On triggering and control of cnidocyst discharge

Ultrastructural and in vivo observations in several planktonic coelenterates show that the cnidocystes, always associated with a receptor pole (the cnidocil itself or an accessory sensory cell) may have no link with the nervous system, or may be associated with it.

On the basis of these observations we propose that the cnidocytes of physonect siphonophores can be placed in two categories of receptor‐effectors both sensitive to stimuli received by the cnidocil. The cnidocytes of the first category, with defensive functions, localized on the pneumatophore, nectophores and bracts, have no link with the nervous system and are typical independent receptor‐effectors. In contrast, those of the second category whose distribution on the fishing tentacles and gastrozolds, implies predatory functions, are connected with the nervous system which may modulate or control their thresholds of excitability. Thus, in one and the same animal, depending on their function, the cnidocytes may or may not be linked with the nervous system, a difficult and long debated question in most of the cnidarians.     

Organization of the nervous system of physonectid siphonophores

Summary An antiserum to the sequence Arg-Phe-amide (RFamide) was used to stain the nervous systems of various physonectid siphonophores. In the stem of Nanomia bijuga, this antiserum stained an ectodermal nerve net, which was interrupted, at regular intervals, by transverse collars of neurons. Injection of Lucifer yellow into the giant axon of the stem showed that this axon was dye-coupled to an ectodermal nerve net that resembled the RFamide-positive network. Ectodermal nets of neurons were also found in the pneumatophore, gastrozooids, tentacles and tentilla. At the junctions of the pneumatophore, the gastrozooids, the dactylozooids and the gonozooids with the stem, and at the junctions of tentacles and tentilla, collars or rings of neurons occurred. The stem was connected to the phyllozooids and nectophores by muscular lamellae, which were bordered by chains of neurons. At the margin of the nectophores, an immunoreactive nerve ring was found. Connected to this ring and located in theseitliche Zapfen (sidely-located patche), were two agglomerations of nerve cells. On the upper side of the bell margin, positioned at 90° relative to the seitliche Zapfen, a delta-shaped neuronal structure was found. This structure was connected to the nerve ring and was associated with a muscle, which ran a short distance along the exumbrellar surface.The nervous systems of Agalma elegans, Forskalia edwardsi, Forskalia leuckarti and Halistemma rubrum resembled that of Nanomia bijuga in all major respects.     

Antisera to the sequence Arg-Phe-amide visualize neuronal centralization in hydroid polyps

Summary Antisera to the sequence Arg-Phe-amide (RF-amide) have a high affinity to the nervous system of fixed hydroid polyps. Whole-mount incubations of several Hydra species with RFamide antisera visualize the three-dimensional structure of an ectodermal nervous system in the hypostome, tentacles, gastric region and peduncle. In the hypostome of Hydra attenuata a ganglion-like structure occurs, consisting of numerous sensory cells located in a region around the mouth opening and a dense plexus of processes which project mostly radially towards the bases of the tentacles. In Hydra oligactis an ectodermal nerve ring was observed lying at the border of hypostome and tentacle bases. This nerve ring consists of a few large ganglion cells with thick processes forming a circle around the hypostome. This is the first direct demonstration of a nerve ring in a hydroid polyp.Incubation of Hydractinia echinata gastrozooids with RFamide antisera visualizes an extremly dense plexus of neuronal processes in body and head regions. A ring of sensory cells around the mouth opening is the first group of neurons to show RFamide immunoreactivity during the development of a primary polyp. In gonozooids the oocytes and spermatophores are covered with strongly immunoreactive neurons.All examples of whole-mount incubations with RF-amide antisera clearly show that hydroid polyps have by no means a diffuse nerve net, as is often believed, and that neuronal centralization and plexus formation are common in these animals. The examples also show that treatment of intact fixed animals with RFamide antisera is a useful technique to study the anatomy or development of a principal portion of the hydroid nervous system.     

Fine structure of the ciliated epidermis on the tentacles ofOwenia fusiformis (Polychaeta,Oweniidae)

Summary The ultrastructure of the epithelium on the oral surface of the tentacles ofOwenia fusiformis has been studied by light and electron microscopy. Unspecialized monociliated cells are the dominant cell type of the epithelium; however, biciliated epidermal cells, monociliated mucous cells and non-ciliated cells are also present. In all of the ciliated cells each cilium is provided with a diplosomal basal body and two striated rootlets. The cytological features of this unspecialized epithelium strongly resemble those of similar epithelia in the Phoronida, Brachiopoda and Hemichordata. So far,O. fusiformis is the only polychaete known to possess an unspecialized monociliated epidermis. Other characters ofOwenia are also mentioned. The relationship between the nervous system and the epidermal cells on the tentacles is examined. It is proposed thatOwenia should assume an important role in discussions of the phylogeny of the Polychaeta.I wish to thank Dr. R.M. Rieger for his inspiration and guidance during the course of this study. Ms. Wilma Hanton provided technical assistance for the electron microscopy. This work was supported by NSF Grant # GB-42211 to R.M. Rieger     

VARIATION IN HYDRA'S TENTACLE NUMBERS AS A FUNCTION OF TEMPERATURE

Chlorohydra uiridissima whose tentacle number is altered at different temperatures, was studied to see how other developmental variables changed as a function of temperature. The results suggest that temperature is instrumental in establishing the size of bud and tentacle primordia, but the number of primordia present may play a limiting role.

Animals were cultured at 18, 23 and 28°C and shifted between the extreme temperatures. Large animals with 8 tentacles, small animals with 5 tentacles, and intermediate animals with 6 and 7 tentacles served as parents. Buds and parents were monitored daily and scored for numbers of buds and tentacles.

Temperature, not parental size, determined the size of the buds. At the lower temperature buds were produced more slowly and initiated less frequently, but occurred in greater numbers per parent and had more tentacles than at the higher temperatures. The duration of bud development also increased at lower temperature, but at the lowest temperature the duration of bud development was not correlated with tentacle numbers on buds.

Changes in the frequency of bud initiation and the duration of bud development induced by changing temperature did not parallel changes in the number of tentacles produced on buds. Animals shifted from 18°C to 28°C underwent rapid increases in the rate of bud initiation and rapid shortening in the duration of bud development, while animals shifted from 28°C to 18°C underwent equally rapid changes in the opposite directions. The number of tentacles produced on buds, however, changed slowly to that characteristic of buds acclimated to the new temperatures. The frequency of bud initiation and the duration of bud development, therefore, do not determine tentacle number.

The number of tentacles already present seems to limit possibilities for adding new tentacles. Parents with five tentacles were especially likely to undergo upward changes in their tentacle number while parents with eight tentacles were resistant to such changes.     

Ontogeny of the fused tentacles in three species of ommastrephid squids (Cephalopoda, Ommastrephidae)

Abstract. The tentacles of ommastrephid squids fuse during embryonic development and remain fused as they grow through hatching, but eventually separate to become two fully functional adult tentacles. The external anatomy of individuals at several post‐hatching ontogenetic stages of three species of ommastrephid squids (Ommastrephes bartramii, Sthenoteuthis oualaniensis, and Hyaloteuthis pelagica) was examined using scanning electron microscopy and morphometrics. The fusion of the transverse muscle mass of the tentacles was examined using light microscopy. Five ontogenetic stages of tentacle separation were defined based on landmark features such as the extent of the fusion and the presence of suckers or sucker buds at the distal tip. The total tentacle length and fused tentacle length reached a maximum when the dorsal mantle length (ML) equaled 3–4 mm (H. pelagica) or 4–6 mm (O. bartramii, S. oualaniensis), and then decreased with increasing ML. The average split length (measured from the base of the tentacles to the point of tentacle fusion) increased gradually with increasing ML, and the separate tentacle diameter was roughly half the diameter of the fused portion at all sizes. In all three species, separation of the fused tentacles began earlier in development (2–3‐mm ML) and was more advanced at smaller sizes than previously reported. The sizes presented here are conservative because excess epithelium at the location of the split may disguise the actual site of separation. Post‐separation tentacles were much shorter than the arms, and the carpal region appeared torn in 2 of the 4 specimens of S. oualaniensis examined. Finally, none of the original distal tip suckers were retained on the post‐separation tentacles of S. oualaniensis. These observations are consistent with the hypothesis that the tentacles separate gradually then rupture at the “wrist” (presumptive carpus), and argue against the possibility of prey capture by the fused tentacles.     

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.     

Ultrastructure of the adhesive tentacles of the limnomedusa Vallentinia gabriella (Hydrozoa,Olindiadidae)

Summary The specialized adhesive exumbrellar tentacles of the limnomedusa Vallentinia gabriella were examined by light microscopy and scanning and transmission electron microscopy. The adhesive region first differentiates some distance from the tentacle tip. As differentiation proceeds the distal part is reduced and the adhesive region comes to lie at the tentacle tip. The adhesive epithelium consists of flagellated and non-flagellated glandular cells, a few nematocytes, and a nerve plexus. The glandular cells are characterized by electron-dense granules and bundles of microtubules. The microtubules, being anchored to the mesoglea, are oriented parallel to the longitudinal axis of the cell and extend up to the cell apex. It can be assumed that the microtubules are involved in the transport of secretory granules to the cell apex. Bundles of neurites run adjacent to the mesoglea between the basal processes of the glandular cells. The neurites form interneural synapses and synapses with glandular cells. It is suggested that detachment of the specialized adhesive tentacles is under nervous control.     

Behavioural response of the scyphozoan jellyfish Aurelia aurita (L.) upon contact with the predatory jellyfish Cyanea capillata (L.)

Underwater manipulative experiments were carried out in situ to investigate the sensibility of the jellyfish Amelia aurita (L.) to contact with the tentacles of Cyanea capillata (L), commonly known as a predator on A. aurita. Movements of individual medusae touched by tentacles of C. capillata and other objects were video‐recorded during SCUBA dives. The behavioural variable studied was change in swim pulse frequency. The results showed that A. aurita was highly susceptible to the tentacles of C. capillata and responded with an increased swim pulse frequency when touched at the umbrellar margin but not at the central exumbrella. Contact with other objects also induced a behavioural response in A. aurita.     

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.     

Osservazioni istologiche,istochimiche ed ultrastrutturali sui tentacoli di Vaginulus borellianus (Colosi), Gastropoda Soleolifera

Riassunto Le ricerche condotte sui tentacoli di Vaginulus borellianus hanno permesso di stabilire: a) l'esistenza nella porzione distale dei tentacoli (superiori ed inferiori) di un epitelio sensoriale di tipo olfattivo costituito da neuroni bipolari, cellule di sostegno e cellule basali. I prolungamenti distali dei primi due tipi di cellule si presentano particolarmente interessanti: nei recettori olfattivi si rinvengono ciglia e microvilli, numerosi e lunghi; nelle cellule di sostegno robuste digitazioni apicali percorse da uno spesso filamento assile, b) l'assenza, sia nel tentacolo superiore che in quello inferiore, di un complesso neuroendocrino del tipo di quello riscontrato in Helix, Arion ed altri Stilommatofori, c) la presenza nel tentacolo inferiore di una ghiandola unica, tubulare, in comunicazione con l'esterno e con una secrezione di tipo mucoso, simile a quella dei mucociti di altre superfici del corpo, o della ghiandola del piede di questa o di altre specie di molluschi.

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Histological, histochemical and ultrastructural observations on the tentacles of Vaginulus borellianus (Colosi)

Summary The tentacle structure of Vaginulus borellianus (Colosi) has been examined with the ligth and electron microscope; the following conclusions have been reached: a) The olfactory sensory epithelium in the distal portion of both tentacles (superior and inferior) is composed of sensory neurons, supporting cells, and basal cells. The dendrites of the neurons bear cilia and a great number of microvilli apically, whereas the apical portion of the supporting cells is formed by cylindrical plasmatic processes. These are long and thick and have an axial filament. b) The cytochemistry and ultrastructure of the unicellular glands enclosed within the dermo-muscular layer of the tentacles indicate that a neuroendocrine complex of the kind described in Helix, Arion and other Stylommatophores does not occur in this species. c) The gland located in the lower tentacles (belonging to Semper's organ) is a tubular formation, with its opening at the outer surface. Its secretion, mostly mucous, is very similar to that of mucocytes and foot glands of this species and other molluscs.

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Lavoro eseguito con un Contributo del Consiglio Nazionale delle Ricerche.     

On the anatomy of the central nervous system and the morphological value of the anterior end appendages of Ampharetidae, Pectinariidae and Terebellidae (Polychaeta)

Earlier papers dealing with the anatomy of the central nervous system of ampharetids, pectinariids and terebellids were studied. On this basis a re‐investigation appeared necessary: statements in the literature about the structure of the brain and the innervation of the appendages of the anterior end were incomplete and contradictory. In the present paper, the brain, the circum‐oesophageal connectives and the innervation of, inter alia, the tentacular membrane (including the dorsal ridge), the buccal tentacles, the alimentary canal, the nuchal organs and the branchiae (when present) of Amphicteis gunneri, Anobothrus gracilis, Melinna cf. cristata, Pectinaria auricoma, P. belgica, P. koreni, Petta pusilla, Pista cristata, Eupolymnia nebulosa, Thelepus cincinnatus and Polycirrus medusa are described. The results are summarized in schematic diagrams and compared with each other and with the central nervous system of other polychaetes. It is concluded that the ampharetids, the pectinariids and the terebellids bear no antennae and no palps and that their buccal tentacles belong to the alimentary canal. It is emphasized that all attempts to range their cephalic nervous system into previously proposed common and general schemes of ‘the polychaete nervous system’ seem totally fruitless.     

Morphological,ultrastructural and histochemical investigation of epipodial sensory structures of Haliotis tuberculata (Gastropoda: Haliotidae)

In this study, we describe the microstructure and ultrastructure of the epipodial papillae and epipodial tentacles of Haliotis tuberculata using light and electron microscopy. The epipodial papillae vary morphologically; they are subdivided into several subpapillae whose surface is covered by small micropapillae. The epipodial tentacles are large extendable conically elongated structures whose surface is differentiated in two regions: the dorsal region with long corrugated folds, and a ventral region composed of three parts, a basal part with the same structure as the dorsal, a middle part with shorter corrugated folds and an apical part with large micropapillae. Although the thin sections and ultrastructure examination show that the epithelium of both organs is morphologically similar and composed of supporting cells, sensory cells and different types of secretory cells, there is a certain specialization in their secretory product. Although the epithelium of both structures was positive for acidic glycoconjugates, the tentacle epithelium was also positive for neutral sugars. Further specific differences were revealed by lectin histochemistry. Because papillae and tentacles can be extended or retracted depending on environmental conditions, they probably have tactile and olfactory functions.     

The neuromuscular system of the cyclostome bryozoan Crisia eburnea (Linnaeus, 1758)

The cyclostome bryozoans constitute an old and divergent group of bryozoans, whose muscle and nervous systems are poorly known. The entire neuromuscular system of the cyclostome Crisia eburnea is here mapped with phalloidin, DAPI and antibodies directed against acetylated α-tubulin and serotonin. Innervation of most muscles as well as the ganglion of C. eburnea is described, and several new details are reported, for example, on the additional and branched ectodermal muscles of the cystid, the presence of subtentacular muscles, the retractor muscles being distinctly striated and the presence of an additional pair of lateroabfrontal nerves in the proximal part of the tentacles. The serotonin-like immunoreactivity in the nervous system of C. eburnea shares many features with those of the other bryozoans studied so far, which probably reflects a common ancestry of the neural architecture. However, the nervous system shows somewhat less complexity compared to that of the sister clade, Eurystomata, and contains fewer cells and nerves compared to the cyclostome Cinctipora which has much larger zooids and more than eight tentacles. No interzooidal neural connections were found in C. eburnea, which is in agreement with the individual response of the zooids.     

Elektronenmikroskopische Untersuchungen an den Oralcirren und der Haut vonBranchiostoma lanceolatum

Praeoral tentacles and epidermis of the anterior body region ofBranchiostoma lanceolatum Pallas have been investigated by electron microscopy. The epidermis of the praeoral tentacles and the anterior body region are mono-layered and cohere by strong denticulations of the adjoining cell walls. Vertical secretory vesicles at the cell surface give off mucous substances. The secretory vesicles are found only in the body epithelium. Between epithelium cells both epithelia contain two different secondary sensilla types.B. lanceolatum is the lowest chordate in which taste buds of the praeoral tentacles have been found. The taste buds overtop the surface of the epithelium. The praeoral tentacles are stiffened by a skeleton rod, situated asymmetrically and build up in layers. The skeleton rod is surrounded by connective tissue, which includes a coelomic space. Axon bundles of different strength are situated in the connective tissue. Not only the taste buds but also singular sensilla types are innervated by these axon bundles. The relatively strong basement lamina is partially zonated and contains pores. An antagonistically arranged layer of collagen fibres of varying thickness occurs below the basement lamina.     

Localization and distribution of nitric oxide synthase and other neuronal markers in the podia of Holothuria arguinensis

The organization of the nervous system of the holothurian podia—the tentacles, papillae, and tube feet—is still poorly understood, which limits the development of functional studies. Knowledge of nitric oxide (NO) signaling in sea cucumbers is nonexistent, although it is known to play an important role in many essential biological functions, including neurotransmission, throughout the animal kingdom. The objective of this study was to characterize the holothurian podia in Holothuria arguinensis. To this end, we used classical histology, nitric oxide synthase (NOS) distribution, using NADPH‐diaphorase histochemistry and NOS immunostaining, and neuronal immunohistochemistry. Our results revealed an abundant distribution of NO in the nervous components of the holothurian podia, suggesting an important role for NO as a neuronal messenger in these structures. Nitrergic fibers were intensely labeled in the longitudinal nerve and the nerve plexus surrounding the stem, but were more weakly labeled in the mesothelium. NOS was also found in scattered cell bodies and abundant fibers in the podia terminal end (i.e., the discs in tentacles and tube feet, and the pointed conical structures in the papillae), with evident neuronal projections to the bud surface, especially in the tentacles. The podia terminal end was the most specialized area and was characterized by a specific nervous arrangement, consisting of a distinct nerve plate, rich in cells and fibers containing potential sensory cells staining positively for neuronal markers, which makes this the most likely candidate to be a chemosensory region and an important candidate for future exploration.     

Ultrastructure of the tentacles of Themiste lageniformis (Sipuncula)

Summary An ultrastructural study of the tentacles of Themiste lageniformis (Sipuncula) was conducted as part of a larger study of head metamorphosis in the species.The oral surface of the tentacles is constructed of a multiciliated, pseudostratified, columnar epithelium while the aboral surface is an unciliated, cuboidal epithelium. Intraepidermal mucous cells lie near the junction of the oral and aboral regions. The basal portion of the epidermal cells is embedded in a thick, collagenous extracellular matrix which contains outer circular muscles, inner longitudinal muscles, the main tentacular nerve and its branches. Three tentacular canals are present and are lined by peritoneum. Hemerythrocytes and coelomocytes flow through the lumen of the canals in a regular pattern.Justification for the designation of the tentacular canals as coelomic rather than vascular is discussed.     

Prodigies of propagation: the many modes of clonal replication in boloceroidid sea anemones (Cnidaria,Anthozoa, Actiniaria)

Summary

Diverse modes of clonal propagation were documented in tiny zooxanthellate sea anemones from the tropical Pacific. All were boloceroidids, as indicated by the tentacles' basal sphincter and the animals' swimming behavior. In one species, single tentacles were pinched off at the sphincter, shed into the coelenteron, and brooded there while regenerating into minute new polyps in ~4 days. Within a day of release, the propagules fed on live prey and swam by lashing the tentacles. A similar process occurs in another species studied, Bunodeopsis medusoides. In a third species a previously undescribed mode of replication was seen. These anemones bore a primary cycle of tentacles that engaged actively in feeding and swimming, were not shed, and showed no sign of producing polyps. Alternating with these tentacles were fan-like clusters of shorter tentacles that were relatively inactive in feeding and swimming. Despite the sphincter at the base of each of these clustered tentacles, they were never shed singly; instead, each cluster separated as a unit that then regenerated into a new polyp. Two other replicative modes were observed in similar, minute boloceroidid anemones collected together in the same habitat: longitudinal fission, not previously reported in boloceroidids, and pedal scission. Modes of replication in these actinians are more diverse than once thought, but the selective forces behind this variation are so far unexplored. These prolific anemones may regularly be taking advantage of their combination of swimming and regenerative abilities to achieve dispersal, not only by sexually produced larvae, but also by cloned polyps.