A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa,Echinodermata) and its bearing on the evolution of early crinoids

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well‐preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of Eumorphocystis suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.     

The morphology of the lung of the African lungfish,Protopterus aethiopicus

Summary The lung of the African lungfish (Protopterus aethiopicus) is paired, long and cylindrical. It is situated on the dorsal aspect of the coelomic cavity ventral to the ribs. Much of the gas exchange tissue is found in the proximal aspect of the lung with the caudal part largely taken up by a centrally situated air-duct with a few large peripherally located alveoli. Interalveolar septa, arranged at differing hierarchical levels from the air-duct, subdivide the lung into alveoli, the gas exchange compartments. The alveolar surface is covered by some cells characterized by microvilli on their free surface, while others are devoid of such structures. The general organization of the lung of Protopterus aethiopicus is similar to that of the other genera of Dipnoi, Neoceratodus and Lepidosiren, with the interalveolar septa increasing the surface area for gas exchange through pulmonary compartmentation. The abundant septal smooth muscle fibres and elastic tissue may contribute to the physiomechanical compliance of the lung. The undifferentiated alveolar pneumocytes and the double capillary system, observed in Protopterus, in general appear to characterize the very primitive lungs of the lower air-breathing vertebrates.     

Ultrastructure of the somatic portion of the gonads in asteroids,with emphasis on flagellated-collar cells and nutrient transport

Somatic portions of gonads in two phanerozonian sea-stars, Ctenodiscus crispatus and Hippasteria phrygiana, were similar in all aspects of gross structure and histology seen previously in both forcipulate and spinulosan asteroids. For the first time, detailed ultrastructural observations have been made of cells and tissues that reveal several features believed to be of universal occurrence in the gonads of asteroids. These include flagellated-collar cells in the visceral peritoneum and other coelomically derived epithelia, muscular-flagellated-collar cells in the visceral peritoneum and genital coelomic (perihaemal) sinus, the digestion of collagen fibers by cells in the connective tissue layer, and the intimate relationship of the genital haemal sinus and the entire germinal epithelium. Structural and functional compartmentalization are discussed in relation to major activities of the gonad throughout the annual reproductive cycle. The distinctive ultrastructure and current generation of flagellated-collar cells found in the visceral peritoneum are analyzed relative to their role in nutrient transport to gonadal tissues. The single flagellum of each flagellated-collar cell beats in coordination with those on neighboring cells to produce extremely rapid, oriented currents of coelomic fluid. The form of beating in an individual flagellum is planar, and the resulting synchronized activity of many adjacent flagella is non-metachronal; both of these characteristic aspects of current production have, thus far, been encountered together only in the Echinodermata. Flagellated-collar cells are efficient in generating currents which mix contents of the coelomic fluid, and they can presumably supply themselves with nutrients. It is concluded that nutrients so obtained are generally not passed through the wall of the gonad to the germinal epithelium and, as a result, have little to do with nutrition of somatic and germinal cells of the germinal epithelium. Alternatively, well-developed genital portions of the haemal system of the sea-star are advanced as the major channels supplying nutrients to germinal epithelia during gametogenesis.     

Ultrastructure of the eyespot in three polychaete trochophore larvae (Annelida)

Summary The eyespot is structurally similar in trochophore larvae of Harmothoe imbricata, Serpula vermicularis and Spirobranchus giganteus. In the receptor cell parallel lamellae lie below a tuft of microvilli which extends into a hollow in one side of the pigment cell. In 1-eyed trochophores this space connects with the outside via a small pore. In eyes preserved during the day there is evidence of a membrane breakdown in both lamellae and microvilli as well as indications of a membrane-fragment disposal system involving the receptor cell, the pigment cell and the eyespot pore. The orientation of the eyespot of S. giganteus is the reverse of that found in S. vermicularis, a situation that may be associated with ecologically significant differences in behaviour.     

The Class Asteroidea (Echinodermata): Fossils and the Base of the Crown Group

Because of limited research, a generally accepted hypothesishas not emerged for the phylogeny of the Asteroidea. The fossilrecord is a potential source of needed data, although fossilasteroids are rare, and they tend to be poorly preserved. Emphasis in the taxonomy of both recent and fossil asteroidshas been on characters visible from the exterior, and paleontologistshave sought to fit even the most ancient (i.e., Ordovician)specimens into taxonomic ordinal schemes devised for recentasteroids. Animal form and arrangement of body wall ossiclesof Paleozoic asteroids can be similar to those of younger species,thereby suggesting close affinities, yet ambulacral arrangementsindicate clear separation of Paleozoic stem groups from thecrown group. Traits taken from the ambulacral column that mark crown-groupasteroids include presence of dorsal podial pores (which allowedtransfer of the ampullae to the arm interior), an offset arrangementof ambulacrals on the adambulacrals, and increased complexityof the articulation structures between ambulacrals and adambulacrals.Transfer of ampullae to the arm interior provided protectionand more space for ampullae within the arm, as well as spacewithin the furrow and between the ambulacral and adambulacralossicles for elaboration of the soft tissues that enhance armmotion.     

Scanning electron microscopy of the respiratory surfaces of Saccobranchus (=Heteropneustes) fossilis (Bloch)

Summary The gill secondary lamellae are generally covered with epithelial cells whose outer surfaces form numerous microvilli. The surface of the primary lamellae is characterised by microridges. A particular type of surface sculpturing seems to be associated with given cell boundaries.Further evidence for the derivation of the air tube and fans which guard its entrance by modification of the basic gill structure has been obtained from both the gross surface architecture and microstructure of the individual cell surfaces. Secondary lamellae are represented by stubby projections which generally have a biserial arrangement. The outer surfaces of the epithelia overlying the capillaries of these respiratory islets are coated with microvilli as in the secondary lamellae. On the other hand, the relatively smooth-surfaced lanes between groups of respiratory islets have a microridged surface similar to that of the primary gill lamellae.It is suggested that previous estimates of surface area, and consequently diffusing capacities of the air-breathing organ, have been low in view of the increased surface, due to both their gross and microstructure. Estimates for gill surface area may need very little correction as the spaces between the microvilli and microridges are probably filled with mucus under normal conditions.We thank Mr. John Clements for his excellent technical assistance and the Department of Botany, Bristol University for the use of their scanning electron microscope     

Ultrastructural observations on the gills of polychaetes

The gills of several polychaete species belonging to 9 families were studied by scanning and transmission electron microscopy. The surface epithelium is covered by a thin cuticle which is invaded by microvilli penetrating the epicuticle in certain species. Some epithelial cells bear cilia, others are mucus-producing cells. The ciliary cells may be arranged in rows and maintain a constant flow of water over the gills. The distance between external water and blood stream differs considerably according to the species investigated. InMalacoceros the gills are characterized by closed afferent and efferent subepithelial vessels, which correspond to tubular invaginations of the coelomic wall. These vessels are lined by the basement lamina of the coelothelial cells, which are of the epitheliomuscular type. The vessels are open in the gills of other polychaetes and release the blood stream into a system of spaces immediately below the epidermis (e.g. in the branchial lamellae ofPectinaria andTerebellides). In several species the blood comes into very intimate contact with the cuticle (e.g. in the gill filaments ofDendronereides), but also in these animals both are separated by a very small epidermal layer.Supported by DFG Sto 75/3-6.     

Morphological comparison of the ampullae of Lorenzini of three sympatric benthic rays

This study investigated and compared the morphology of the electrosensory system of three species of benthic rays. Neotrygon trigonoides, Hemitrygon fluviorum and Maculabatis toshi inhabit similar habitats within Moreton Bay, Queensland, Australia. Like all elasmobranchs, they possess the ability to detect weak electrical fields using their ampullae of Lorenzini. Macroscopically, the ampullary organs of all three species are aggregated in three bilaterally paired clusters: the mandibular, hyoid and superficial ophthalmic clusters. The hyoid and superficial ophthalmic clusters of ampullae arise from both dorsal and ventral ampullary pores. The dorsal pores are typically larger than the ventral pores in all three species, except for the posterior ventral pores of the hyoid grouping. Ampullary canals arising from the hyoid cluster possessed a quasi‐sinusoidal shape, but otherwise appeared similar to the canals described for other elasmobranchs. Ultrastructure of the ampullae of Lorenzini of the three species was studied using a combination of light, confocal and electron microscopy. All possess ampullae of the alveolar type. In N. trigonoides and M. toshi, each ampullary canal terminates in three to five sensory chambers, each comprising several alveoli lined with receptor and supportive cells and eight to 11 sensory chambers in H. fluviorum. Receptor cells of all three species possess a similar organization to those of other elasmobranchs and were enveloped by large, apically nucleated supportive cells protruding well into the alveolar sacs. The luminally extended chassis of supportive cells protruding dramatically into the ampullary lumen had not previously been documented for any elasmobranch species.     

Ultrastructure of coelomic lining in echinoderm podia: significance for concepts in the evolution of muscle and peritoneal cells

Summary Ultrastructural data are presented on the histological organization of coelomic lining in the podia of ten species of the five major groups of extant echinoderms. Further evidence of the incorporation of podial retractor muscle cells (myocytes) into a monociliated myoepithelial coelomic lining is provided. In the podia of the crinoid Nemaster rubinginosa and the ophiuroid Ophiophragmus wurdemani as well as in the feeding tentacles of the holothurian Leptosynapta tenuis, coelomic linings are organized as simple myoepithelia consisting of non-contractile peritoneal cells (peritoneocytes) and myocytes. Coelomic linings in the holothurian Thyonella gemmata, the echinoids Eucidaris cf. tribuloides and Lytechinus variegatus, and the asteroids Asterias forbesi and Astropecten sp. are pseudostratified or bipartite pseudostratified myoepithelia consisting of subapical myocytes and apically situated peritoneocytes. The ophiuroid podia of Ophioderma brevispinum and Ophiothrix angulata exhibit transitions from simple myoepithelia to partially pseudostratified epithelia. Intermediate forms between the extremes in myoepithelial organization also occur in the podial lining of single species (e.g. Eucidaris cf. tribuloides). These data supplement recent ultrastructural studies on the podial lining of echinoderms and, in conjunction with published ultrastructural data on the myoepithelial organization of other coelomic linings in echinoderms and in other coelomates, suggest myoepithelial organization of the coelomic lining is a plesiomorph feature in Bilateria.     

Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida)

Temereva, E.N., Malakhov, V.V. and Yushin, V.V. 2011. Ultrastructural study of oogenesis in Phoronopsis harmeri (Phoronida). —Acta Zoologica (Stockholm) 92 : 241–250. The successive stages of oogenesis in Phoronopsis harmeri were examined by electron microscopy methods. During the oogenesis, each oocyte is encircled by vasoperitoneal (coelomic) cells forming a follicle. The previtellogenic oocytes are small cells which accumulate ribosomes for future synthesis; their cytoplasm contains characteristic clusters of mitochondria and osmiophilic particles resembling a germ plasm of other metazoans. The cytoplasm of the vitellogenic oocytes includes numerous mitochondria, cisternae of the rough endoplasmic reticulum, Golgi bodies and annulate lamellae. The synthesis of three types of inclusions was observed: strongly osmiophilic granules (lipid droplets) as a prevalent component, distinctly larger granules surrounded by membrane (proteinaceous yolk) and numerous large vesicles with pale flocculent content. No inclusions which could be unequivocally interpreted as the cortical granules were detected. The surface of the vitellogenic oocytes is covered by microvilli which increase in number and length during development. The oogenesis in Phoronida may be interpreted as follicular because of close association of oocytes with the vasoperitoneal tissue. However, well‐developed synthetic apparatus together with a strongly developed microvillous surface and absence of endocytosis indicate a clear case of autosynthetic vitellogenesis. Thus, in phoronids, there is a combination of simply developed follicle and autosynthesis that, apparently, is plesiomorphic character.     

Fine Structure of Tentacles,Arms and Associated Coelomic Structures of Cephalodiscus gracilis (Pterobranchia,Hemichordata)

The tentacles of the pterobranch Cephalodiscus, a hemisessile ciliary feeder, originate from the lateral aspects of the arms and are covered by an innervated epithelium, the majority of its cells bearing microvilli. Each side of a tentacle has two rows of ciliated cells and additional glandular cells. The coelomic spaces in the tentacles are lined by cross-striated myoepithelial cells, allowing rapid movements of the tentacles. One, possibly two, blood vessels accompany the coelomic canal. On their outer sides the arms are covered by a simple ciliated epithelium with intra-epithelial nerve fibres; the inner side is covered by vacuolar cells. On both sides different types of exocrine cells occur. The collar canals of the mesocoel are of complicated structure. Ventrally their epithelium is pseudostratified and ciliated; dorsally it is lower and forms a fold with specialized cross-striated myoepithelial cells of the coelomic lining. Arms, tentacles, associated coelomic spaces and the collar canal of the mesocoel are considered to be functionally interrelated. It is assumed that rapid regulation of the pore width is possible and even necessary when the tentacular apparatus is retracted, which presumably leads to an increase of hydrostatic pressure in the coelom.     

Morphometric and ultrastructural comparison of the olfactory system in elasmobranchs: The significance of structure–function relationships based on phylogeny and ecology

This study investigated the relationship between olfactory morphology, habitat occupancy, and lifestyle in 21 elasmobranch species in a phylogenetic context. Four measures of olfactory capability, that is, the number of olfactory lamellae, the surface area of the olfactory epithelium, the mass of the olfactory bulb, and the mass of the olfactory rosette were compared between individual species and groups, comprised of species with similar habitat and/or lifestyle. Statistical analyses using generalized least squares phylogenetic regression revealed that bentho‐pelagic sharks and rays possess significantly more olfactory lamellae and larger sensory epithelial surface areas than benthic species. There was no significant correlation between either olfactory bulb or rosette mass and habitat type. There was also no significant difference between the number of lamellae or the size of the sensory surface area in groups comprised of species with similar diets, that is, groups preying predominantly on crustaceans, cephalopods, echinoderms, polychaetes, molluscs, or teleosts. However, some groups had significantly larger olfactory bulb or rosette masses than others. There was little evidence to support a correlation between phylogeny and morphology, indicating that differences in olfactory capabilities are the result of functional rather than phylogenetic adaptations. All olfactory epithelia exhibited microvilli and cilia, with microvilli in both nonsensory and sensory areas, and cilia only in sensory areas. Cilia over the sensory epithelia originated from supporting cells. In contrast to teleosts, which possess ciliated and microvillous olfactory receptor types, no ciliated olfactory receptor cells were observed. This is the first comprehensive study comparing olfactory morphology to several aspects of elasmobranch ecology in a phylogenetic context. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Vibrio splendidus persister cells induced by host coelomic fluids show a similar phenotype to antibiotic-induced counterparts

Persister cells are dormant variants of regular cells that are multidrug tolerant and have heterogeneous phenotypes; these cells are a potential threat to hosts because they can escape the immune system or antibiotic treatments and reconstitute infectious. Skin ulcer syndrome (SUS) frequently occurs in the sea cucumber (Apostichopus japonicus), and Vibrio splendidus is one of the main bacterial pathogens of SUS. This study found that the active cells of V. splendidus became persister cells more readily in the presence of A. japonicus coelomic fluids. We showed that the A. japonicus coelomic fluids plus antibiotics induce 100-fold more persister cells in V. splendidus compared with antibiotics alone via nine sets of experiments including assays for antibiotic resistance, metabolic activity, and single-cell phenotypes. Furthermore, the coelomic fluids-induced persister cells showed similar phenotypes as the antibiotic-induced persister cells. Further investigation showed that guanosine pentaphosphate/tetraphosphate (henceforth ppGpp) and SOS response pathway involved in the formation of persister cells as determined using real-time RT-PCR. In addition, single-cell observations showed that, similar to the antibiotic-induced V. splendidus persister cells, the coelomic fluids-induced persister cells have five resuscitation phenotypes: no growth, expansion, elongation, elongation and then division, and elongation followed by death/disappearance. In addition, dark foci formed in the majority of persister cells for both the antibiotic-induced and coelomic fluids-induced persister cells. Our results highlight that the pathogen V. splendidus might escape from the host immune system by entering the persister state during the process of infection due to exposure to coelomic fluids.     

Surface morphology and functions of pharyngeal structures in the larval lamprey Petromyzon marinus

To gain insight into the multiple functions of a complex biological structure, the morphology of the pharynx of the larva (ammocoete) of the lamprey Petromyzon marinus was investigated with scanning electron microscopy and histochemistry (PAS and Alcian blue). Features studied include the gills, the parabranchial chambers external to the gills, intrapharyngeal ciliary tracts, the ridged pharyngeal roof, the floor, and the intrapharyngeal taste buds. Significant findings are: (1) All (nonciliated) cells lining these structures are covered with microvilli or microridges. The pattern and packing density of these membrane features vary among different pharyngeal structures. The lumenar membranes of pharyngeal lining cells overlie a mucous prosecretion in the apical cytoplasm, suggesting that the microvilli/ridges on these membranes function to anchor mucus. (2) Patterns of microvilli/ridges on the gill respiratory lamellae differ among ammocoetes of different species. (3) Pharyngeal osmo-regulatory cells (“chloride cells”) could not be identified on the basis of the microvillus/ridge pattern. (4) Two types of ciliary tracts are present within the pharynx. One has tall (x = 13 μm) and densely packed cilia, whereas the other has shorter (x = 7 μm) and less densely packed ones. Because mucus covers both types of tracts their function appears to involve the transport of mucus. (5) Food particles were found on the lateral surfaces of the gill filaments and on the surfaces of the parabranchial chambers. It appears that goblet cells in the epithelia of these regions secrete mucus in which the particles are trapped.     

Scanning electron microscopy of epithelial surfaces of the sea anemone Acontiophorum niveum (Phylum Cnidaria): Class anthozoa

Epithelium of the sea anemone Acontiophorum niveum is modified into four general arrangements of microappendages: (1) uniform microvilli covering pedal disc and column, (2) an interspersion of microvilli, ciliary cones, and kinocilia on tentacles, (3) flagella among an understory of microvilli from the oral disc, actinopharynx, filaments, and acontia, and (4) sparse flagella among irregular microvilli from endoderm. These arrangements are similar to those described previously in the epithelia of other anthozoans.     

The paracrystalline structure of an insect rhabdomere (Calliphora erythrocephala)

Summary By use of a modified fixation technique, the receptor cells of the compound eye of the blowfly Calliphora erythrocephala were found to contain a regular, paracrystalline array of alternating rows of hexagonally shaped microvilli. The receptor cells R1 to R6 have a cell-specific number of microvilli per row in a cross section. Every microvillus has a filament cluster connecting the axial skeleton with the microvillar membrane. This cluster is preferentially right-left oriented relative to the longitudinal axis of the microvillar array. Three adjacent microvilli are interconnected by an electron-dense substance. A mirroring technique indicated that this intermicrovillar structure consists of three subunits, although these subunits could not be conclusively demonstrated by classical densitometry or image subtraction techniques. The electron-dense substance can be seen in all cross sections of the proximal and distal parts of the microvilli. They are cylindrical structures separating the microvilli along their entire length. It is suggested that these cylindrical aggregates contain an enzymatic complex separating the rhodopsin-containing microvillar membrane into six compartments.     

On the ampullary organs of the South-American paddle-fish Sorubim lima (Siluroidea,Pimelodidae)

Summary Ampullary organs were found in the epidermis of the paddle-fish Sorubim lima; they are distributed all over the skin surface of the fish but are particularly densely grouped in the head region and on the dorsal surface of the paddle. Histological and electron microscopical observations show that their structure is similar to the type of cutaneous ampullary organs characteristic of other Siluroidea. Composed of a relatively large mucus-filled ampulla, the organ possesses a short and narrow canal which leads to the outer epidermal surface. The wall of the ampulla is formed of several layers of flat epidermal cells. In general four sensory cells, each one surrounded by supporting cells, compose the sensory epithelium at the bottom of the ampulla. The inner surface of the sensory cells in contact with the ampullary mucus bears only microvilli. The contact between the nerve endings and the sensory cells show the characteristic structure of an afferent neuro-sensory junction. Two ampullae are innervated in some cases by the same afferent nerve fibre.The author expresses her gratitude to Dr. Szabo for his scientific advice during her stay in Gif sur Yvette     

Some evidence for the ampullary organs in the European cave salamander Proteus anguinus (Urodela,Amphibia)

Summary The multicellular epithelial organs in Proteus anguinus, which Bugnion (1873) assumed to be developing neuromasts, have been analyzed by lightand electron-microscopy. Their fundamental structure consists of single ampullae with sensory and accessory cells with apical parts that extend into the pit of the ampulla, and of a short jelly-filled canal connecting the ampulla pit with the surface of the skin. The organs are located intra-epithelially and are supported by a tiny dermal papilla. The cell elements of sensory epithelium are apically linked together by tight junctions. The free apical surface of the sensory cell bears several hundred densely packed stereocilia-like microvilli whereas the basal surface displays afferent neurosensory junctions with a pronounced round synaptic body. The compact uniform organization of the apical microvillous part shows a hexagonal pattern. A basal body was found in some sensory cells whereas a kinocilium was observed only in a single cell. The accessory cells have their free surface differentiated in a sparsely distributed and frequently-forked microvilli. The canal wall is built of two or three layers of tightly coalescent flat cells bordering on the lumen with branching microvilli. The ultrastructure of the content of the ampulla pit is presented.In the discussion stress is laid on the peculiarities of the natural history of Proteus anguinus that support the view that the morphologically-identified ampullary organs are electroreceptive. The structural characteristics of ampullary receptor cells are dealt with from the viewpoint of functional morphology and in the light of evolutionary hypotheses of ampullary organs.     

Ultrastructure of tentacles in the sipunculid worm <Emphasis Type="Italic">Thysanocardia nigra</Emphasis> Ikeda, 1904 (Sipuncula)

The ultrastructure of the tentacles was studied in the sipunculid worm Thysanocardia nigra. Flexible digitate tentacles are arranged into the dorsal and ventral tentacular crowns at the anterior end of the introvert of Th. nigra. The tentacle bears oral, lateral, and aboral rows of cilia; on the oral side, there is a longitudinal groove. Each tentacle contains two oral tentacular canals and an aboral tentacular canal. The oral side of the tentacle is covered by a simple columnar epithelium, which contains large glandular cells that secrete their products onto the apical surface of the epithelium. The lateral and aboral epithelia are composed of cuboidal and flattened cells. The tentacular canals are lined with a flattened coelomic epithelium that consists of podocytes with their processes and multiciliated cells. The tentacular canals are continuous with the radial coelomic canals of the head and constitute the terminal parts of the tentacular coelom, which shows a highly complex morphology. Five tentacular nerves and circular and longitudinal muscle bands lie in the connective tissue of the tentacle wall. Similarities and differences in the tentacle morphology between Th. nigra and other sipunculan species are discussed.Original Russian Text Copyright © 2005 by Biologiya Morya, Maiorova, Adrianov.