Ultrastructure and formation of the body cavity lining in Phoronis muelleri (Phoronida, Lophophorata)

Among other characteristics a trimeric coelomic compartmentation consisting of an anterior protocoel, followed by a mesocoel and a posterior metacoel is traditionally believed to substantiate the sister-group relationship between Lophophorata and Deuterostomia, together forming the Radialia. As molecular data cannot support this hypothesis a reanalysis of the coelomic cavities in Phoronida is undertaken, because corresponding coelomic compartmentation is widely accepted to support the Radialia hypothesis. A coelomic cavity can be recognized on the ultrastructural level because its lining is a true epithelium with polarized cells interconnected by apical adherens junctions. This study reveals that neither in larval nor adult Phoronis muelleri (Phoronida) an anterior cavity with such a lining is present. What on the light microscopic level leads to the impression of a cavity inside the larval episphere, actually is an enlarged subepidermal extracellular matrix with an amorphous, presumably gel-like filling, into which several muscle cells are embedded. Larvae, thus, possess only one coelomic cavity, the large trunk coelom of the larva which is adopted in the adult organization. The second coelomic cavity of adult P. muelleri, the lophophore coelom, develops as a double-layer of epithelialized mesodermal cells at the base of the adult tentacle buds and becomes fluid filled during metamorphosis. Like the larval episphere, larval tentacles and most parts of the blastocoel are filled by an amorphous matrix. Reanalysis of the literature and comparison with Brachiopoda and Bryozoa allows the hypothesis that a protocoel is lacking in all Lophophorata, and that merely two unpaired coelomic cavities, one tentacle and one trunk coelom, can be assumed for the ground pattern of this taxon. These results do not provide further evidence for the Radialia hypothesis, but also do not contradict it. Accepted: 28 August 2000     

Body cavities in bryozoans: Functional and phylogenetic implications

Based on morphological evidence, Bryozoa together with Phoronida and Brachiopoda are traditionally combined in the group Lophophorata, although this view has been recently challenged by molecular studies. The core of the concept lies in the presence of the lophophore as well as the nature and arrangement of the body cavities. Bryozoa are the least known in this respect. Here, we focused on the fine structure of the body cavity in 12 bryozoan species: 6 gymnolaemates, 3 stenolaemates and 3 phylactolaemates. In gymnolaemates, the complete epithelial lining of the body cavity is restricted to the lophophore, gut walls, and tentacle sheath. By contrast, the cystid walls are composed only of the ectocyst-producing epidermis without a coelothelium, or an underlying extracellular matrix; only the storage cells and cells of the funicular system contact the epidermis. The nature of the main body cavity in gymnolaemates is unique and may be considered as a secondarily modified coelom. In cyclostomes, both the lophophoral and endosaccal cavities are completely lined with coelothelium, while the exosaccal cavity only has the epidermis along the cystid wall. In gymnolaemates, the lophophore and trunk cavities are divided by an incomplete septum and communicate through two pores. In cyclostomes, the septum has a similar location, but no openings. In Phylactolaemata, the body cavity is undivided: the lophophore and trunk coeloms merge at the bases of the lophophore arms, the epistome cavity joins the trunk, and the forked canal opens into the arm coelom. The coelomic lining of the body is complete except for the epistome, lophophoral arms, and the basal portions of the tentacles, where the cells do not interlock perfectly (this design probably facilitates the ammonia excretion). The observed partitioning of the body cavity in bryozoans differs from that in phoronids and brachiopods, and contradicts the Lophophorata concept.     

Organization of the epistome in <Emphasis Type="Italic">Phoronopsis harmeri</Emphasis> (Phoronida) and consideration of the coelomic organization in Phoronida

Results provided by modern TEM methods indicate the existence of the lophophoral and trunk coelomes but not of the preoral coelom in Phoronida. In the present work, the epistome in Phoronopsis harmeri was studied by histological and ultrastructural methods. Two kinds of cells were found in the frontal epidermis: supporting and glandular. The coelomic compartment is shown to be inside the epistome. This compartment has a complex shape, consists of a central part and two lateral branches, and contacts the lophophoral coelom, forming two complete dissepiments on the lateral sides and a partition with many holes in the center. TEM reveals that some portions of the incomplete partition are organized like a mesentery, with the two layers of cells separated by ECM. The myoepithelial cells of the coelomic lining form the circular and radial musculature of the epistome. Numerous amoebocytes occur in the coelom lumen. The tip of the epistome and its dorso-lateral parts lack a coelomic cavity and are occupied by ECM and muscle cells. The fine structure of the T-shaped vessel is described, and its localization inside lophophoral coelom is demonstrated. We assert that the cavity inside the epistome is the preoral coelom corresponding to the true preoral coelom of the larva of this species. Proving this assertion will require additional study of metamorphosis in this species. To clarify the patterns of coelom organization in phoronids, we discuss the bipartite coelomic system in Phoronis and the tripartite coelomic system in Phoronopsis.     

The phylogenetic position of Brachiopoda—a comparison of morphological and molecular data

Analyses of rRNA and rDNA among Metazoa result in a hypothesis of a sistergroup relationship of Brachiopoda and certain spiralian taxa, whereas analyses of morphological data imply that Brachiopoda show affinities to Deuterostomia within the Radialia. Regarding Brachiopoda as a derived spiralian taxon must be followed by a reinterpretation of the evolution of distinct brachiopod morphological characters—like cleavage pattern, coelom or larva. The experimental insertion of a monophyletic taxon consisting of Brachiopoda and Phoronida into a widely accepted phylogenetic tree of Spiralia leads to the hypothesis that at least trimeric organization, mesosomal tentacular apparatus and heterogeneously assembled metanephridia are products of convergent evolution in Brachiopoda plus Phoronida and Deuterostomia. The hypothesis of a radialian nature of Brachiopoda and Phoronida, as implied by morphological data, remains as the most parsimonious possibility to explain the evolution of seven regarded characters (cleavage pattern, larva, tentacular apparatus, coelom, metameric segmentation, metanephridia and chaetae) in Brachiopoda. Due to the conflicting results of both methods a hitherto undetected systematical problem is discussed possibly hindering data comparability. If the course of evolution can principally be inferred from the information preserved in recent and fossil animals, the results should be congruent in the analyses of both, molecular and morphological data.     

Ultrastructure of the coelom in the brachiopod Lingula anatina

The organization of the coelomic system and the ultrastructure of the coelomic lining are used in phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians. In the current study, the organization of the coelom of the lophophore in the brachiopod Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (= canals). The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium, alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and podocyte‐like cells. Connections between cells of the coelomic lining are provided by adherens junctions, tight‐like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges. The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated myoepithelium can be considered as the ancestral type of coelomic lining. The many different kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial cells and muscle cells.     

Ultrastructure of the Coelomocytes in the Tentacular Coelom of Thysanocardia nigra Ikeda, 1904 (Sipuncula)

Free-floating coelomocytes in the tentacular coelomic cavity of the sipunculan Thysanocardia nigra Ikeda, 1904, were studied using light interference contrast microscopy and scanning and transmission electron microscopy. The following coelomocyte types were distinguished: hemerythrocytes, amoebocytes, and two morphological types of granular cells. No clusters of specialized cells that had been reported to occur in the trunk coelom of Th. nigra were found in the tentacular coelom. The corresponding types of coelomocytes from the tentacular and trunk coelomic cavities were shown to differ in size. These two coeloms are completely separated in sipunculans.     

Ultrastructure of the body cavities in juveniles and adults of the appendicularian Oikopleura gracilis (Tunicata,Chordata) suggests how the heart may have evolved in tunicates

The organization of the body cavities is an important morphological trait that can be used for establishing the phylogenetic relationships between different groups of animals. In the present study, the hemocoel and coelomic systems of 10‐hr‐old juveniles and adults of the hermaphroditic oikopleurid Oikopleura gracilis were examined using light and transmission electron microscopy. The trunk hemocoel in 10‐hr‐old juveniles was represented by small clefts containing layers of extracellular matrix of adjacent tissues or interstices with migrating primordial germ syncytium. The wide hemocoel in the tail contained extracellular strands, subdividing the hemocoel into hemal sinuses. In adults, a large hemocoel appeared in the trunk and tail, and also contained extracellular strands. The hermaphroditic gonad was surrounded by its own lining, separating it from the hemocoel. The gamete‐filled cavity in the ovary and testis appeared only at late‐stage gonadogenesis, when the pre‐spawning reduction of syncytium occurred in the gonads. The true coelom in 10‐hr‐old juveniles and adults was represented by the pericardium. The lining of the pericardium consisted of myoepithelial and peritoneal cells. In the myoepithelial cells of 10‐hr‐old juveniles, myofibrils had been formed. The myoepithelial cells of adults had several parallel rows of completely differentiated myofibrils. The substantial reduction of the coelomic and circulatory systems in O. gracilis evidently results from the extreme shortening of ontogeny in appendicularians. Development in O. gracilis from early juvenile to adult involves the following steps, which also suggest how the tunicate heart may have evolved: a single‐layered coelomic sac gives rise to a grooved pericardium with an open hemal sinus (simple heart). In ascidians, this simple heart in turn gives rise to a closed tubular, double‐layered heart–pericardial complex, with a separate pericardial cavity and a closed heart, whose wall is formed by specialized myocardium.     

Ultrastructure of the body cavity lining in a secondary acoelomate,Microphthalmus cf. Listensis westheide (Polychaeta: Hesionidae)

The organization of the body cavity lining in selected regions of the juvenile and adult of the interstitial hesionid polychaete Microphthalmus cf. listensis is described. Tissues comprising the body cavity lining in the juvenile consist of somatic and splanchnic circular and longitudinal muscles and undifferentiated cells. Somatic and splanchnic cell layers exhibit epithelial ( = eucoelomate) organization in the pharyngeal region. In the midbody, some undifferentiated cells exhibiting mesenchymal organization persist among the epithelially organized somatic and splanchnic cells, forming a gradation between eucoelomate and acoelomate tissue organizations. A coelomic cavity is absent. Tissues comprising the body cavity lining of the adult consist of somatic and splanchnic circular and longitudinal myocytes and coelenchymal cells. Coelenchymal cells are shown from serial section analysis to be mesenchymal in organization and derived from the somatic peritoneum. A 30–65-nm coelomic cavity lies between the apices of somatic and splanchnic cell layers in the pharyngeal region. In the anterior setigerous segments, the coelom is reduced to a narrow cavity surrounded by coelenchymal cells lying midventrally between the paired ejaculatory ducts. There is a regional obliteration of the splanchnic musculature in the posterior segments so that apices of the coelenchymal cells lie in direct apposition to the basal extracellular matrix of the gut. The coeom is only present middorsally as a 0.7-μm-wide cavity. Although the coelomic cavity is highly reduced in the adult, the body cavity lining still reveals its origin from the epithelial ( = eucoelomate) organization. The findings of this study illustrate possible organizational intermediates in the evolution of the acoelomate from the eucoelomate condition in annelids.     

Ultrastructure of the body cavities in Phylactolaemata (Bryozoa)

Only species belonging to the bryozoan subtaxon Phylactolaemata possess an epistome. To test whether there is a specific coelomic cavity inside the epistome, Fredericella sultana, Plumatella emarginata, and Lophopus crystallinus were studied on the ultrastructural level. In F. sultana and P. emarginata, the epistome contains a coelomic cavity. The cavity is confluent with the trunk coelom and lined by peritoneal and myoepithelial cells. The lophophore coelom extends into the tentacles and is connected to the trunk coelom by two weakly ciliated coelomic ducts on either side of the rectum. The lophophore coelom passes the epistome coelom on its anterior side. This region has traditionally been called the forked canal and hypothesized to represent the site of excretion. L. crystallinus lacks an epistome. It has a simple ciliated field where an epistome is situated in the other species. Underneath this field, the forked canal is situated. Compared with the other species, it is pronounced and exhibits a dense ciliation. Despite the occurrence of podocytes, which are prerequisites for a selected fluid transfer, there is no indication for an excretory function of the forked canal, especially as no excretory porus was found. J. Morphol. 2009. © 2008 Wiley‐Liss, Inc.     

Development of head and trunk mesoderm in the dogfish,Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures

Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance for understanding the developmental body plan of vertebrates and its evolutionary origin. In particular, the segmentation of the mesoderm is central to the problem. The shark embryo has provided a canonical morphological scheme of the head, with its epithelialized coelomic cavities (head cavities), which have often been regarded as head somites. To understand the evolutionary significance of the head cavities, the embryonic development of the mesoderm was investigated at the morphological and histological levels in the shark, Scyliorhinus torazame. Unlike somites and some enterocoelic mesodermal components in other vertebrates, the head cavities in S. torazame appeared as irregular cyst(s) in the originally unsegmented mesenchymal head mesoderm, and not via segmentation of an undivided coelom. The mandibular cavity appeared first in the paraxial part of the mandibular mesoderm, followed by the hyoid cavity, and the premandibular cavity was the last to form. The prechordal plate was recognized as a rhomboid roof of the preoral gut, continuous with the rostral notochord, and was divided anteroposteriorly into two parts by the growth of the hypothalamic primordium. Of those, the posterior part was likely to differentiate into the premandibular cavity, and the anterior part disappeared later. The head cavities and somites in the trunk exhibited significant differences, in terms of histological appearance and timing of differentiation. The mandibular cavity developed a rostral process secondarily; its homology to the anterior cavity reported in some elasmobranch embryos is discussed.     

Morphologische untersuchungen zum archicoelomatenproblem

Zusammenjassung Es kann festgestellt werden, daß Untersuchungen der mikroskopischen Anatomic adulter Phoronis ijimai, Oka eine Eigenständigkeit des Epistoms sowie des zu diesem Körperteil gehörenden Coeloms ergeben. Ph. ijimai besitzt also eine archimere Gliederung. Der vorderste Abschnitt, das Epistom, kann mit dem Prosoma, sein Coelom mit dem Axocoel des Archicoelomatenbauplans homologisiert werden. Entsprechend sind die folgenden Körperabschnitte homolog mit Mesosoma and Metasoma resp. Hydrocoel und Somatocoel.Die Archicoelomaten-Natur der Phoronidea ist damit auch im Hinblick auf die Körpergliederung sicher.

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Morphological investigations about the Archicoelomate-ProblemI. The body segmentation in Phoronis ijimai Oka (Phoronidea)

Summary Investigations of the microscopic anatomy in adult Phoronis ijimai Oka can be said to reveal autonomy of the epistome and of the coelom belonging to this section of the body. Ph. ijimai thus possesses an archimeric segmentation. The front section, the epistome can be compared to the prosoma, its coelom to the axocoel in archicoelomate structure. In the same way the succeeding segments are comparable with mesosoma and metasoma or hydrocoel and somatocoel respectively.Allocation of Phoronidae to the Archicoelomates is thus also justified by the body segmentation.

     

Ultrastructure of hatchling chaetognaths (Ferosagitta hispida): Epithelial arrangement of the mesoderm and its phylogenetic implications

The trunk and tail mesoderm of hatchling chaetognaths consists of a simple myoepithelium containing four stereotypically arranged cell types, each matching in position a specific adult tissue. The trunk mesoderm includes lateral cells, longitudinal muscle cells, dorsal and ventral medial cells, and peri-intestinal cells. These correspond, respectively, to the lateral fields, longitudinal body wall muscles, dorsal and ventral perimysial cells, and periintestinal muscles of adults. Because the developing intestine does not extend into the tail, tail cells equivalent in position to peri-intestinal cells in the trunk are designated mesenterial cells. Numerous small spaces situated among the apices of hatchling mesodermal cells have the same position relative to surrounding cells as both the coelomic cavities of early embryos and the adult body cavities. We infer that these spaces in hatchlings expand and coalesce to form the definitive adult body cavities, and that these spaces and the adult body cavities derive from the embryonic coeloms. Because hatchlings lack mesenchymal mesoderm, we infer that all adult mesodermal tissues develop by elaboration of the coelomic lining of hatchlings. Because hatchlings lack cells corresponding to the squamous peritoneocytes overlying the body wall muscles of adults, we conclude that peritoneocytes are specialized adult cells that are not equivalent to cells of the embryonic coelomic lining. Finally, hatchlings contain a complete trunk/tail septum. This observation contradicts reports that this septum forms several days after hatching. It also weakens arguments that chaetognaths are bimeric rather than trimeric. © 1994 Wiley-Liss, Inc.     

The physiological function of the coelom in starfish larvae and its evolutionary implications

The coelom in the bipinnaria larva of Asterias acts as a buoyancy tank. The concentrations of magnesium and sulphate in the coelomic fluid are lower than in seawater, reducing the density. The coelomic epithelium is a secretory epithelium, probably secreting sodium or chloride ions that then draw in the counter ion and water. The rate of urine production is very high for an isotonic marine animal, compensating for the large surface/volume ratio of the coelom. This function would account for the precocious development of the coelom and its association with an excretory duct. It is proposed that the coeloms of other pelagic larvae such as the actinotroch of Phoronis and of echiuran larvae have a similar function and that this may have been an original function of the coelom, although in many phyla this function has been modified or lost.     

Nephridial development and body cavity formation in <Emphasis Type="Italic">Artemia salina</Emphasis> (Crustacea: Branchiopoda): no evidence for any transitory coelom

The Ecdysozoa-hypothesis on the origin of arthropods questions the homology of segmentation in arthropods, onychophorans, and annelids. The implication of convergent gain of metamery in these groups seems to conflict particularly with the correspondence in the development of serial coelomic cavities and metanephridia. Ultrastructural studies of the mesoderm development in Onychophora revealed that main correspondence with the state in annelids concerns the involvement of epithelial lining cells of the embryonic coelomic cavities in the formation of the visceral and somatic musculature. The significance of this correspondence, however, remained unclear as comparable data on the state in arthropods were still missing. Developmental studies on selected representatives covering all major arthropod subgroups aim to fill in this gap. Data were raised by a combination of transmission electron microscopy and fluorescent stainings of the muscular system and nuclei for the anostracan crustacean Artemia salina. In this species, putative transitory coelomic cavities proved to be absent in all trunk segments. In the second antennal and second maxillary segments small, compact nephridial anlagen develop into a sacculus and excretory duct. The sacculus originates from the terminal cells of the nephridial duct, which is formed in advance. The lumen of the sacculus is inconspicuous in its earliest functional stage and later enlarges to a bulb; it accordingly represents no remnant of any primarily large coelomic cavity. The muscular system is entirely formed prior to and independent of coelomic or nephridial anlagen. Visceral and somatic mesoderm already separate in the caudal body region. Transitory segmental clusters of mesodermal cells are composed of somatic cells only and accordingly represent no “somites”. Our observations overall do not provide any support for the homology of coelomic cavities in annelids and arthropods.     

Ultrastructure of coelomic organization in annelids

Summary A comparative ultrastructural study of trunk segments of a variety of small polychaetes (body diameter less than 250 rn) was undertaken to determine whether small body size is correlated with deviations from the typical annelid pattern of coelomic organization. Peritoneum is never found covering oblique or parapodial muscles traversing the body cavity. Aside from this, two major patterns of body cavity organization are seen: 1. spacious body cavity with varying extent of peritoneal lining (complete, partial, absent) and 2. the body tending toward the acoelomate condition, as a result of the expansion of lining cells or the lack of initial cavity formation. The significance of these anatomical variations is discussed with respect to functional and phylogenetic considerations.Abbreviations BL Basal lamina - CC Coelenchyme - CM Circular muscle - CO Coelom - CT Cuticle - DM Dorsal mesentery - DV Dorsal blood vessel - EP Epidermis - HC Hemocyte - HD Hemidesmosome - IN Intestine - IS Intestinal sinus - LM Longitudinal muscle - NC Ventral nerve cord - NE Nephridium - OM Oblique muscle - OO Oocyte - PT Peritoneum - ST Stomach - VM Ventral mesentery - VV Ventral blood vessel     

The origin of the coelom in Brachiopoda and its phylogenetic significance

The origin of the mesoderm and the subsequent formation of the coelom in the larvae of the brachiopod species Notosaria nigricans and Calloria inconspicua is documented in detail at the ultrastructural level. During gastrulation, the blastocoel is completely displaced by the invaginating archenteron. Initial mesoderm formation was observed in late wedge-shaped to early three-lobed stages in both species. Proliferation of mesodermal cells from the archenteral epithelium mainly occurs in the dorsolateral (C. inconspicua) and caudolateral (N. nigricans) parts of the archenteral wall. Thus, a compact mesodermal cell mass pushes its way towards the subepidermal basal lamina. During further development of the larva, the mesoderm is separated from the archenteral epithelium by an extracellular matrix secreted frontad from behind. As a result, a single coelomic anlage is formed. The initial mesoderm in both species is of archenteral/endodermal origin. Considering endodermal origin as the crucial character for enterocoely, coelom formation through proliferation of a compact, endodermally derived mesodermal cell mass in Brachiopoda is clearly identified as enterocoely. Endodermal origin of mesoderm and, therefore, of the coelomic epithelium is hypothesised as a synapomorphy of Brachiopoda and Deuterostomia. As a consequence: (1) Brachiopoda and Deuterostomia are considered sister groups, (2) Brachiopoda group within Radialia and (3) lophophorates (”Tentaculata”) remain as a paraphyletic grouping. Accepted: 26 November 1999     

When an epithelium ceases to exist – an ultrastructural study on the fate of the embryonic coelom in Epiperipatus biolleyi (Onychophora, Peripatidae)

It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda.     

Functional anatomy of the respiratory system of Branchipolynoe species (Polychaeta, Polynoidae), commensal with Bathymodiolus species (Bivalvia, Mytilidae) from deep-sea hydrothermal vents

 The gills of three species of Branchipolynoe have been studied in order to better understand the morphological and anatomical adaptations of their respiratory system. These Polynoidae live commensally inside the pallial cavity of different species of Bathymodiolus (Mytilidae), found clustered near deep-sea hydrothermal vents and cold seeps, and which harbor chemolithoautotrophic bacteria in their gills. As the mussels exploit hydrothermal fluid, the pallial cavity is perfused with a sulfide-rich hydrothermal water. The gills of Branchipolynoe species are well-developed branched outgrows of the body wall, located on the parapodia, and filled with coelomic fluid. They do not contain blood vessels. Living animals are red, due to the presence of extracellular hemoglobins in the coelom. The gill epidermis is made of supporting cells and a few ciliated cells arranged in longitudinal rows along the branches. Myoepithelial and ciliated cells line the interior of the coelomic cavity which contains the respiratory pigments. Coelomic fluid circulation inside the gills and body cavity is probably facilitated by both the cilia and myoepithelial contractions. The cuticle, the epidermis, and the coelomic epithelium are completely devoid of bacteria. The gill surface areas per unit body weight and the minimum diffusion distances, between external milieu and coelomic hemoglobins, have been calculated and compared with data already obtained on vascular gills of littoral or hydrothermal species of Polychaeta. In Branchipolynoe species, the respiratory surface area is very large, similar to that of a free-living hydrothermal species Alvinella pompejana, and the minimum diffusion distance is short, similar to that of the littoral species Arenicola marina. Although the organization of these coelomic gills in Branchipolynoe species is totally different from that of usual vascular gills, their characteristics represent a unique and effective respiratory system in Polynoidae which has adapted to the hypoxic and sulfide-rich micro-habitat which probably holds in the mantle cavity of vent mussels. In the gill epidermis, numerous secondary and large compound lysosomes are present which might be involved in sulfide detoxification. Accepted: 5 August 1998     

The embryological origin of the juxtatesticular body in jawfishes (Opistognathidae)

A male accessory sex organ, termed the juxtatesticular body (JTB), is located in the posterior part of the trunk, outside the coelomic cavity, lying ventral to the urinary ducts and dorsal to the urinary bladder and testes in jawfishes. Its microscopic structure is unusual for an accessory sex organ because it is highly vascularized, organized in small follicles, and ductless. The embryological origin of the JTB and the development of the urogenital apparatus was studied in juveniles of Opistognathus whitehurstii and O. maxillosus . Both sexes possess a structure located outside the coelomic cavity in the posterior part of the trunk. In females this structure showed the same histological organization as the kidney, however in males it was different and recognized as the JTB. The degree of development of the JTB followed that of the testes, being represented in youngest recognizable males only by a small mass of mesenchymal cells while it was fully developed in males with spermatogenic testes. In most immature males renal structures, such as tubules and glomeruli, were found in the dorsal part of this structure. On the basis of anatomical and cytological features a nephrogenic origin for the JTB is proposed.     

The microscopic anatomy and ultrastructure of the contractile vessel in the sipunculan Themiste hexadactyla (Satô, 1930) (Sipuncula: Sipunculidea)

The microscopic anatomy and ultrastructure of the contractile vessel of the sipunculan Themiste hexadactyla (Satô, 1930) from Vostok Bay (the Sea of Japan) were studied by histological and electron microscope methods. The ultrastructural features of the internal (endothelium) and external (coelothelium) lining of the contractile vessel are described and illustrated. Numerous macromolecular filters, the so-called “double diaphragms,” were found in the external coelothelium facing the cavity of the trunk coelom. This suggests a possible filtration from the tentacular coelom into the trunk coelom though the contractile vessel wall. The microscopic peculiarities of the main tube of the contractile vessel and its numerous lateral branches twining around several internal organs are described in detail. The contractile vessel is polyfunctional: it can act as the main reservoir for the cavity fluid during the withdrawal of the tentacular crown and performs the functions of the distribution system in sipunculans.