Transformations of the axial complex of ophiuroids as a result of shifting of the madreporite to the oral side

In comparison with Asteroidea, the axial complex of ophiuroids has some important features, which are the result of shifting of the madreporite from the aboral side to the oral side. In contrast to Asteroidea, the stone canal of ophiuroids connects with the water ring from the outside, not from the inside. In Ophiuroidea, the somatocoelomic perihaemal coelom is closer to the mouth than the axocoelomic ring. The water ring of ophiuroids is shifted to the oral side relative to the perihaemal coelomic rings. The genital coelom and gastric haemal ring are located on the outer side of the axial complex, whereas in Asteroidea, they are located on the inner side. The pericardial part of the axial organ is situated on the oral side. The interradial sections of the genital coelom and genital haemal ring are descended to the oral side. Our hypothesis considers that the ancestors of ophiuroids turned the aboral side of the animal to the substratum. It caused shifting of the madreporite to the oral side and closing of the anus.     

Axial complex of Crinoidea: Comparison with other Ambulacraria

The anatomy of Crinoidea differs from that of the other modern echinoderms. In order to see, whether such differences extend to the axial complex as well, we studied the axial complex of Himerometra robustipinna (Himerometridae, Comatulida) and compared it with modern Eleutherozoa. The axial coelom is represented by narrow spaces lined with squamous coelothelium, and surrounds the extracellular haemocoelic lacunae of the axial organ. The latter is located, for the most part, along the central oral-aboral axis of the body. The axial organ can be divided into the lacunar and tubular region. The tubular coelomic canals penetrating the thickness of the axial organ have cuboidal epithelial lining, and end blindly both on the oral and aboral sides. The axial coelom, perihaemal coelom, and genital coelom are clearly visible, but they connect with the general perivisceral coelom and with each other via numerous openings. The haemocoelic spaces of the oral haemal ring pass between the clefts of the perihaemal coelom, and connect with the axial organ. In addition, the axial organ connects with intestinal haemal vessels and with the genital haemal lacuna. Numerous thin stone canaliculi pierce the spongy tissue of the oral haemal ring. They do not connect with the environment. On the oral side, each stone canaliculus opens into the water ring. The numerous slender tegmenal pores penetrate the oral epidermis of the calyx and open to the environment. Tegmenal canaliculi lead into bubbles of the perivisceral coelom. Some structures of the crinoid axial complex (stone canaliculi, communication between different coeloms) are numerous whereas in other echinoderms these structures are fewer or only one. The arrangement of the circumoral complex of Crinoidea is most similar to Holothuroidea. The anatomical structure and histology of the axial complex of Crinoidea resembles the “heart-kidney” of Hemichordata in some aspects.     

The morphology of the axial complex and associated structures in Asterozoa (Asteroidea,Echinoidea, Ophiuroidea)

The representatives of Asterozoa (Asteroidea, Echinoidea, and Ophiuroidea) have a similar structural plan of the axial complex with minor differences within each class; this structural scheme substantially differs from that in Crinozoa and Holothurozoa. The axial complex consists of the coelomic organs and the haemocoel (blood) structures, which are morphologically and functionally integral. The coelomic organs are the stone canal, axial coelom, perihaemal coeloms (axocoel perihaemal ring and somatocoel perihaemal ring), water ring, and pericardial and genital coeloms. These organs are closely associated with the epigastric and hypogastric coeloms and with the perioral coelomic ring. The haemocoel structures of the axial complex include the oral haemal ring, heart, axial organ, genital haemal ring, and gastric haemal ring. The epineural canals of echinoids and ophiuroids are of a noncoelomic nature. They are formed by the invagination of the ectoneural cord and closing of the epidermis above it. The possible functions of the axial complex in Asterozoa are blood circulation and excretion.     

The Structure and Origin of the Plasmodium of Rhopalura ophiocomae(Phylum Orthonectida)

Abstract Adult orthonectids develop from germinal cells within a cytoplasmic matrix called a plasmodium. This is generally assumed to be formed by the parasite. In the case of Rhopalura ophiocomae, which lives in the brittle star Amphipholis squamata, the plasmodia occupying the perivisceral coelom are closely associated with the walls of the genital bursae or the gut, and they are covered by peritoneum. They have been reported to contain scattered small nuclei distinct from those within germinal cells, embryos, and adults, but the results of the present study indicate that such nuclei probably do not exist. Furthermore, electron micrographs show that some plasmodia are in continuity with the cytoplasm of contractile cells that lie beneath the peritoneum of a genital bursa or the gut of the host. The matrix of a plasmodium of R. ophiocomaeappears, therefore, to consist of cytoplasm of a contractile cell. It is proposed that after a contractile cell has been entered by an infective cell of the parasite, it hypertrophies, bulging progressively farther into the perivisceral coelom and lifting up the peritoneum, which remains in intimate contact with it.     

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral–aboral axis, involving local mesoderm–ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left–right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal–vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom–vestibule interaction that establishes the adult oral–aboral axis.     

Haemal and coelomic circulatory systems in the arms and pinnules ofFlorometra serratissima (Echinodermata: Crinoidea)

Summary The haemal and coelomic circulatory systems in arms and pinnules of a stalkless crinoid are described by transmission electron microscopy, and the coelomic topography is revealed by scanning electron microscopy of corrosion casts and peritoneal surfaces. In addition, the route of the coelomic circulation in the living crinoid is shown by injection of carmine particles, and sites of peritoneal phagocytosis are demonstrated by injection of latex beads. The most important morphological findings are: the controversial hyponeural circulation is haemal and not coelomic; peritoneal ciliation is general and not limited to the cells of the ciliated pits; and occur smooth muscle cells occur below the peritoneum. Carmine particles injected into the central body coelom rapidly travel outward toward the arm and pinnule tips via the aboral canals; the particles return to the central body via the subtentacular canals. Latex beads injected intracoelomically are taken up by peritoneal cells throughout the subtentacular, genital and aboral canals. The possible functions of the haemal and coelomic circulatory systems of crinoids are discussed.     

The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model

Morris, V.B., Selvakumaraswamy, P., Whan, R., and Byrne, M. 2011. The coeloms in a late brachiolaria larva of the asterinid sea star Parvulastra exigua: deriving an asteroid coelomic model. —Acta Zoologica (Stockholm) 92 : 266–275. The coeloms and their interconnexions in a late pre‐metamorphic brachiolaria larva of a sea star are described from the series of images in the frontal, transverse and sagittal planes obtained by confocal laser scanning microscopy. A larval, brachial coelom connects with the coeloms of the adult rudiment that lie posteriorly. The connexion is through the anterior coelom, which lies over the head of the archenteron, to the right anterior coelom and then to the left posterior coelom through the ventral horn of the left posterior coelom. The right posterior coelom is a separate coelom. The hydrocoele is on the larval left side separated from other coeloms except for a connexion to the anterior coelom. On the larval right side, the anterior coelom and right anterior coelom connect with the pore canal that opens to the exterior at the hydropore. From these coeloms, we derived an asteroid coelomic model comprising the larval left and right coeloms linked over the head of the archenteron by a common anterior coelom. The asymmetry of the hydrocoele and the left posterior coelom on the left side linked through the common anterior coelom to the right side, with the external opening, translates into the oral and aboral coeloms of the adult stage. The coelomic model has application in the search for morphological homology between the echinoderm classes and the deuterostome phyla.     

Origin of Germ Cells and Early Differentiation of Gonads in the Starfish, Asterina pectinifera

The origin of the germ cells and the development of the genital system in the annually spawning starfish, Asterina pectinifera , were studied by light and electron microscopy. Characteristic germ cells were first characterized in gonads after spawning: the gonia are larger than somatic cells, have large nuclei (with electron-lucent nucleoplasm), and show mitochondrial aggregation associated with nuage (electron-dense bodies). In young starfish without gonads similar cells were detected in the haemal sinus, where they were termed primordial germ cells (PGCs). Brachiolariae and metamorphosed juveniles had a cellular cluster in the coelomic epithelium, near the hydroporic canal. The cluster was comprised of cells endowed with the above-mentioned characteristics of the germ cells. The germ cell counts indicated that PGCs migrate from the aboral haemal sinus near the hydroporic canal, through the haemal sinus to the gonads, where they settle, proliferate, and differentiate into gonia.     

The effect of age at metamorphosis on the transition from larval to adult suspension‐feeding of the slipper limpet Crepidula fornicata

Slipper limpets use different ciliary feeding mechanisms as larvae and adults. Veliger larvae of Crepidula fornicata developed part of the adult feeding apparatus, including ctenidial filaments, neck lobe, and radula, before metamorphosis, but ctenidial feeding did not begin until well after loss of the larval feeding apparatus (velum) at metamorphosis. Earlier initiation of ctenidial feeding by individuals that were older larvae when metamorphosis occurred suggests continued development toward ctenidial feeding during delay of metamorphosis. Early juveniles produced a ciliary current through the mantle cavity and moved the radula in a grasping action before they began to capture algal cells on mucous strands or form a food cord. Either early juveniles could not yet form mucous strands or they delayed their production until development of other necessary structures. The neck canal for transporting food from ctenidium to mouth cannot develop before velar loss. In their first feeding, juveniles fed much like the adults except that the neck canal was less developed and the path of the food cord toward the mouth sometimes varied. As suspension feeders, calyptraeids lack the elaborations of foregut that complicate transition to juvenile feeding for many caenogastropods, but a path for the food cord must develop after velar loss. Why individuals can initiate ctenidial feeding sooner when they are older at metamorphosis is not yet known. The juveniles became sedentary soon after metamorphosis and were not observed to feed by scraping the substratum with the radula, in contrast to the first feeding by juveniles of another calyptraeid species, observed by Montiel et al. ( 2005 ).     

On the Significance of Larval and Juvenile Morphology for Suggesting Phylogenetic Relationships of the Vestimentifera

SYNOPSIS. Examination of larvae and juveniles ofRidgeia hasrevealed the presence of a prototroch, metatroch and neurotroch.A complete digestive tract is present in juveniles, the esophagealportion of which passes through the brain. The major longitudinal,pulsatile blood vessel is on the side opposite the nerve cordand, along with the longitudinal blood vessel adjacent to thenerve cord, is situated in the medial mesentery separating lateralcoelomic cavities. The posterior body region is multisegmented,and the lateral cavities of each segment are separated by amedial mesentery. Newly forming segments exhibit paired spacesthat expand to form coelomic cavities of that segment. All ofthese characters are held in common with the Annelida and stronglysuggest that the nerve cord is ventral and the major pulsatileblood vessel is dorsal; the brain is, in effect, a circumesophagealganglion; and coelom formation is by schizocoely and mesodermalorigin is teloblastic. It is suggested that the Vestimentiferaare allied with the gastroneuralians and may well have arisennear the base of the phylogenetic line leading to the Annelida.Avenues of future research, likely to shed more light on theposition of the Vestimentifera in invertebrate phylogeny, aresuggested     

Ultrastructure of Axial Vascular and Coelomic Organs in Comasterid Featherstars (Echinodermata: Crinoidea)

Abstract The spongy body of Davidaster rubiginosa, D. discoidea, and Comactinia meridionalis, is an axial haemal plexus consisting of two structurally similar, but positionally distinct, regions: an oral circumesophageal part and an aboral part which lies lateral to the axial organ. The axial organ is a large axial blood vessel which is infiltrated by hollow cellular tubes lined with monociliated epithelial cells. The spongy body plexus is a tangle of small blood vessels overlain by podocytes and myocytes. The spongy body and the axial organ are situated in the axial coelom, which is confluent with the perivisceral coelom, the water vascular system, and the parietal canals. The parietal canals open to the exterior via ciliated tegmenal ducts and surface pores. The crinoid spongy body is morphologically similar to the axial gland of asteroids, ophiuroids, and echinoids (AOE). Although the axial glands of these three classes of echinoderms are mutually homologous structures, the homology of the crinoid spongy body and the AOE axial gland is questionable because of differences in organization and developmental origin. Alternatively, the crinoid spongy body may be homologous to asteroid gastric haemal tufts, which are podocyte-covered blood vessels suspended in the perivisceral coelom. The functional organization of the spongy body suggests a filtration nephridium and predicts an excretory function. An alternative hypothesis is that the spongy body is a site of nutrient transfer from the blood vascular system to the perivisceral coelom.     

An 83-kDa embryonic-type nuclear antigen is detected within the germinal vesicles of oocytes of the ascidianHalocynthia roretzi

Summary Monoclonal antibodies were raised against germinal vesicles which were isolated from fully grown oocytes of the ascidianHalocynthia roretzi. Immunoblot analyses revealed that one of the antibodies, designated Hgv-2, recognized a single band with a molecular weight of about 83 kDa. The antibody, visualized by indirect immunohistochemistry, reacted only with the germinal vesicles of oocytes and did not react with test cells, follicle cells, and other somatic cells of the gonad. During embryogenesis the antigenicity was found in interphase nuclei of all embryonic cells. The antibody did not react with chromosomes or the mitotic apparatus. The antigenicity was retained by interphase nuclei of larval cells, but it disappeared from nuclei of juveniles about 7 days after metamorphosis.     

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.     

Relationships Between Invertebrate Phyla Based on Functional-Mechanical Analysis of the Hydrostatic Skeleton

SYNOPSIS. The phylogeny of the major groups of deuterostomecoelomates—the chordates,hemichordates and echinoderms—isdiscussed based on a mechanical-functional analysis of the hydrostaticskeleton and associated structures. The basic approach is tofirst establish transformation series of individual featuresand of functional complexes of features and secondto determinetheir "Lesrichtung" by showing the direction of increased economy(i.e., better adaptation) with respect to environmental factors.It is argued that a metameric coelom is primitive with respectto an oligomeric one and that the ancestral form of the deuterostomecoelomates is a metameric, coelomate worm-like animal with acomplex set of circular, transverse andlongitudinal body muscles.The coelom plus the complex body musculature formed the hydrostaticskeleton.The sequence of structural modifications leading to chordatesis: (a) appearance of the notochord; (b) specialization of thedorsal longitudinal muscles with a reduction and disappearanceof the transverse and circular muscles; (c) simultaneous appearanceof the dorsal hollow nerve cord; (d) development of a postanaltail; and (e) appearance and specialization of the branchialbasket with gill slits as a filter feeding apparatus. The primitivechordate would be most similar to the lancelet (Acrania). Tunicatesare advanced chordates specialized forsessile life and lostmost chordate features in the adult, but retained them in thelarvae as adaptations for active dispersal. Enteropneusts (acornworms) are another advanced group specialized for burrowingin fine sediments and that evolved the anterior proboscis asa peristaltic burrowing organ. The notochord was lost as wasthe dorsal nerve cord and segmented conditionof the coelom.A collar originated as a means to prevent discharged water fromre-entering themouth. Pterobranchs arose from enteropneustlikeforms; their major structural changes are reduction of the branchialbasket and modification of the collar into tentacles which areassociated with life in a closed tube. Finally, echinodermsarose from a pterobranch-like ancestor by specializing for sessilelife and feeding with tentacles and by final loss of the branchialbasket. Groups such as the tunicates, hemichordates and echinodermscould be eliminated as ancestral forms within the deuterostomecoelomates because the evolution of acraniates and vertebratesfrom each of these groups would involve the appearance of gillslits before the notochord and/or the evolution of a metamericcoelom from an oligomeric one, both of which are exceedinglyimprobable. Central to the methods used to establish the transformationseries of features and their direction of evolutionary change(Lesrichtung) are functional (mechanical) analysis and adaptiveinterpretation of features; hence, functional-adaptive analysesare an integral and essential part of the methodology of phylogeneticinvestigation.     

Attachment and metamorphosis of the cheilo-ctenostome bryozoan bugula neritina (linné)

The structure, attachment and subsequent metamorphosis of larvae of the marine bryozoan Bugula neritina were studied by light and electron microscopy. Two points of larval anatomy are of special significance to proper interpretation of the metamorphosis:

• 1 Two cytologically similar blastemal tissues, each laden with free ribosomes, occur as parts of the apical organ complex. The upper blastema directly contacts the larval surface, forming the non-ciliated rows of the apical organ. The lower blastema is internal and is oral to and contiguous with the upper blastema.
• 2 The epidermal tissues of the larva are joined in the following sequence, beginning at the aboral pole: a. apical organ complex; b. apical-connecting cell; c. infolded pallial sinus epithelium; d. vesicular-connecting cell; e. aboral vesicular epithelium; f. corona; g. oral vesicular epithelium; and i., j., and k. internal sac neck, wall and roof regions.

The initial stages of metamorphosis involve a complex sequence of morphogenetic movements, including:

• 1 eversion of the internal sac, permanently attaching the larva to the substrate;
• 2 inrolling of the aboral vesicular epithelium, corona, oral vesicular and ciliated epithelia, and neck region of the internal sac into the larval interior; concomitantly the pallial sinus epithelium evaginates;
• 3 loss of connection between the invaginated tissues and the surface;
• 4 fusion of the pallial sinus epithelium with the wall region of the internal sac, maintaining the integrity of the body surface;
• 5 retraction of the apical organ complex and invagination of the pallial sinus epithelium with the simultaneous elevation of the internal sac wall region to the aboral pole.

At the conclusion of these events the preancestrular surface is covered by the wall and roof regions of the internal sac. Cells of the wall region form the epidermis of the body wall except for the attachment disc and secrete a cuticular exoskeleton that is secondarily calcified; the attachment disc is formed by the roof region of the internal sac. Internally, the ectodermal upper blastema differentiates into the lophophore and digestive tract of the ancestrular polypide, while the lower blastema forms the lining of the lophophoral coelom and the splanchnic (but not the somatic) lining of the visceral coelom. The visceral somatic peritoneum is formed from cells that may originate from the mesodermally derived pigmented cells of the larva to which they are similar in pigmentation and cytology. Such a composite derivation of a coelomic lining has not been described previously.     

Regeneration of a complex of structures of the ophiuroid <Emphasis Type="Italic">Amphipholis kochii</Emphasis> Lütken, 1872 (Ophiurae) after disk autotomy

Regeneration of structures (genital bars and ligaments) which enables ophiuroids to throw away the aboral part of the disc was studied in this work. The succession of disc separation during autotomy has been fixed. It has been revealed that upon restoration of genital bars and ligaments regeneration occurs due to migration of cells from tissues remaining after autotomy, but not due to differentiation of cell elements. The cell sources of regeneration of the studied structures are fibroblasts and sclerocytes.     

Coelomogenesis during the abbreviated development of the echinoid Heliocidaris erythrogramma and the developmental origin of the echinoderm pentameral body plan

The development of the coeloms is described in an echinoid with an abbreviated larval development and shows the early morphogenesis of the coeloms of the adult stage. The development is described from images obtained by laser scanning confocal microscopy. The development in Heliocidaris erythrogramma is asymmetric with a larger left coelom forming on the larval-left side and a smaller right coelom forming on the larval-right side. The right coelom forms after the development of the left coelom is well advanced. The hydrocoele forms from the anterior part of the left coelom. The five lobes of the hydrocoele from which the pentamery of the adult derives take shape on the outer, distal wall of the anterior part of the left coelom. The hydrocoele separates from the more posterior part of the left coelom, which becomes the left posterior coelom. The lobes of the hydrocoele are named, based on the site of the connexion of the stone canal to the hydrocoele. The mouth is assumed to form by penetration through only the outer, distal wall of the hydrocoele and the ectoderm. Both larval and adult polarities are evident in this larva. A comparison with coelomogenesis in the asteroid Parvulastra exigua, which also has an abbreviated development, leads to predictions of homology between the echinoderm and chordate phyla that do not require the hypothesis of a dorsoventral inversion event in chordates.     

The Fine Structure of the Trophont and Stages in Telotroch Formation in Circolagenophrys ampulla (Ciliophora,Peritrichida)1

ABSTRACT. Ultrastructural studies of the trophont of the epizooic loricate peritrich, Circolagenophrys ampulla, show that the body conforms to the basic peritrich pattern. The lorica is dome-shaped, and the trophont is joined to it by attachment organelles. A single row of barren aboral kinetosomes is present. In telotroch formation, as cytokinesis proceeds, a band of aboral kinetosomes develops, running posteroventrally in an arc from the base of the epistomial disc. In one instance, postciliary microtubules were seen associated with the kinetosomes of the adoral polykinety in a dividing organism. In the fully developed telotroch there are several distinctive structures. In the midaboral region there is a scopula with numerous barren kinetosomes in the epiplasm underlying the pellicle. Surrounding the rim of the aboral surface is a tripartite fringe which overlies the base of the aboral ciliary girdle. The outer layer of this fringe contains regularly spaced electron-dense striations and the middle region contains microfilaments. The aboral ciliary girdle forms a complete ring. It is composed of diagonal rows of kinetosomes, 8–9 in each row. Striated fibers run between the rows of kinetosomes. They bend at the ends of the rows and continue for some distance below the outer rim of the aboral surface. Running beside each striated fiber is a band of paracrystalline material. Several distinctive structures are associated with the kinetosomes and striated fibers of the aboral girdle. In the telotroch many of the adoral cilia are absent but the adoral kinetosomes are still present. The possible functions of the specializations of the aboral surface in settlement of the telotroch, and the relationship between telotroch formation and the molting behavior of the crustacean host are discussed.