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.     

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.     

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.     

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 aboral ring and the development of early gonads in the echinoid Paracentrotus lividus (Echinodermata, Echinoidea)

Abstract. The development of the genital apparatus is described for the echinoid Paracentrotus lividus. This apparatus derives from the aboral ring, an annular structure that includes an inconspicuous coelom and, in juveniles, the germinal rachis. The germinal epithelium grows out from the germinal rachis, and the gonadal wall and coelom in early (tubular) gonads share similarities with their equivalents in the aboral ring. The original germinal rachis regresses to form a genital cord one cell wide in late juveniles. A genital cord was observed in a few field-collected adult individuals (>40 mm test diameter).     

Ultrastructural evidence of the excretory function in the asteroid axial organ (Asteroidea,Echinodermata)

The ultrastructure of the axial organ of Asterias amurensis has been studied The organ is a network of canals of the axial coelom separated by haemocoelic spaces. The axial coelom is lined with two types of monociliary cells: podocytes and musculo-epithelial cells. Podocytes form numerous basal processes adjacent to the basal lamina on the coelomic side. Musculo-epithelial cells form processes running along the basal lamina. Some bundles of these processes wrapped in the basal lamina pass through haemocoelic spaces between neighboring coelomic canals. It is hypothesized that the axial organ serves for filtration of fluid from haemocoelic spaces into the axial coelom cavity, from which urine is excreted through the madreporite to the exterior.     

Fine structure of the axial complex of Sphaerechinus granularis (Lam.) (Echinodermata: Echinoidea)

Summary Three regions of the axial complex in Sphaerechinus granularis can be distinguished: 1) The axial organ which protrudes from one side of the axial sinus; the sinus septum which separates the sinus from the body cavity and encloses the stone canal; the pulsating vessel which runs along the inside of the axial organ. 2) The blindly-ending terminal sinus in which the pulsating vessel broadens out to the contractile terminal process. 3) The ampulla of the stone canal which connects the axocoel and water vascular system and which opens out through the madreporite.A single-layered, monociliated coelomic epithelium surrounds all regions of the axial complex. This epithelium contains smooth muscle cells at the contractile areas. Canaliculi, surrounded by basal lamina, are formed through infolding of epithelia; they end blindly in the fluid and connective tissue -matrix of the inner structures.The lacunae of the dorso-ventral mesentery connect the periesophageal and the perianal haemal ring with the axial organ. The axial organ contains many coelomocytes rich in pigment and granules. These coelomocytes are separated into compartments by elastic fibres. Phagocytosis of whole cells and transformational stages of coelomocytes suggest storage and degradation functions. An excretory function via the water vascular system is also suggested.     

The aboral haemal system of the sea star,Asterias rubens (Echinodermata,Asteroidea): An ultrastructural and histochemical study

Summary The aboral parts of the haemal system of the sea star Asterias rubens are described, based on light and electron microscopy. These parts are (1) the mesenteric strands along the pyloric caeca and the pyloric stomach, (2) the gastric haemal tufts, and (3) the aboral haemal ring. The mesenteric haemal strands are very limited in size and distribution and, therefore, do not seem to have a major function in nutrient translocation. The myoepithelial cells of the gastric haemal tufts have the typical features of choanocytes; their ultrastructural characteristics corroborate the possible absorptive role of the gastric haemal tufts. The myoepithelial cells of the aboral haemal ring often show distinct apical bulbs of cytoplasm suggesting apocrine secretion of PAS-positive materials which are found in the surrounding aboral coelomic ring. These cells contain large stores of particulate glycogen and typical 1–2 m electrondense globules.The ground substance of the haemal tissues, which contains collagen fibers, reticular fibrils, and numerous amoeboid phagocytes, has been analyzed histochemically. Sulfated glycosaminoglycans are almost completely absent; the predominant components are polysaccharides, proteins, and/or glycoproteins. Lipids have not been demonstrated.The possible functions of the haemal tissues and associated coelomic channels are discussed.     

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.     

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.     

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.     

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.     

Tracing the evolution of the holothurian body plan through stem‐group fossils

The fossil echinoderm Palaeocucumaria, from the early Devonian Hunsrück Slate of southwestern Germany, has been studied using both traditional techniques and X‐ray microtomography, and its anatomy clarified. Phylogenetic analysis shows that it is a stem‐group holothurian with a combination of characters that help understand how the modern (crown‐group) holothurian body plan developed. Echinoids and holothurians have evolved along different paths, by differential growth of the larval‐ and rudment‐derived body regions. Palaeocucumaria shows that late stem‐group holothurians had a water vascular organization with a single external madreporite and calcified stone canal leading to the aboral end of the peripharyngeal coelom, and five primary radial water vessels that gave rise to tentacle‐like tube‐feet. This fossil data, in combination with a molecular phylogeny based on 18 s‐like rRNA gene sequence data, is used to order evolutionary steps in the making of the crown‐group holothurian body plan. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 670–681.     

The Fine Structure of the Stalk of the Pentacrinoid Larva of a Feather Star,Comanthus japonica (Echinodermata: Crinoidea)

The stalk of the pentacrinoid larva of a feather star (Comanthus japonica) is described for the first time by transmission electron microscopy. One end of the stalk bears the calyx and the other end is cemented to the substrate by attachment cement consisting of a meshwork of 5 nm filaments. The stalk is supported by a scries of skeletal ossicles pierced by a central canal: short intercolumnal ligaments connect adjacent skeletal ossicles and central through-going ligaments run the length of the central canal. At the end of the stalk nearest the calyx, the chambered organ and the closely associated axial organ are histologically similar to those of adult crinoids. Presumed neurosecretory neurons are associated with the intercolumnal ligaments, and the following kinds of nerves run down the central canal: (1) a large stalk nerve in each of the five interradii; (2) smaller coelomic nerves in each of the five radii in association with the epithelium of tubular aboral extensions of the chambered organ; (3) a very small nerve associated with the aboral extension of the axial organ in the stalk axis. This axial organ extension is surrounded by a haemal channel. Because of the small size of the stalk, none of the nerves or the haemal channel were described in previous light microscopic studies. The discussion gives special attention to the controversial motility of the pentacrinoid stalk.     

Towards a Better Understanding of the Origins of Microlens Arrays in Mesozoic Ophiuroids and Asteroids

Echinoderms are characterized by a calcite endoskeleton with a unique microstructure, which is optimized for multiple functions. For instance, some light-sensitive ophiuroids (Ophiuroidea) and asteroids (Asteroidea) possess skeletal plates with multi-lens arrays that are thought to act as photosensory organs. The origins of these lens-like microstructures have long been unclear. It was recently proposed that the complex photosensory systems in certain modern ophiuroids and asteroids could be traced back to at least the Late Cretaceous (ca. 79 Ma). Here, we document similar structures in ophiuroids and asteroids from the Early Cretaceous of Poland (ca. 136 Ma) that are approximately 57 million years older than the oldest asterozoans with lens-like microstructures described thus far. We use scanning electron microscopy, synchrotron tomography, and electron backscatter diffraction combined with focused ion beam microscopy to describe the morphology and crystallography of these structures in exceptional detail. The results indicate that, similar to Recent light-sensitive ophiuroids, putative microlenses in Cretaceous ophiuroids and asteroids exhibit a shape and crystal orientation that would have minimized spherical aberration and birefringence. We suggest that these lens-like microstructures evolved by secondary deposition of calcite on pre-existing porous tubercles that were already present in ancestral Jurassic forms.     

Origin of bilateral-symmetrical animals (Bilateria)

The paper is an attempt to attack the old problem of the origin of Bilateria by the methods of evolutionary tetrad (i.e. combination of comparative anatomy, comparative embryology, paleontology, and molecular biology). Three groups of theories of classical comparative anatomy (planulod-turbellarian, archicoelomate, and metameric) are discussed. Comparative embryology brings out clearly that the ventral side of embryo comes from the blastoporal region in all groups of Bilateria (except Chordata, where the blastoporal region corresponds to the dorsal side that is come out of the upside-down morphology of chordates) and mouth and anus comes from the anterior and posterior ends of elongated blastopore. From the point of view of paleontology, some of vendian metazoans demonstrate transitional conditions between the Radiata and Bilateria. Vendian bilaterians are metameric organisms with normal or asymmetric position of segments and could be pictured as "bilateral coelenterates" creeping on the oral surface. In Cnidaria, the expression of homologues of "Brachyury", "goosecoid", and "fork head" genes are revealed in the circular region around the mouth. In Bilateria, these genes are expressed along the elongated blastopore and around the mouth and anus. These results support the old conception on the amphistomic origin of mouth and anus as well as the homology between the oral disc of cnidarians and ventral side of Bilateria. The combination of four mentioned approaches enables us to propose the conception of the origin of Bilateria from vendian bilateral coelenterates with numerous metameric pouches of gastral cavity. Bilaterian ancestors crawled on the oral disc (= ventral side). These ancestors gave rise to both phanerosoic cnidarians and triploblastic bilaterians. Cnidarian ancestors attached to bottom by the aboral pole with the resulting degradation of aboral nerve ganglion. Bilateral symmetry of anthozoans is considered to be primitive feature for cnidarians. In case of triploblastic Bilateria, the elongated blastopore closed in the middle and subdivided into mouth and anus (amphistomy) and gastral pouches separated from the central part of gastral cavity and transformed to metameric coelomic chambers. The primary bilaterians are supposed to be complicated organisms having coelom and segmentation. The complexity of primary Bilateria provides an explanation for the abundance of highly organized organisms (arthropods, mollusks etc.) in Cambrian time. It is postulated that Ctenophora is the only group recent eumetazoans with primary axial symmetry.