Ultrastructure of Adult Gonads and Development and Structure of the Larva of Rhabdopleura normani (Hemichordata: Pterobranchia)

Sexually mature adults, embryos and larvae of the pterobranch Rhabdopleura normani from Bermuda were studied with light and electron microscopy. The sexes are separate among the zooids of a colony, but a given colony may contain females and males. In zooids of either sex the single gonad is associated with a large haemal sinus in the trunk sac and is displaced laterally (to the right or to the left). The wall of the gonad is composed of three layers: an outer metasomal peritoneum, an internal lining of germinal epithelium and an intervening genital haemal sinus. The mature gametes lie in the lumen within the gonad. The spermatozoon is characterized by an elongate nucleus, no obvious acrosome, a long mitochondrial filament in a midpiece appendix and a single flagellum with a 9+2 axoneme. Females brood 200 μm eggs and embryos in their distinctive, basally coiled tubes. The yolky eggs undergo radial cleavage and develop into ciliated, lecithotrophic, oblong larvae (400 μm in length) that are characterized by: (1) yellow coloration peppered with black pigment spots; (2) a deep ventral depression; (3) a posterior adhesive organ; (4) an anterior apical sensory organ; (5) an evenly ciliated epitdermis. The ventral depression is not invaginating endoderm, but is instead a glandular epithelium that evidently secretes the larval cocoon and the adult tube. Internally, the peritoneum of the coelomic cavities begins to split from the periphery of a large, central mass of yolky mesenchyme cells. The larva swims using cilia, but also undergoes contractions, evidently powered by the peritoneal cells, which constitute a myoepithelium. The discussion considers pterobranch affinities with other deuterostomes and with lophophorates.     

The prosicular stage of Rhabdopleura (Pterobranchia: Hemichordata)

After settling, the larva of Rhabdopleura surrounds itself with a collagenous dome. Later, the zooid breaks through the wall of the dome and builds the horizontal tube part of the coenecium on to the dome.
The dome is a layered structure, unknown in other parts of the coenecium. whereas the horizontal tube is made up of rings in the classical manner of the adult coenecium. The construction of these two parts is different. The techniques used to reinforce the horizontal tube show a marked similarity to the cortical bandages recently described in the fossil graptolites, and give support to the claim that they are ancestral to Rhabdopleura. There are two sorts of early horizontal tube, one is a straight tube, and the other is longer and coiled. The hole in the dome through which the zooid emerges to build the horizontal tube is probably produced by a chemical boring of the zooid, and supports the hypothesis that the zooids can bore holes in shells and corals.     

An intertidal Rhabdopleura (Hemichordata, Pterobranchia) from Fiji

Rhabdopleura has been discovered living in coral rubble on reefs in Fiji. The habitat is unusual, being the underside of coral boulders in the intertidal zone. Some features of the environment and the fauna associated with the Rhabdopleura are briefly described. The Rhabdopleura zooids exhibit other modes of tube building besides the regular cylindrical horizontal and erect tubes. The colony ramifies through interstices in the dead coral and the zooids can line larger cavities within the coral with coenecial tissue. It is probably these cavities, together with the overall porosity of coral, that retain enough water to keep the zooids alive while the intertidal reef flat is exposed to the air. Within the depths of the coral the black stolons are frequently naked. The species is probably R. normani Allman.     

Erect tube growth in Rhabdopleura compacta (Hemichordata: Pterobranchia) from off Start Point, Devon

Patterns of tube construction in the upright tubes of Rhabdopleura compacta are described. Tube building is seen to be a highly regular process, with growth extending over more than one season. Participation in the building of any one tube can involve multiple generations of zooids. Spatial awareness in a zooid adding new material to a pre-existing tube can be demonstrated. This shows that the construction of the tube is strictly determined, either by environmental or genetic mechanisms, rather than being a function of developing zooid morphology, as previously suggested, or random processes.     

Cephalodiscus reproductive biology (Pterobranchia,Hemichordata)

Sexually mature adults and embryos and larvae of Cephalodiscus nigrescens and C. gracilis were studied by light and electron microscopy. Contrary to claims in the literature, individual coenecial cavities are inhabited by colonies of up to 15 joined zooids and not by single individuals, which is important for the interpretation of the mode of life of the related fossil group the graptolites. Some aspects of the reproductive apparatus and reproduction in Cephalodiscus are reported. The ultrastructure of the spermatozoon is described for the first time. Coelom formation is by schizocoely. The structure of the larva at several developmental stages is illustrated. Not all fertilised eggs are destined to become motile larvae and some develop into zooids omitting the motile stage. The lumen of the oviduct is much larger than previously supposed. Spermatozoa are shed into the cavity of the coenecium. It is proposed that fertilisation takes place within the coenecium. The ultrastructure of the enigmatic black ‘Comma Body’ is described and a reproductive function is proposed. Budding takes place from a base common to several zooids. This base probably also serves as an attachment foot. Large masses of yolk have been discovered within the coelom of some zooids and muscle stalks. It is inconceivable that a colony of Cephalodiscus nigrescens could survive unless it spent most of its life outside the coenecium.     

The dormant buds of Rhabdopleura compacta (Hemichordata)

Rhabdopleura has an overwintering stage that consists of two layers of cells surrounding a central yolk mass. This cellular part is surrounded by a thick electron dense capsule which is secreted by the bud itself. The capsule is probably impervious and protective to its contents. Blood vessels join the buds to the zooids of the colony. They form the probable route of transfer of yolk from the zooids to the dormant bud. The capsule of the dormant bud has some structural features in common with the black stolon of the adult zooids. The black stolon is probably formed in a manner similar to that which made the fusellar fabric of the periderm of fossil graptolities.     

The habitat and behaviour of Cephalodiscus gracilis (Pterobranchia, Hemichordata) from Bermuda

Cephalodiscus gracilis lives in shallow water around Bermuda. The zooids secrete a transparent coenecium. Several zooids can be attached to a common point. The zooids may be of differing maturity, having from none to five pairs of arms. The mature zooids feed by extending their arms like meridians around a globe with the tentacles of adjacent arms interdigitating to make a spherical filter net. Feeding currents are induced by cilia. The mucus flows along the external surfaces of the arms, around the collar and into the mouth. The rejection current runs on the inside surface of the arms. The rejected material is stored in pellets near the arm tips. It is'flicked'away at intervals.
The larvae are found in densely pigmented stalks attached to the common sucker. The zooids also reproduce by budding.     

Graptolite (Hemichordata,Pterobranchia) preservation and identification in the Cambrian Series 3

Due to inadequate preservation, pterobranchs are often difficult to identify in the fossil record, and a better understanding of preservational modes and diagenetic and metamorphic effects is needed for their recognition. Pterobranch hemichordates are common in Cambrian Stage 5 and younger sedimentary rocks, but are frequently overlooked. Often, pterobranch hemichordate colonies have been considered to be algal remains or hydroids. Re‐examination of Cambrian Burgess Shale algae reveals that the genera Yuknessia and Dalyia can be recognized as putative early representatives of pterobranch hemichordates. Distinct fusellar construction of the individual zooidal tubes and branching of the creeping proximal part of the colonies are found in the morphologically similar rhabdopleurid pterobranch genus Sphenoecium. The erect tubes of Sphenoecium do not branch and can reach a length of several centimetres. The development of the fusellar construction in this taxon shows a highly irregular development of the suture patterns, but a fairly consistent height of the individual fuselli. The taxon is widely distributed in the Cambrian Series 3, but has regularly been identified as a hydroid or an alga. Sphenoecium wheelerensis from the Cambrian Wheeler Shale of Utah is described as new.     

Fossilization processes of graptolites: insights from the experimental decay of Rhabdopleura sp. (Pterobranchia)

Laboratory experiments documenting the decomposition pattern of extant organisms are used to reconstruct the anatomy and taphonomy of fossil taxa. The subclass Graptolithina (Hemichordata: Pterobranchia) is a significant fossil taxon of the Palaeozoic era, represented by just one modern genus, Rhabdopleura. The rich graptolite fossil record is characterized by an almost total absence of fossil zooids. Here we investigated the temporal decay pattern of Rhabdopleura sp. tubes, stolons and single zooids removed from the tubarium. Tubes showed decay after four days, when fuselli began to separate from the tube walls. This rapid loss may explain the absence of fuselli from some graptolite fossils. The black stolon did not show decay until day 155. One day after their removal, zooids quickly decomposed in the following temporal sequence: (1) tentacles; (2) ectoderm; (3) arms; (4) gut; (5) cephalic shield, leading to complete disappearance of recognizable body parts in the majority of experimental zooids within 64–104 h. The most resistant zooid features to decay (61 days) were black‐pigmented granules. These results indicate that tubes and the black stolon would persist for weeks across death, transport and burial, whereas a complete decay of zooid features occurs in few days, providing an explanation for the overall poor record of fossil graptolite zooids and suggesting that recorded silhouettes of fossil zooids may be attributed to fossil decay‐resistant pigments.     

Feeding and gut contents in Cephalodiscus nigrescens (Hemichordata, Pterobranchia) from the Weddell Sea

The gut of the pterobranch hemichordate Cephalodiscus nigrescens contains plankton of sizes from less than 1 μm to over 100 μm in diameter. Some of the smaller plankton are clumped together in spherical bolus that is mucus-bound. Most plankton types known from the habitat are respresnted amongst the gut contents.     

On the Fine Structure of the Alimentary Tract of Cephalodiscus gracilis (Pterobranchia,Hemichordata)

The U-shaped alimentary tract of Cephalodiscus is of exclusively epithelial structure; on the basis of fine structural criteria the entire tract can be divided into two large subdivisions: an anterior one with mouth, mouth cavity, pharynx and oesophagus, and a posterior one with stomach and intestine. The anterior subdivision is built up of a relatively uniform, innervated, pseudostratified, ciliated epithelium with mucus cells which are concentrated in the initial parts of the mouth cavity. Cilia and mucus presumably constitute a mechanism transporting food particles into the stomach. In the area of the gill slits specific vacuolated cells occur which may lend rigidity to the walls of the slits. The gastric epithelium consists of prismatic cells characterized by, among others, large inclusion bodies, which may represent digestive vacuoles, small dense rod-shaped granules and an elaborate system of microridges, at the base of which abundant endocytotic vesicles occur. The dorsal gastric pouch contains cells rich in rough ER and secretory granules, probably containing digestive enzymes. Thus morphological evidence points both to intra- and extracellular digestion. The intestinal epithelium resembles that of the stomach, however, it is lower, its organelles are fewer and it bears, beside cilia, mainly microridges, which towards its distal end become sparse. Both in the gastric and intestinal epithelium small granulated cells have been found, which presumably represent endocrine cells.     

The structures of the tentacles of Rhabdopleura compacta (Hemichordata) with special reference to neurociliary control

Summary The tentacle of Rhabdopleura compacta (Hemichordata) consists of two layers of cells surrounding a central coelomic cavity. The two layers of cells are separated by a cell free basement lamella.The tentacles on the arms of Rhabdopleura bear three longitudinal rows of cilia. The ciliated cells are closely associated with bundles of nerve fibres, and between some of the cells and nerve fibres there are synapses. The peripheral regions of the ciliated cells are joined to one another by desmosomes. Tonofibrils join some of these desmosomes to the kinetosomes of the cilia.The nerve fibres are confined to the ectodermal layer and the muscle cells to the layer of cells within the basement lamella. In the ectodermal layer besides ciliated cells there are mucus cells, densely pigmented cells, and green bodies. The function of these last two types of cells is secretory. Most of the epithelial cells have microvilli upon their free borders.I wish to thank Professor J. Z. Young F. R. S. for enthusiastic advice and encouragement. Dr. R. Bellairs generously provided the facilities for electron microscopy. Mr. R. Moss gave excellent technical and photographic assistance. Dr. A. Stebbing of the Plymouth Marine Biological Laboratory helped me to obtain and to identify the specimens. Professor D. W. James kindly allowed me to use his facilities for interference microscopy.     

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.     

Fine Structure of Heart,Pericardium and Glomerular Vessel in Cephalodiscus gracilis M'Intosh, 1882 (Pterobranchia,Hemichordata)

Heart, pericardium and glomerular vessel of Cephalodiscus gracilis have been studied with the electron microscope. The lumen of the heart is lined by a basal lamina and an associated epithelium, composed of myoepithelial cells with well developed thin and thick myofilaments. The heart is located in the pericardial cavity, which is deliminated by the pericardium. The latter is composed of two flat layers of myoepithelia with fused basal laminae. The outer layer of the pericardium is the protocoelomic lining, and the inner layer is the ‘parietal’ pericardial epithelium. The myoepithelium forming the heart wall can be considered to represent the ‘visceral’ pericardial epithelium. The spacious glomerular vessel is lined by a basal lamina, on which typical podocytes rest. These cells indicate that ultrafiltration takes place through the wall of the glomerular vessel. The lumen of the vessel contains fine granular material (presumably precipitated blood proteins), fibrils with a faint cross striation, suggesting that they represent collagen, and stellate cells, which in part line the vessel. Since ultrafiltration requires hydrostatic pressure, it is inferred that the blood flow is from the dorsal region then through the heart and into the glomerular vessel.     

Fine structure of eyespots in tornarian larvae (Phylum: Hemichordata)

Summary The eyespots of tornariae of enteropneusts (Ptychodera flava from Hawaiian waters and an unknown species from southern California) were studied by electron microscopy. An ocellus is composed of two types of cells: sensory and supportive. The former is characterized by a bulbous cilium (with 9+2 axoneme) at its distal end, one or sometimes two arrays of microvilli from its sides below the cilium, and a basal axon. The latter features large, clear vesicles which presumably contained the reddish-orange pigment seen in the ocellus of a living larva. Five-day old tornariae of P. flava are positively phototactic. Both cilium and microvilli may function as photoreceptors. The tornarian ocellus studied is compared with eyespots of other invertebrates, and the evolutionary significance of its putative photoreceptors is discussed.We acknowledge the kind assistance of Drs. Michael G. Hadfield, University of Hawaii, and Russel L. Zimmer, Santa Catalina Marine Biological Laboratory, and the support of grant 10292 from the USPHS.     

Structure of the digestive tract of tornaria larva inEnteropneusta (Hemichordata)

The ultrastructure of the digestive tract of tornaria larva of enteropneusts was investigated. It showed that the digestive tract consists of three parts: esophagus, stomach, and intestine. The esophagus epithelium consists of two types of multiciliated epithelial cells and solitary muscle cells. Axonal tracts and neurons were found in the ventral wall of the esophagus. The cardiac sphincter contains an anterior band of strongly ciliated cells and a posterior band of cells with long vacuolized processes which partition the sphincter lumen. The stomach consists of three cell types: (1) cells with electron-opaque cytoplasm, bearing a fringed border on their apical sides; (2, 3) sparse cells with electron-light cytoplasm and different patterns of apical microvilli. Cells of the pyloric sphincter bear numerous cilia and almost no microvilli. The intestine consists of three parts. The anterior part is formed of multiciliated cells which bear the fringed border. The middle part consists of flattened cells bearing rare cilia and vast numbers of mace-like microvilli. The posterior part of the intestine is formed of cells bearing numerous cilia and few microvilli. Muscle cells were not found in either stomach or intestine epithelium. One noticed that the structure of the digestive tract of enteropneust tornaria larva differs from that of echinoid pluteus larva.     

Morphology and development of a new species of Balanoglossus (Hemichordata: Enteropneusta: Ptychoderidae) from Shimoda, Japan

The morphology and development of a new species of the genus Balanoglossus belonging to the family Ptychoderidae are described in detail. This acorn worm was collected from the sandy seashore in the cove near Shimoda Marine Research Center, University of Tsukuba (Shimoda, Shizuoka Prefecture, Japan). This acorn worm is easily distinguished from other balanoglossids by a characteristic hepatic region. There are two kinds of hepatic saccules: large, dark-colored saccules in the anterior region and small, light-colored saccules in the posterior region. Between the two subregions of the hepatic region, there is a small region that has no or tiny saccules. This species does not form distinct burrows or mounds of casts. The breeding season is in winter. The process of embryogenesis from fertilization to metamorphosis was observed. A shift in seawater temperature from about 13 degrees C to about 9 degrees C induced gamete release. Two days after fertilization, embryos hatched and became typical feeding tornaria larvae. Two months after fertilization these larvae metamorphosed into juveniles and began their benthic life.     

Metamorphosis of Hydractinia echinata (Cnidaria) is caspase-dependent

Apoptotic cell death plays an important role in many developmental pathways in multicellular animals. Here, we show that metamorphosis in the basal invertebrate Hydractinia echinata (Cnidaria) depends on the activity of caspases, the central enzymes in apoptosis. Caspases are activated during metamorphosis and this activity can be measured with caspase-3 specific fluorogenic substrates. In affinity labelling experiments 23/25 kDa bands were obtained, which represented active caspase. Specific inhibition of caspase activity with caspase-3 inhibitors abolished metamorphosis completely, reversibly and in a dose-dependent manner. This suggests that caspase activity is indispensable for metamorphosis in Hydractinia echinata.     

Metamorphosis of cinctoblastula larvae (Homoscleromorpha, porifera)

The metamorphosis of the cinctoblastula of Homoscleromorpha is studied in five species belonging to three genera. The different steps of metamorphosis are similar in all species. The metamorphosis occurs by the invagination and involution of either the anterior epithelium or the posterior epithelium of the larva. During metamorphosis, morphogenetic polymorphism was observed, which has an individual character and does not depend on either external or species specific factors. In the rhagon, the development of the aquiferous system occurs only by epithelial morphogenesis and subsequent differentiation of cells. Mesohylar cells derive from flagellated cells after ingression. The formation of pinacoderm and choanoderm occurs by the differentiation of the larval flagellated epithelium. This is possibly due to the conservation of cell junctions in the external surface of the larval flagellated cells and of the basement membrane in their internal surface. The main difference in homoscleromorph metamorphosis compared with Demospongiae is the persistence of the flagellated epithelium throughout this process and even in the adult since exo- and endopinacoderm remain flagellated. The antero-posterior axis of the larva corresponds to the baso-apical axis of the adult in Homoscleromorpha.