temperatures are discussed.     

Ultrastructure of the pericardium in chitons (Mollusca: Polyplacophora), in relation to filtration and contraction mechanisms

Summary The pericardium in Lepidopleurus asellus (Spengler), Tonicella marmorea (Fabricius), T. rubra L., Ischnochiton albus L., and Calleochiton laevis (Montagu), species taxonomically far apart, is described. It consists of a flat, simple epithelium facing the pericardial cavity, a basement membrane, a muscle layer with two types of muscle fibres, nerve processes, glio-interstitial cells, and fibrocytes, embedded in a loose collagen matrix. The epithelium in L. asellus and I. albus have convoluted lateral cell borders, and in L. asellus very long basal cell processes are seen. Type 1 muscle fibres resemble smooth molluscan muscle. Type 2 muscle fibres resemble cardiac muscle fibres in chitons. Nerve processes associated with glio-interstitial cells and cell processes, run free in the matrix. Synapses in type 1 fibres are covered with glio-interstitial cell processes, lacking in type 2 muscle fibres synapses.This work was supported by grants from the Norwegian Research Council for Science and the Humanities     

Structure of the prostomial appendages and the central nervous system in the Protodrilida (Polychaeta)

Summary The prostomial appendages and the central nervous system have been investigated by electron microscopy in Protodriloides chaetifer, P. symbioticus, Protodrilus haurakiensis, P. oculifer, P. ciliatus, P. helgolandicus, P. adhaerens, Saccocirrus krusadensis and S. papillocereus. The tentacles are highly developed, mobile sensory structures and consist of cuticle, epidermis, a different number of intraepithelial nerves, a small blind-ending blood vessel and a bundle of longitudinal muscle fibres. An internal canal is only present in Protodrilus and Saccocirrus species. On the tentacles seven types of sensory cells have been found including different multiciliated and uniciliated sensory cells with cilia penetrating the cuticle, sensory cells with non-penetrative cilia, phaosomes and basal ciliated sensory cells. The latter are described for the first time in polychaetes. From the specific pattern of innervation by up to five nerves originating close to the brain from the dorsal and ventral roots of the circumoesophageal connectives it is evident that the prostomial appendages represent palps. In the palps the nerve fibres form neuroneuronal, myoneuronal and epithelioneuronal synapses. The brain also gives rise to the stomatogastric nerves and various dorsal nerves. The palp canals are separated from the surrounding tissue by a prominent extracellular matrix. The wall is formed by muscle cells. The centre is usually completely filled with the cell bodies of these muscle fibres and large coelenchyme-like cells. These cells move freely in the canals and they are very likely the structural basis for the hydroskeletal function of the canals. The canals are completely separated from other body cavities and fluid is probably driven into the canals from the blood vascular system via podocytes located in a specific zone in the prostomium. In particular, the structure of the central nervous system with its nerves, the pattern of innervation of the palps and the palp canal system are compared with those of other polychaetes with special emphasis to the Spionida, the taxon presumed to include the sister group of the Protodrilida.     

The fine structure of the muscle system in the female of the orthonectid Intoshia variabili (Orthonectida)

The contractile system of the female Intoshia variabili (Orthonectida) consists of smooth muscles. The attachment of the longitudinal muscle fibres at the anterior and the posterior tips of the body is rather peculiar, accomplished by means of elongated terminal muscle cells piercing through several ciliated cells. In the last ciliated cell, the muscle cell invaginates the ciliated cell basal membrane almost up to the ciliated cell surface. Here, around the protrusion terminus, there is an electron‐dense zone in contact with the cilia rootlets.     

Substance P immunoreactivity in sensory structures and the central and pharyngeal nervous system of Stenostomum leucops (Catenulida) and Microstomum lineare (Macrostomida)

Immunoreactivity (IR) obtained by monoclonal antibodies to substance P (SP) was studied in the asexually reproducing microturbellarians Stenostomum leucops and Microstomum lineare. The IR pattern was studied by confocal and ordinary fluorescence microscopy. In both species, IR occurs in the brain in peripheral cells, neuropilar fibres, in longitudinal cords and in the pharyngeal nervous system. The IR patterns reveal neuroanatomical details not observed with other neuroactive substances. In both species, immunopositive cells send fibers to the ciliary pits. In M. lineare, additional fibres run to more frontally located sensory structures. In S. leucops, two pharyngeal nerve rings are visualized. The pharyngeal nerve ring close to the surface associated with symmetrical immunopositive cell pairs is demonstrated for the first time, while the deeper-lying pharyngeal nerve ring has been previously demonstrated by antibodies to the molluscan cardioactive peptide FMRF-amide. Two cells with strong IR are connected by short fibres to the pharyngeal nerve ring in M. lineare. In the developing new individuals, i.e., the zooids of M. lineare, IR to SP is first revealed in nerve fibres growing out from parental lateral nerve cords towards the centre of the worm where the new brain commissure will appear. Immunopositive cells in the brain periphery and close to the developing ciliary pits appear later. Simultaneous staining by antibodies to SP and 5-HT shows that IR to SP appears later than IR to 5-HT.     

Distribution of PACAP (pituitary adenylate cyclase-activating polypeptide)-like and helospectin-like peptides in the teleost gut

Pituitary adenylate cyclase-activating polypeptide (PACAP) and helospectin are two vasoactive intestinal polypeptide (VIP)-related neuropeptides that have recently been demonstrated in the mammalian gut; the aim of this study was to reveal their occurrence and localisation in the gastrointestinal tract, swimbladder, urinary bladder and the vagal innervation of the gut of teleosts, using immunohistochemical methods on whole-mounts and sections of these tissues from the Atlantic cod, Gadus morhua and the rainbow trout, Oncorhynchus mykiss. Both PACAP-like and helospectin-like peptides were present in the gut wall of the two species. Immunoreactive nerve fibres were found in all layers but were most frequent in the myenteric plexus and along the circular muscle fibres. Immunoreactivity was also demonstrated in nerves innervating the swimbladder wall, the urinary bladder and blood vessels to the gut. Immunoreactive nerve cell bodies were found in the myenteric plexus of the gut and in the muscularis mucosae of the swimbladder. In the vagus nerve, non-immunoreactive nerve cells were surrounded by PACAP-immunoreactive fibres. Double staining revealed the coexistence of PACAP-like and helospectin-like peptides with VIP in all visualized nerve fibres and in some endocrine cells. It is concluded that PACAP-like and helospectin-like peptides coexist with VIP in nerves innervating the gut of two teleost species. The distribution suggests that both PACAP and helospectin, like VIP, are involved in the control of gut motility and secretion.     

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.