Comparative analysis of the circulatory system in Amphipoda (Malacostraca, Crustacea)

We present data on the haemolymph vascular system (HVS) in four representatives of the major amphipod lineages Gammaridea, Hyperiidea and Caprellidea based on corrosion casting and three‐dimensional reconstructions of histological semi‐thin sections. In all these species the HVS comprises a dorsal pulsatile heart, which is continued in the body axis by the anterior and posterior aortae. The heart is equipped with three pairs of incurrent ostia. The number of cardiac arteries that lead off the heart varies among species: in the studied Gammaridea four pairs occur, in Hyperia galba only the three posterior pairs of cardiac arteries occur, while in Caprella mutica cardiac arteries are absent. In all the studied species the posterior aorta leads as a simple tube into the pleon attached to the dorsal diaphragm. The anterior aorta runs from its origin in the anterior part of the second thoracic segment into the cephalothorax. Both pairs of antennae have an arterial supply off the anterior aorta. An overview of previously studied species including our present findings shows the amphipod HVS to be relatively uniform and the gammarid form is discussed as being closest to the ground pattern of Amphipoda.     

Peptidergic control of the heart of the stick insect, Baculum extradentatum

The dorsal vessel of the Vietnamese stick insect, Baculum extradentatum, consists of a tubular heart and an aorta that extends anteriorly into the head. Alary muscles, associated with the heart, are anchored to the body wall with attachments to the dorsal diaphragm. Alary muscle contraction draws haemolymph into the heart through incurrent ostia. Excurrent ostia lie on the dorsal vessel in the last thoracic and in each of the first two abdominal segments. Muscle fibers are associated with these excurrent ostia. Crustacean cardioactive peptide (CCAP)- and proctolin-like immunoreactivity is present in axons of the segmental nerves that project to the dorsal vessel, and in processes extending over the heart and alary muscles. Proctolin-like immunoreactive processes are also localized to the valves of the incurrent ostia and to the excurrent ostia. Neither the link nerve neurons, nor the lateral cardiac neurons, stain positively for these peptides. Physiological assays reveal dose-dependent increases in heart beat frequency in response to CCAP and proctolin. Isolating the dorsal vessel from the ventral nerve cord led to a change in the pattern of heart contractions, from a tonic, stable heart beat, to one which was phasic. The tonic nature was restored by the application of CCAP.     

Morphology of the haemolymph vascular system in Tanaidacea and Cumacea: - implications for the relationships of "core group" Peracarida (Malacostraca; Crustacea)

Tanaidacea and Cumacea are crucial for understanding the phylogenetic relationships of "core group" peracarids. Here, the haemolymph vascular system in three tanaidacean and four cumacean species was studied on the basis of histological sections and 3D reconstruction. The circulatory organs in Tanaidacea include a tubular heart which extends through most of the thorax. It is extended into the cephalothorax by an anterior aorta. Haemolymph enters the heart through one to two pairs of incurrent ostia. Up to five pairs of cardiac arteries emanate from the heart to supply viscera in the body cavity. In the anterior cephalothorax, the aorta forms a pericerebral ring from which the arteries for the brain and the antennae branch off. In Cumacea, the heart is shorter but more voluminous. In all cumaceans studied, five pairs of cardiac arteries supply the thoracopods and the pleon. The single pair of ostia is situated in the centre of the heart. The anterior aorta runs into the anterior cephalothorax where it supplies the brain and antennae. This paper provides a general comparative discussion of all available data from the literature and the data provided herein. In certain details, the haemolymph vascular system of the Tanaidacea resembles that of Amphipoda, and some correspondences between Cumacea and Isopoda are pointed out. These findings might support a closer relationship between the latter two taxa while they show no support for an amphipod/isopod clade.     

The circulatory system in Phreatoicidea: implications for the isopod ground pattern and peracarid phylogeny

The circulatory systems of four species of Phreatoicidea and two species of Oniscidea were studied on the basis of serial semi-thin sections and a corrosion cast method. A 3D computer reconstruction was used to visualize the circulatory organs in the head of the Phreatoicidea. In the Phreatoicidea, the circulatory system consists of a longitudinal dorsal heart extending from the third thoracic to the border between the fourth and fifth pleonal segments. It is equipped with two pairs of asymmetrically arranged ostia, while five pairs of lateral cardiac arteries and an unpaired anterior aorta extend from the heart. Entering the head, the aorta is accompanied by the two first lateral arteries, which supply the muscles of the mandibles. Four pairs of arteries branch off the aorta to supply both pairs of antennae, the eyes, and sinuses in the head. In addition, several minute capillaries extend from the aorta to supply the brain. The two oniscidean species were re-investigated with regard to some characters which have been controversially discussed. In these species, the heart extends from the border between the fifth and sixth thoracic segments to the fifth pleonal segment. Five pairs of lateral cardiac arteries and the unpaired anterior aorta lead off the heart. A ventral vessel was not observed. The ground pattern of the circulatory system in isopods is reconstructed with greater reliability through optimisation of its characters based on proposed phylogenetic relationships. The results do not support a phylogenetic position of the Isopoda as basal Peracarida or even basal Eumalacostraca.     

Scanning electron microscopy of the dorsal vessel of Panstrongylus megistus (Burmeister, 1835) (Hemiptera: Reduviidae).

In this study we analyzed the microanatomy of the dorsal vessel of the triatomine Panstrongylus megistus. The organ is a tubule anatomically divided into an anterior aorta and a posterior heart, connected to the body wall through 8 pairs of alary muscles. The heart is divided in 3 chambers by means of 2 pairs of cardiac valves. A pair of ostia can be observed in the lateral wall of each chamber. A bundle of nerve fibers was found outside the organ, running dorsally along its major axis. A group of longitudinal muscular fibers was found in the ventral portion of the vessel. The vessel was found to be lined both internally and externally by pericardial cells covered by a thin laminar membrane. Inside the vessel the pericardial cells were disposed in layers and on the outside they formed clusters or rows.     

Functional morphology of the dorsal vessel in the adult fly Protophormia terraenovae (diptera,calliphoridae)

The morphology and ultrastructure of the contractile tubular vessel acting as the cardiac pump in Protophormia terraenovae flies were analyzed by means of light microscopy, SEM and TEM. The results provide a novel anatomical picture of the two vessel portions, the abdominal heart and the aorta, and lay the foundations for an interpretation of the cardiocirculatory function in the fly. In the thorax, the thin and unchambered aorta is without apertures, while the abdominal heart presents a very small caudal aperture and pairs of lateral ostia, one in each of the five chambers of which it is composed. The ostia of the four more distal chambers are of the incurrent type, which is to say that they act as valves ensuring that hemolymph flows only into the heart. Conversely, the ostia in the most proximal chamber allow hemolymph to flow both into and out of the heart. The entire vessel is composed of a single layer of myofibers that are oriented circularly around the lumen in the abdominal heart and longitudinally in the thoracic aorta. The abdominal heart has a thicker wall, a far more diffused and thick distribution of tracheoles, and a far greater number of mitochondria with respect to the aorta. This arrangement ensures a greater availability of oxygen and energy in the abdominal heart compared to the aorta and leads one to suppose that the high‐ and low‐frequency contractions of the cardiac cycle (Thon, [1982] J. Insect Physiol. 28:411–416) can be attributed to the abdominal heart and the aorta, respectively. J. Morphol. 240:15–31, 1999. © 1999 Wiley‐Liss, Inc.     

Homeotic genes autonomously specify the anteroposterior subdivision of the Drosophila dorsal vessel into aorta and heart

The embryonic dorsal vessel in Drosophila possesses anteroposterior polarity and is subdivided into two chamber-like portions, the aorta in the anterior and the heart in the posterior. The heart portion features a wider bore as compared with the aorta and develops inflow valves (ostia) that allow the pumping of hemolymph from posterior toward the anterior. Here, we demonstrate that homeotic selector genes provide positional information that determines the anteroposterior subdivision of the dorsal vessel. Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) are expressed in distinct domains along the anteroposterior axis within the dorsal vessel, and, in particular, the domain of abd-A expression in cardioblasts and pericardial cells coincides with the heart portion. We provide evidence that loss of abd-A function causes a transformation of the heart into aorta, whereas ectopic expression of abd-A in more anterior cardioblasts causes the aorta to assume heart-like features. These observations suggest that the spatially restricted expression and activity of abd-A determine heart identities in cells of the posterior portion of the dorsal vessel. We also show that Abd-B, which at earlier stages is expressed posteriorly to the cardiogenic mesoderm, represses cardiogenesis. In light of the developmental and morphological similarities between the Drosophila dorsal vessel and the primitive heart tube in early vertebrate embryos, these data suggest that Hox genes may also provide important anteroposterior cues during chamber specification in the developing vertebrate heart.     

Regulation of air and blood flow through the booklungs of the desert scorpion, Paruroctonus mesaensis

Injections of dye, latex and India ink were used to reveal the path of hemolymph circulation through the scorpion booklungs. Fine, branched arteries carry blood directly to muscle and other organs. The blood returns through venous channels to the ventral mesosoma where it passes laterally through the booklungs and into the pneumocardial veins just beneath the pleural cuticle. Blood flows dorsally through these veins to the pericardial sinus and heart. The scorpion has four pairs of booklungs located in the anterior segments of the ventral mesosoma. Each booklung has a spiracle which opens into an atrium enclosed by cuticular membrane. Air passes from the atrium into the booklung lamellae. Agitation of the animal or application of CO(2) causes retraction of the anterior and posterior atrial membrane. This expands the atrial chamber and allows gas exchange in the booklung lamellae. The posterior atrial membrane has a specialized region which forms a springy valve. This normally closes the spiracle unless pulled open by contraction of the attached poststigmaticus muscle. The pectens and receptors within the atrium may mediate the responses to CO(2). Slender hypocardial ligaments containing muscle fibers extend from the heart (dorsal mesosoma) to the booklungs in the ventral mesosoma. Heart movements thus cause dorso-ventral movement of the booklungs. The significance of these movements is as yet unclear. They may increase ventilation, help force blood to the heart and/or agitate the blood and booklung lamellae and thereby aid gas exchange. Passage of blood through the booklungs is regulated by dorsal and ventral muscles attached to the atrium at the lateral edge of the booklung. Contraction of the ventral atrial muscle closes the excurrent channel for passage of blood from the booklung into the pneumocardial vein. Electrical stimulation of the segmentai nerves from the subesophageal and first three abdominal ganglia causes spiracle opening and contraction of muscles attached to the atrial membrane. A previous study showed that these same segmental nerves also modulate heart activity. They thus provide a major pathway for regulation of the respiratory and circulatory systems.     

Ostia, the inflow tracts of the Drosophila heart, develop from a genetically distinct subset of cardial cells.

The homeobox gene tinman and the nuclear receptor gene seven-up are expressed in mutually exclusive dorsal vessel cells in Drosophila, however, the physiological reason for this distinction is not known. We demonstrate that tin and svp-lacZ expression persists through the larval stage to the adult stage in the same pattern of cells expressing these genes in the embryo. In the larva, six pairs of Svp-expressing cells form muscular ostia, which permit hemolymph to enter the heart for circulation, however, more anterior Svp-expressing cells form the wall of the dorsal vessel. During pupation, the adult heart forms from a chimera of larval and imaginal muscle fibers. The portion of the dorsal vessel containing the larval ostia is histolyzed and the anterior Svp-expressing cells metamorphose into imaginal ostia. This is the first demonstration that the significant molecular diversity of cardial cells identified in the embryonic heart correlates with the formation of physiologically and functionally distinct muscle cells in the animal. Furthermore, our experiments define the cellular changes that occur as the larval heart is remodeled into an imaginal structure in an important model organism.     

Accessory hemolymph pump in the mesothoracic legs of locusts, (Schistocerca gregaria forskal) (Orthoptera,Acrididae)

An accessory pulsatile organ located in the mesothoracic legs pumps hemolymph towards the tip of the leg ventrally and towards the body near the dorsal side. It consists of a muscle attached to the ventral side of the trochanter and to the central region of a transverse connective tissue diaphragm located at the trochanter-femur border. The diaphragm has a ventral outlet that permits efferent hemolymph flow through a narrow femoral sinus. A second dorsal outlet allows the afferent countercurrent back to the thorax through a separate hemolymph channel. During abdominal ventilation, the pumping rhythm of the legfn2heart is neurally synchronized with abdominal ventilation. Expiratory pressure expands tracheal air sacs in the ventral trochanter and helps driving hemolymph out of this space. In idle periods of resting ventilation, an autonomous myogenic rhythm of the leg–heart can maintain hemolymph circulation in the mesothoracic leg without neural control.     

Morphology of the dorsal vessel in the abdomen of the blood-feeding insect Rhodnius prolixus

The dorsal vessel (DV) in the abdomen of the blood-feeding insect Rhodnius prolixus was divided functionally into two regions, the heart, into which haemolymph entered the DV through four pairs of ostia located in abdominal segment VII, and the aorta, along which the haemolymph was propelled from abdominal segment VI to the thorax. Osmium-fixed whole mounts revealed the DV to consist of spirally arranged striated muscle fibers and to possess two rows of ventrally attached longitudinal fibers extending the length of the abdomen. Seven pairs of alary muscles were found attached to the DV in the posterior abdominal segments. Contractions of the alary muscles attached to the ventral surface of abdominal segments VII and VIII served to expand the heart. Electron microscopy revealed the DV to consist of a thin layer of contractile elements surrounded by an inner (intima) and outer (adventitia) connective tissue layer. Embedded in the intima along each lateral side of the DV were two large groups of endocardial cells extending the length of the DV. A small group of pericardial cells was embedded in the adventitia along the mid-ventral side of the DV, and clusters of pericardial cells were found attached to the alary muscles. Nerve terminals were found only on the heart: they contained agranular synaptic vesicles approximately 30 nm in diameter and densely stained granules approximately 100-120 nm in diameter. These structural components are discussed in relation to the role of the DV in circulation.     

The circulatory system and its spatial relations to other major organ systems in Spelaeogriphacea and Mictacea (Malacostraca, Crustacea) – a three-dimensional analysis

Spelaeogriphacea and Mictacea are enigmatic taxa within malacostracan crustaceans that play a pivotal role in peracarid phylogeny. Anatomical data on both taxa that are suitable for use in cladistic analyses are still scarce. Here, we provide for the first time detailed three-dimensional information on the major organ systems of Spelaeogriphacea and Mictacea (the circulatory system, the digestive system, and the central nervous system) using semithin sections in combination with computer aided three-dimensional reconstruction techniques. The digestive system in both Spelaeogriphus lepidops and Mictocaris halope is made up of a short oesophagus leading to a voluminous stomach chamber. Posteriorly, a pylorus is attached to the stomach chamber. An antechamber of the midgut glands is situated at the transition into the midgut, from which up to four tubular midgut glands emanate. The midgut is a straight tube running through the body terminating in a short hindgut. The central nervous system in the cephalothorax is made up of a brain and a suboesophageal ganglion. Both species show some reduction of the protocerebrum caused by the lack of eyes. The circulatory system is made up of a tubular heart that is situated in the thorax. It is equipped with two pairs of incurrent ostia in S. lepidops and one pair in M. halope . The only artery leading off the heart is the anterior aorta, which runs into the cephalothorax. A dilation is formed between the brain and the anterior stomach wall, into which oesophageal dilator muscles are internalized. The function of this so-called 'myoarterial formation a' as an accessory pulsatile structure in the anterior cephalothorax of these animals is discussed.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 629–642.     

Structure and function of spiny lobster ligamental nerve plexuses: Evidence for synthesis,storage, and secretion of biogenic amines

Three pairs of ligaments support not only the heart of spiny lobsters but also ligamental nerve plexuses, the complex terminal aborizations of segmental nerves. Segmental nerves 1–4 project from the thoracic ganglia into the pericardial cavity and ultimately ramify along the strands of the anterior, medial, and posterior ligaments. In each branch, a core of large axons sends fibers to terminate in a surrounding cortex of fine and varicose secretory processes. Electron micrographs reveal at least five distinct populations of granule-filled neuronal profiles, many with vesicles clustered at membrane thickenings adjacent to the epineural sheath. The ligamental nerve plexuses synthesize and accumulate octopamine, dopamine, 5-HT, and acetylcholine. Octopamine and 5-HT are predominant, comprising 33% and 65%, respectively, of the synthetic activity devoted to the four amines. Thus, the anatomy, ultrastructure, and neurochemistry of the ligamental nerve plexuses establishes their homology with the pericardial organs of other Crustacea. Octopamine and 5-HT are released by a Ca⁺⁺-dependent mechanism upon electrical stimulation of preterminal nerve trunks, and, in vivo, would be swept immediately through ostia into the heart. These observations, when considered with known effects of octopamine and 5-HT on crustacean cardiac activity, neuromuscular transmission, muscle tension, and cyclic AMP metabolism provide a strong case for hormonal actions at target sites throughout the animal. Segmental nerve processes in the dorsal nerve trunk ramify into a plexus around the dorsal nerve apparatus, a small muscular bulb that lies recessed in the cardiac surface. The dorsal nerve, carrying excitatory and inhibitory input to the cardiac ganglion directly through the bulb's hollow interior. The apparatus synthesizes and contains acetylcholine and the three amines mentioned above. In situ, it may beat rhythmically out of phase with the heart.     

Complex innervation of three neck muscles by motor and dorsal unpaired median neurons in crickets

Summary In the crickets, Gryllus campestris and Gryllus bimaculatus, the innervation of the dorso-ventral neck muscles M62, M57, and M59 was examined using cobalt staining via peripheral nerves and electrophysiological methods. M62 and M57 are each innervated by two motoneurons in the suboesophageal ganglion. The four motoneurons project into the median nerve to bifurcate into the transverse nerves of both sides. M62 and M57 are the only neck muscles innervated via this route. These bifurcating axon-projections are identical to those of the spiracular motoneurons in the prothoracic ganglion innervating the opener and closer muscle of the first thoracic spiracle in the cricket. The morphology of their branching pattern is described. The neck muscle M57 and the opener muscle of the first thoracic spiracle are additionally innervated by one mesothoracic motoneuron each, with similar morphology. These results suggest, that in crickets, the neck muscles M57 and M62 are homologous to spiracular muscles in the thoracic segments. The two neck muscles M62 and M59 (the posterior neighbour of M57) receive projections from a prothoracic dorsal unpaired median (DUM) neuron that also innervates dorsal-longitudinal neck muscles but not M57. In addition, one or two mesothoracic DUM neurons send axon collaterals intersegmentally to M59. This is the first demonstration of the innervation of neck muscles by DUM neurons.     

The Hox gene abdominal-A specifies heart cell fate in the Drosophila dorsal vessel

The Drosophila melanogaster dorsal vessel is a linear organ that pumps blood through the body. Blood enters the dorsal vessel in a posterior chamber termed the heart, and is pumped in an anterior direction through a region of the dorsal vessel termed the aorta. Although the genes that specify dorsal vessel cell fate are well understood, there is still much to be learned concerning how cell fate in this linear tube is determined in an anteroposterior manner, either in Drosophila or in any other animal. We demonstrate that the formation of a morphologically and molecularly distinct heart depends crucially upon the homeotic segmentation gene abdominal-A (abd-A). abd-A expression in the dorsal vessel was detected only in the heart, and overexpression of abd-A induced heart fate in the aorta in a cell-autonomous manner. Mutation of abd-A resulted in a loss of heart-specific markers. We also demonstrate that abd-A and sevenup co-expression in cardial cells defined the location of ostia, or inflow tracts. Other genes of the Bithorax Complex do not appear to participate in heart specification, although high level expression of Ultrabithorax is capable of inducing a partial heart fate in the aorta. These findings for the first time demonstrate a specific involvement for Hox genes in patterning the muscular circulatory system, and suggest a mechanism of broad relevance for animal heart patterning.     

The circulatory system in Mysidacea--implications for the phylogenetic position of Lophogastrida and Mysida (Malacostraca, Crustacea)

The morphology of the circulatory organs in Mysida and Lophogastrida (traditionally combined as Mysidacea) is revisited investigating species so far unstudied. In addition to classical morphological methods, a newly developed combination of corrosion casting with micro computer tomography (MicroCT) and computer aided 3D reconstructions is used. Lophogastrida and Mysida show a highly developed arterial system. The tubular heart extends through the greater part of the thorax and is connected with the ventral vessel via an unpaired descending artery. It is suggested that a distinct ostia pattern supports the monophyly of Mysidacea. The cardiac artery system is more complex in Lophogastrida than in Mysida, consisting of up to 10 pairs of arteries that supply the viscera. In both taxa, an anterior and posterior aorta leads off the heart. In the anterior part of the cephalothorax the anterior aorta forms dilations into which muscles are internalized; these structures are called myoarterial formations. One of these myoarterial formations can also be found in all the other peracarid taxa but not in other Malacostraca.     

Growth of the jugular lymph sacs and establishment of the topography of the cervical portion of the thoracic duct during early human ontogenesis

In 196 human embryos, prefetuses, fetuses and newborns, by means of a complex of morphological methods, development of the jugular lymphatic sacs and the process of settling of the thoracic duct cervical part topography have been studied. The jugular lymphatic sac anlages take place on the 6th week of the development. From the lymphatic cleft, situating in the mesenchyme near the anterior cardinal veins, multichambered cavities laid with endotheliocytes are forming,--the jugular lymphatic sacs. Connection of the initially close lymphatic sacs with the venous system takes place secondarily by the end of the embryonic period of development. In the area of the sac ostia a valve is formed, that makes morphological premises for unidirected lymph flow into the venous system. The lymph nodes developing at the place of the reducing jugular lymphatic sacs, ensure formation: from the left jugular lymphatic sac--the cervical part of the thoracic duct, from the right jugular lymphatic sac--the right lymphatic duct and the jugular and the subclavicular lymphatic trunks. Variability in the form and topography of these structures are determined both by the form and construction of the jugular lymphatic sacs and by developmental peculiarities of the lymph nodes at their place. The process of settling of the thoracic duct cervical part topography depends on age changes of its size and form, as well as on development of structures situating nearby, and by the time of birth it is not completed.     

Morphology of the pupal heart,adult heart,and associated tissues in the fruit fly,Drosophila melanogaster

The early pupal heart of the fruit fly Drosophila melanogaster has recently been the subject of intense physiological and molecular work, yet it has not been well described, nor has it been compared with the heart of the adult fly. In the work reported here, the hearts of adults and early pupae of D. melanogaster were studied by scanning and transmission electron microscopy and by light microscopy. The hearts of adults and early pupae both consist of a tube of circular striated muscle one cell in thickness. The alary muscles, which suspend the heart, are more delicate in the adult compared to the early pupa. The pericardial cells in both early pupae and adults are connected to the heart by connective tissue radiating from the alary muscles or dorsal diaphragm. We confirm that four major changes occur in the heart during metamorphosis: 1) a conical chamber is formed de novo in the first and second abdominal segments; 2) the adult heart curves to conform to the contour of the abdomen; 3) a layer of longitudinal striated muscle appears on the ventral surface of the heart; 4) a fourth pair of ostia is added to the three already present in the early pupa; and note additionally that 5) the ostia appear as simple openings in the heart of the early pupa but are valve‐like in the adult. J. Morphol. 240:225–235, 1999. © 1999 Wiley‐Liss, Inc.     

Larval morphology of the lesser housefly, Fannia canicularis

The morphology of all larval instars of Fannia canicularis (Linnaeus) (Diptera: Fanniidae) is documented using a combination of light and scanning electron microscopy. The following structures are documented for all instars: antennal complex; maxillary palpus; facial mask; cephaloskeleton; ventral organ; anterior spiracle; Keilin's organ; posterior spiracle; fleshy processes, and anal pad. Structures reported for the first time for all instars include: two pairs of lateral prominences on the prothoracic segment; additional ventrolateral prominences on the second thoracic segment, and a papilla at the base of the posterior spiracle. Other structures reported for the first time are anterior spiracles in the first instar and a serrated tip on the mouthhook in the second instar. A trichoid sensillum on the posterior spiracular plate, representing a sensory organ otherwise unknown in the Calyptratae, is described in the second and third instars. Results are discussed and compared with existing knowledge on dipteran larval morphology.     

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

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