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.     

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.     

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.     

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.     

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.     

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.     

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).     

The fine structure of the axial organ of the feather star Nemaster rubiginosa (echinodermata: crinoidea)

The fine structure of all the constituent cell type of the crinoid axial organ is described (coelomic epithelial cells, muscles, free cells, gland cells and neurons) ; also described is the fine structure of the extracellular haemal fluid. The gland cells of the glandular tubules of the axial organ have the characteristic fine structure of protein exporting cells and may produce granular and filamentous components of the haemal fluid. The neurons (perikarya and axons) of the axial organ may possibly be neurosecretory, since they are filled with electron-dense granules. Homologies between the axial organs of different echinoderm classes are discussed.     

Zum Nachweis des Axialkomplexes bei Holothurien (Echinodermata)

On the basis of a histological investigation concerning 16 species of 4 holothurian orders, the homology of the so-called problematic canal with the axial sinus on the one side and the dorso-axial haemal strand with the axial gland on the other side is discussed. Combining these results with former investigations, the following conclusions are drawn. There is no axial complex as an organic unit represented within the class Holothuroidea. Nevertheless, this axial complex, formerly thought to be missing, is locally disintegrated into two parts, which are isolated from each other within the dorsal mesentery. One part is the stone canal—protocoelampulla—the other one the axial sinus—axial gland. Within the holothurians there is a tendency towards reduction of the protocoelampulla and the axial sinus. This recently aberrant structure of the axial complex can be deduced from an echinoid "plan".     

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 role of the coelom as a hydrostatic skeleton in lingulid brachiopods

Anatomical and experimental studies of the perivisceral coelom and pedicel of Lingula ana lina indicate that the coelomic fluid functions as a hydrostatic skeleton in respect of valve and pedicel movements, valve opening always being associated with positive pressures. The perivisceral coelom is surrounded by a body wall containing circumferential muscle fibres, whilst all muscles passing between the valves (principally adductor and oblique fibres) are located within the body wall. These two sets of muscles function similarly to the circular and longitudinal muscles of a classical hydrostatic skeleton.
Pressure recordings from the lumen of the pedicel and perivisceral coelom, during opening or rotary movements of the valves, were similar and showed pressure pulses of up to 0.8 kPa. During the initial stages of burrowing, pulses of up to 2.5 kPa were observed when the valves were being pressed into the sand. These values are well within the capability of the circumferential muscles of the body wall.     

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.     

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.     

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.     

Anatomy of the ovaries of the starfish Asterias rubens (Echinodermata)

Summary The ovaries of the starfish Asterias rubens were studied histologically and ultrastructurally. The reproductive system in female specimens consists of ten separate ovaries, two in each ray. Each ovary is made up of a rachis with lateral primary and secondary folds: the acini maiores and acini minores. The ovarian wall is composed of an outer and an inner part, separated by the genital coelomic sinus. The ovarian lumen contains oocytes in various phases of oogenesis, follicle cells, nurse cells, phagocytosing cells and steroid-synthesizing cells.Oogenesis is divided into four phases: (i) multiplication phase of oogonia, (ii) initial growth phase of oocytes I, (iii) growth phase proper of oocytes I, and (iv) post-growth phase of oocytes I. The granular endoplasmic reticulum and the Golgi complex of the oocytes appear to be involved in yolk formation, while the haemal system, haemal fluid and nurse cells may also be important for vitellogenesis. The haemal system is discussed as most likely being involved in synchronizing the development of the ovaries during the annual reproductive cycle and in inducing, stimulating and regulating the function of the ovaries.Steroid-synthesizing cells are present during vitellogenesis; a correlation between the presence of these cells and vitellogenesis is discussed.     

The Development of the Brachiopod Crania (Neocrania) anomala (O. F. Müller)and its Phylogenetic Significance

The freely spawned eggs of Crania go through radial cleavage, embolic gastrulation, and the posteroventral part of the archenteron forms mesoderm through modified enterocoely. The blastopore closes in the posterior end of the larva. The ciliated, lecithotrophic larva has four pairs of coelomic pouches and three pairs of dorsal setal bundles. At metamorphosis, the larva curls ventrally by contraction of a pair of midventral muscles, which are extensions of the first pair of coelomic sacs; the larva attaches by the epithelium just behind the closed blastopore. The brachial valve is secreted by the middle part of the dorsal epithelium and the pedicle valve is secreted by the attachment epithelium. The second pair of coelomic sacs develop small attachment areas at the edge of the dorsal valve and become the lophophore coelom (mesocoel); the third pair of coelomic sacs become the body coelom (metacoel) with the adductor muscles. The posterior position of the closing blastopore is characteristic of deuterostomes. The ventral curving of the settling larva and the formation of both valves from dorsal epithelial areas indicate that the brachiopods have a very short ventral side as opposed to the phoronids. It is concluded that both groups have originated from a creeping ancestor with a straight gut.     

An ultrastructural study of relationships between the ovarian haemal system,follicle cells,and primary oocytes in the sea star,Asterias rubens

Summary The genital haemal sinus, present throughout the gonad wall of sea stars, is supposed to be the site of ultimate accumulation of nutrients for the germinal epithelium. Early vitellogenic pear-shaped oocytes are attached to this sinus by stalk-like processes. The ultrastructure of this association and of the oocyte-follicle cell complex is described with emphasis on mechanisms involved in oocyte nutrition.The genital haemal sinus, and sometimes portions of the surrounding genital coelomic sinus, contain a fine granular ground substance and amoeboid cells. Material similar to the haemal ground substance also fills vacuities in the inner basal laminae of the haemal sinus and intervenes between this layer and adjacent germinal and follicle cells in the ovarian lumen.Vitellogenesis is first detectable as numerous vacuoles accumulate within the oocyte-stalk near the haemal sinus; they contain flocculent material and often fuse with adjacent lysosome-like vacuoles. As vitellogenesis proceeds, oocytes develop complex and tenuous connections with the haemal sinus. These consist of a network of pseudopodia that interdigitate with thin sheet-like extensions of follicle cells. These cells are attached to the oolemma by microfilamentous processes and contain regularly arranged concentrations of glycogen granules and well developed rough endoplasmic reticulum.It is concluded, (1) that follicle cells provide each oocyte with a compartmentalized microenvironment within the ovarian lumen, (2) that such compartments are intimately associated with the nutrient laden haemal sinus, and (3) that nutritive and vitellogenic substances, derived extragonadally and stored temporarily in the ovarian wall, can pass through the oocyte-stalk.