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.     

Functional morphology of the tentacles and tentilla of Coeloplana bannworthi (Ctenophora, Platyctenida), an ectosymbiont of Diadema setosum (Echinodermata, Echinoida)

 The tentacular apparatus of Coeloplana bannworthi consists of a pair of tentacles which bear, on their ventral side, numerous tentilla. Each tentacle extends from and retracts into a tentacular sheath. Tentacles and tentilla are made up of an axial core covered by an epidermis. The epidermis includes six cell types: covering cells, two types of gland cells (mucous cells and granular gland cells), two types of sensory cells (ciliated cells and hoplocytes), and collocytes, this last cell type being exclusively found in the tentilla. The core is made up of a fibrillar matrix, the mesoglea, which is crossed by nerve processes and two kinds of smooth muscle cells. Regular muscle cells are present in both the tentacles and tentilla while giant muscle cells occur exclusively in the tentilla. The retraction of the tentacular apparatus is an active phenomenon due to the contraction of both types of muscle cells. The extension is a passive phenomenon that occurs when the muscle cells relax. Tentacles and tentilla first extend slightly due to the rebound elasticity of the mesogleal fibers and then drag forces exerted by the water column enable the tentacular apparatus to lengthen totally. Once the tentacles and tentilla are extended, gland cells, sensory cells, and collocytes are exposed to the water column. Any swimming planktonic organism may stimulate the sensory cilia which initiates tentillum movements. Pegs of hoplocytes can then more easily contact the prey which results in a slight elevation of the nearby collocytes, the last being responsible for gluing the prey to the tentilla. Accepted: 1 April 1997     

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.     

Distribution of catecholamine-containing,serotonin-like and neuropeptide FMRFamide-like immunoreactive neurons and processes in the nervous system of the actinotroch larva ofPhoronis muelleri (Phoronida)

Summary Glyoxylic-acid-induced fluorescence of catecholamines and antibodies against serotonin and FMRFamide were used to study the distribution of putative neurotransmitters in the actinotroch larva ofPhoronis muelleri Selys-Longchamps, 1903. Catecholamines occur in the neuropile of the apical ganglion, in the longitudinal median epistome nerves, in the epistome marginal nerves, and in the nerve along the bases of the tentacles. The tentacles have laterofrontal and latero-abfrontal bundles of processes that form two minor nerves along the lateral ciliary band of the tentacles, and a medio-frontal bundle of processes. Monopolar cells are located on the ventro-lateral part of the mesosome. Processes are located along the posterior ciliary band and as a reticulum in the epidermis. Serotonin-like immunoreactive cells and processes are located in the apical ganglion, in the longitudinal median epistome nerves, and as a dorsal and ventral pair of bundles along the tentacle bases. Processes from the latter extend into the tentacles as the medioabfrontal processes. The latero-abfrontal processes form a minor nerve along the ciliary band. The dorsal bundles forms the major nerve ring along the tentacles and processes extend from it to the metasome. Processes are located along the posterior ciliary band. FMRFamide-like immunoreactive cells and processes are found in the apical ganglion, in the longitudinal median epistome nerves and as a pair of lateral epistome processes projecting towards the ring of tentacles. In the tentacles, a pair of latero-frontal processes are found; these form a minor nerve along the ciliary band. A band of cells can be seen along the tentacle ring.     

Tentacular apparatus ultrastructure in the larva of Bolinopsis infundibulum (Lobata: Ctenophora)

Borisenko, I. and Ereskovsky, A.V. 2011. Tentacular apparatus ultrastructure in the larva of Bolinopsis infundibulum (Lobata: Ctenophora). —Acta Zoologica (Stockholm) 00 : 1–10. Most ctenophores have a tentacular apparatus, which plays some role in their feeding. Tentacle structure has been described in adults of only three ctenophore species, but the larval tentacles have remained completely unstudied. We made a light and electron microscopic study of the tentacular apparatus in the larvae of Bolinopsis infundibulum from the White Sea. The tentacular apparatus of B. infundibulum larvae consists of the tentacle proper and the tentacle root. The former contains terminally differentiated cells, while the latter contains stem cells and cells undergoing differentiation. The core of the tentacle is formed by myocytes, and its epidermis contains colloblasts (hunting cells), wall cells, degenerating cask cells, refractive vesicles, and ciliated sensory cells. Stem cells, colloblasts, and cask cells at various stages of differentiation and putative myocytes progenitors were revealed in the tentacle root. Two different populations of the stem cells in the tentacle root give rise to epidermal (colloblasts and cask cells) and mesogleal (myocytes) cell lines. Nervous elements, glandular cells, and basal lamina were not found. Step‐by‐step differentiation of colloblasts and cask cells is described.     

Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida)

The ultrastructure of the ciliary apparatus of multiciliated epidermal cells in larval and adult sipunculids is described and the phylogenetic implications discussed. The pelagosphera of Apionsoma misakianum has a dense cover of epidermal cilia on the head region. The cilia have a long, narrow distal part and two long ciliary rootlets, one rostrally and one vertically orientated. The adult Phascolion strombus has cilia on the nuchal organ and on the oral side of the tentacles. These cilia have a narrow distal part as in the A. misakianum larva, but the ciliary rootlets have a different structure. The first rootlet on the anterior face of the basal body is very short and small. The second, vertically orientated rootlet is long and relatively thick. The two ciliary rootlets present in the larval A. misakianum are similar to the basal metazoan type of ciliary apparatus of epidermal multiciliated cells and thus likely represent the plesiomorphic state. The minute first rootlet in the adult P. strombus is viewed as a consequence of a secondary reduction. No possible synapomorphic character with the phylogenetically troublesome Xenoturbella was found.     

Ciliary feeding structures and particle capture mechanism in the freshwater bryozoan Plumatella repens (Phylactolaemata)

Abstract. In contrast to marine bryozoans, the lophophore structure and the ciliary filter‐feeding mechanism in freshwater bryozoans have so far been only poorly described. Specimens of the phylactolaemate bryozoan Plumatella repens were studied to clarify the tentacular ciliary structures and the particle capture mechanism. Scanning electron microscopy revealed that the tentacles of the lophophore have a frontal band of densely packed cilia, and on each side a zigzag row of laterofrontal cilia and a band of lateral cilia. Phalloidin‐linked fluorescent dye showed no sign of muscular tissue within the tentacles. Video microscopy was used to describe basic characteristics of particle capture. Suspended particles in the incoming water flow, set up by the lateral ‘pump’ cilia on the tentacles, approach the tentacles with a velocity of 1–2 mm s^‐1. Near the tentacles, the particles are stopped by the stiff sensory laterofrontal cilia acting as a mechanical sieve, as previously seen in marine bryozoans. The particle capture mechanism suggested is based on the assumed ability of the sensory stiff laterofrontal cilia to be triggered by the deflection caused by the drag force of the through‐flowing water on a captured food particle. Thus, when a particle is stopped by the laterofrontal cilia, the otherwise stiff cilia are presumably triggered to make an inward flick which brings the restrained particle back into the downward directed main current, possibly to be captured again further down in the lophophore before being carried to the mouth via the food groove. No tentacle flicks and no transport of captured particles on the frontal side of the tentacles were observed. The velocity of the metachronal wave of the water‐pumping lateral cilia was measured to be ～0.2 mm s^‐1, the wavelength was ～7 μm, and hence the ciliary beat frequency estimated to be ～30 Hz (～20 °C). The filter feeding process in P. repens reported here resembles the ciliary sieving process described for marine bryozoans in recent years, although no tentacle flicks were observed in P. repens. The phylogenetic position of the phylactolaemates is discussed in the light of these findings.     

Neurobiology of the gorgonian coelenterates,Murcea californca and Lophogorgia chilensis

Summary Diffuse and synaptic nerve nets are present in the coenenchymal mesoglea and ectoderm of Muricea and Lophogorgia colonies. The nerve nets extend into the polyp column and tentacles maintaining a subectodermalmesogleal position. The density of nerve elements is low in comparison with similar nerve nets found in pennatulids.In the column of the polyp anthocodium, and throughout the oral disk region, neurons cross the mesoglea and enter the polyp endoderm. These neurons presumably connect with the endodermal nerve net which innervates the septal musculature. The trans-mesogleal neurons probably represent the connection between colonial and polyp nervous systems.In the tentacles, longitudinal ectodermal musculature is present with an overlying nerve plexus. These muscles and nerves, as well as tentacular sensory cells, are well represented in the oral side of the tentacles only.Presumed sensory cells form ciliary cone complexes in which one cell possesses an apical cilium. The other cells as well as the centrally located nematocyte contribute microvilli to the cone. The basal portion of the sensory cells is drawn into one or more neurite-like processes which enter the ectodermal nerve plexus. Similar processes form synapses with longitudinal muscle cells and nematocytes. The sensory cells of the ciliary cones presumably include chemoreceptors which can activate or modify nematocyst discharge, local muscle twitches, and tentacle bending.This work was supported by Office of Naval Research Contract N00014-75-C-0242, NSF Grant BMS 74-23242 and General Research Funds of the University of California, Santa Barbara. We wish to thank Dr. Steven K. Fisher for the use of facilities in his lab. This paper is part of a thesis to be submitted by R.A.S. to the Department of Biological Sciences, University of California, Santa Barbara in partial fulfillment of the requirements for the Ph. D.     

Tentacle structure in freshwater bryozoans

Tentacles are the main food‐gathering organs of bryozoans. The most common design is a hollow tube of extracellular matrix (ECM), covered with ten columns of epithelial cells on the outside, and a coelothelium on the inside. Nerves follow the ECM, going between the bases of some epidermal cells. The tentacle musculature includes two bundles formed by myoepithelial cells of the coelothelium. The tentacles of freshwater (phylactolaemate) bryozoans, however, differ somewhat in structure from those of marine bryozoans. Here, we describe the tentacles of three species of phylactolaemates, comparing them to gymnolaemates and stenolaemates. Phylactolaemate tentacles tend to be longer, and with more voluminous coeloms. The composition of the frontal cell row and the number of frontal nerves is variable in freshwater bryozoans, but constant in marine groups. Abfrontal cells form a continuous row in Phylactolaemata, but occur intermittently in other two classes. Phylactolaemata lack the microvillar cuticle reported in Gymnolaemata. Abfrontal sensory tufts are always composed of pairs of mono‐ and/or biciliated cells. This arrangement differs from individual abfrontal ciliary cells of other bryozoans: monociliated in Stenolaemata and monociliated and multiciliated ones in Gymnolaemata. In all three groups, however, ciliated abfrontal cells probably serve as mechanoreceptors. We confirm previously described phylactolemate traits: an unusual arrangement of two‐layered coelothelium lining the lateral sides of the tentacle and oral slits in the intertentacular membrane. As previously reported, tentacle movements involved in feeding differ between bryozoan groups, with phylactolaemates tending to have slower movements than both gymnolaemates and stenolaemates, and a narrower behavioral repertoire than gymnolaemates. The morphological and ultrastructural differences between the freshwater species we studied and marine bryozoans may be related to these functional differences. Muscle organization, tentacle and coelom size, and degree of confluence between tentacle and lophophore coeloms probably account for much of the observed behavioral variability.     

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.     

Form and function of tentacles in pteriomorphian bivalves

Tentacles are remarkable anatomical structures in invertebrates for their diversity of form and function. In bivalves, tentacular organs are commonly associated with protective, secretory, and sensory roles. However, anatomical details are available for only a few species, rendering the diversity and evolution of bivalve tentacles still obscure. In Pteriomorphia, a clade including oysters, scallops, pearl oysters, and relatives, tentacles are abundant and diverse. We investigated tentacle anatomy in the group to understand variation, infer functions, and investigate patterns in tentacle diversity. Six species from four pteriomorphian families (Ostreidae, Pinnidae, Pteriidae, and Spondylidae) were collected and thoroughly investigated with integrative microscopy techniques, including histology, scanning electron microscopy, and confocal microscopy. Tentacles can be classified as middle fold tentacles (MFT) and inner fold tentacles (IFT) according to their position with respect to the folds of the mantle margin. While MFT morphology indicates intense secretion of mucosubstances, no evidence for secretory activity was found for IFT. However, both tentacle types have appropriate ciliary distribution and length to promote mucus transportation for cleaning and lubrication. Protective and sensory functions are discussed based on different lines of evidence, including secretion, cilia distribution, musculature, and innervation. Our results support the homology of MFT and IFT only for Pterioidea and Ostreoidea, considering their morphology, the presence of ciliated receptors at the tips, and branched innervation pattern. This is in accordance with recent phylogenetic hypotheses that support the close relationship between these superfamilies. In contrast, major structural differences indicate that MFT and IFT are probably not homologous across all pteriomorphians. By applying integrative microscopy, we were able to reveal anatomical elements that are essential for the understanding of homology and function when dealing with such superficially similar structures.     

Rejection Mechanism of Plectolophous Lophophore of Brachiopod <Emphasis Type="Italic">Coptothyris grayi</Emphasis> (Terebratulida,Rhynchonelliformea)

Brachiopoda is a relict group of invertebrate filter feeders that used a tentacle organ, lophophore, for capturing food particles from the water column. Brachiopod extinction apparently occurred due to low productivity of their filtering organ in comparison with more advanced filter-feeders. Investigation of the filtering mechanism of modern brachiopods is essential to understanding their evolutionary fate. This study is devoted to the rejection mechanism of large waste particles from the plectolophous lophophore of brachiopod Coptothyris grayi. The waste particles gather inside of the lophophore on the outer side of the brachial fold. The particles form rows along frontal grooves of outer tentacles and are carried successively to the tentacle tips and move along them, slimed by mucus. One portion of the particles comes off the lophophore and falls down the mantle, while another part is carried to the abfrontal surface of the tentacles. Due to repeated reversals of abfrontal cilia, the particles wavily move along the abfrontal surface of tentacles. Such movement contributes to the secretion of mucus and the formation of particle clots. The clots come off the lophophore and fall down the mantle. The particles are transported along the mantle by cilia to the anterior part of the mantle margin. Here the ciliary reversals that facilitate secretion of mucus and formation of pseudofeces also take place. The latter takes away from the mantle cavity. Thus, only outer tentacles participate in the rejection of large waste particles from the lophophore. Ciliary reversals of the abfrontal surface of tentacles and the mantle are discovered in brachiopods for the first time. This facilitates the additional secretion of mucus and formation of pseudofeces, easing their exit from the mantle cavity. The results contribute to the knowledge of lophophore function and evolution of tentacle organs in Bilateria.     

The morphology and life cycle of Ophryodendron mysidacii sp. nov. a marine suctorian epibiont on a mysid crustacean

A new species of suctorian protist epibiont of the mysid Schistomysis parkeri is described. The individuals show two types of adult form: elongated and flattened, both with 4-8 tentacular lobes. This new suctorian differs from described species of pro-Ophryodendron group by size, number of tentacular lobes, insertion of the tentacles, union of the lorica with the body, shape of the macronucleus, number of micronuclei and the lack of stalk (adult forms). The life cycle of this species is analysed and a succession pattern of its different stages is proposed.     

Tentacular and oral-disc regeneration in the sea anemone,Aiptasia diaphana. I. Sequential morphological events in distal-end restitution

The complete regeneration of a new oral-disc and tentacles has been observed and described for Aiptasia diaphana. These structures are regenerated quite rapidly: seven to ten days at 20°C. At three days post-amputation, the new primary, secondary, and tertiary tentacle buds begin to develop in direct association with the underlying primary, secondary, and tertiary septae (respectively) of the column, suggesting that the latter organize the form of the regenerating oral-disc. Two days after amputation, the zooxanthellae of the presumptive oral disc arrange themselves into a ring which quite precisely delimits the area from which the tentacle buds will form. In spite of its suggestive proximity, this accumulation of algae plays no role in the induction of tentacle buds as was shown by studying regeneration in anemones which essentially lacked large quantities of these symbiotic algae. Cuts perpendicular to the longitudinal axis of the column result in an equal rate of tentacular regeneration around the entire circumference of the presumptive oral disc. Oblique amputations foster an asynchronous regeneration: the tentacle buds of the distal-most area of the severed column are larger and regenerate much sooner than those of the proximal region. Similar results were obtained by studying anemones which were cut perpendicular to their longitudinal axes at different levels along the column. The data suggest that an oral-aboral gradient exists concerning the time required for the initiation of tentacle budding and the rate of tentacle regeneration.     

Regeneration in youngBunodactis verrucosa Pennant (actinaria,anthozoa)

Summary YoungBunodactis verrucosa Pennant at the 12 tentacle stage are employed to test the applicability of the polar coordinate model to coelenterate regeneration. The animals are cut along every radius into fragments of 3 to 9 segments. Most fragments are patent 3–4 weeks later, but small fragments have a higher mortality rate than large fragments. Some fragments do not regenerate and occasionally tentacles fuse, thereby reducing the number of segments. Small fragments tend to regenerate more tentacies than large fragments, but large fragments may regenerate great numbers of supernumerary tentacles. Twenty-two percent of the fragments restore the missing number of tentacles, while 76% of all fragments produce an even number of tentacles.Fragments restoring the correct numbers of tentacles show a marked tendency to form the correct tentacles (regulative regeneration). Fragments regenerating two less than the number of tentacles already present show a marked tendency to reproduce tentacles of the types already present (miror image formation). Other fragments produce missing segments (forward regeneration), or those already present (reverse regeneration) at lower frequencies.No fragments beginning or ending with the number 1 directive tentacle fail to regenerate entirely, while first cycle segments maximally remote from segment 1 are associated with the absence of regeneration. No fragments beginning or ending with the number 4 directive tentacle fail to undergo forward regeneration, regulate or produce a mirror image when the appropriate number of segments are regenerated. In contrast, segment 4 is associated with a low frequency of reverse regeneration, and second cycle segments cut away from immediate contact with segment 4 show an increase in the frequency of reverse regeneration. Controls through morphogenic substances rather than polar coordinates seem to explain these results. Such substances would control the number and direction of tentacle regeneration.This work was performed while the author was on sabbatical leave from the University of Pittsburgh at the Stazione Zoologica di Napoli. The author gratefully acknowledges the assistance of Mr. Ciro Gargiulo and of Ms. Gisella Princivalli. This work was supported by a travel grant from the United States Italy Cooperative Science Program of the National Science Foundation. The paper is dedicated to Dr. Alberto Monroy whose generosity made it possible     

Brachiopod tentacles: Ultrastructure and functional significance of the connective tissue and myoepithelial cells in Terebratalia

Summary The fine structure of the tentacles of the articulate brachiopod Terebratalia transversa has been studied by light and electron microscopy. The epidermis consists of a simple epithelium that is ciliated in frontal and paired latero-frontal or latero-abfrontal longitudinal tracts. Bundles of unsheathed nerve fibers extend longitudinally between the bases of the frontal epidermal cells and appear to end on the connective tissue cylinder; no myoneural junctions were found. The acellular connective tissue cylinder in each tentacle is composed of orthogonal arrays of collagen fibrils embedded in an amorphous matrix. Baffles of parallel crimped collagen fibrils traverse the connective tissue cylinder in regions where it buckles during flexion of the tentacle.The tentacular peritoneum consists of four cell types: 1) common peritoneal cells that line the lateral walls of the coelomic canal, 2) striated and 3) smooth myoepithelial cells that extend along the frontal and abfrontal sides of the coelomic canal, and 4) squamous smooth myoepithelial cells that comprise the tentacular blood channel.Experimental manipulations of a tentacle indicate that its movements are effected by the interaction of the tentacular contractile apparatus and the resilience of the supportive connective tissue cylinder. The frontal contractile bundle is composed of a central group of striated fibers and two lateral groups of smooth fibers which function to flex the tentacle and to hold it down, respectively. The small abfrontal group of smooth myoepithelial cells effects the re-extension of the tentacle, in conjunction with the passive resiliency of the connective tissue cylinder and the concomitant relaxation of the frontal contractile bundle.The authors wish to express their appreciation to Professor Robert L. Fernald for his advice and encouragement throughout the course of this study. Some of the work was conducted at the Friday Harbor Laboratories of the University of Washington. The authors are indebted to the Director, Professor A.O.D. Willows, for use of the facilities. Part of this study was supported by NIH Developmental Biology Training Grant No. 5-T01-HD00266 and NSF grant BMS 7507689     

Muscle development in squid: Ultrastructural differentiation of a specialized muscle fiber type

The ultrastructural differentiation of two muscle fiber types of the squid Sepioteuthis lessoniana was correlated with development of prey-capture behavior. Transmission electron microscopy was used to document the differentiation of the fast-contracting cross-striated muscle cells of the tentacles and the obliquely striated muscle cells of the arms of specimens sampled at one week intervals from hatching to 5 weeks. By using high-speed video recordings, the ultrastructural differentiation was correlated with changes in prey-capture behavior that occur during development and growth. The ultrastructural analysis focused on the muscle cells of the transverse muscle of the tentacles and the transverse muscle of the arms. For the first 2 weeks after hatching, the tentacle transverse muscle fibers do not show the adult ultrastructure and are indistinguishable from the obliquely striated fibers of the transverse muscle of the arms. Transverse striation of the tentacle muscle cells appears at approximately three weeks and adult ultrastructure is present by 4–5 weeks after hatching. The high-speed video recordings show correlated behavioral changes. During the first 2–3 weeks after hatching, the animals use a different prey-capture mode from the adults; they jet forward and capture the prey with splayed arms and tentacles rather than employing the rapid tentacular strike. © 1996 Wiley-Liss, Inc.     

Functional organization of battery cell complexes in tentacles of Hydra attenuata

Ultrastructural and light microscopic observations on the organization of thick and thin regions of hydra's tentacles, made on serial sections and on whole fixed, plastic-embedded tentacles, reveal the existence of two levels of anatomical order in the tentacle ectoderm: (1) The battery-cell complex (BCC), composed of a single epitheliomuscular cell (EMC) and its content of enclosed nematocytes and neurons; and (2) the battery cell complex ring (BCC ring), an arrangement of 4 or more BCCs into larger units organized as rings around the circumference of the tentacle. All EMCs of the distal tentacle appear to contain batteries of nematocytes, and are, therefore, called “battery cells.” Apart from battery cell complexes and migrating nematocytes, there are no other cell types in the tentacle ectoderm. Battery cells are composed of three distinct regions: the cell body, peripheral attenuated extensions and myonemes. Thick tentacle bands are composed of cell bodies, whereas thin bands are made up of attenuated extensions. Myonemes contribute to both thick and thin regions. It was confirmed that each battery cell has several myonemes, which appear to interdigitate with myonemes of other more proximal and distal battery cells, but not with battery cells of the same BCC ring. Nematocytes have several basal processes. Some processes insert between myonemes and contact the mesoglea; other processes insert into cuplike extensions of myonemes, and are connected to myonemal cups by desmosomal junctions. These observations are discussed in relation to mechanical and electrical aspects of tentacular contraction and bending.     

The phylogenetic position of entoprocta, ectoprocta, phoronida, and brachiopoda

Ectoprocts, phoronids and brachiopods are often dealt with underthe heading Tentaculata or Lophophorata, sometimes with entoproctsdiscussed in the same chapter, for example in Ruppert and Barnes(1994). The Lophophorata is purported to be held together bythe presence of a "lophophore," a mesosomal tentacle crown withan upstream-collecting ciliary band. However, the mesosomaltentacle crown of pterobranchs has upstream-collecting ciliarybands with monociliate cells, similar to those of phoronidsand brachiopods, although its ontogeny is not well documented.On the contrary, the ectoproct tentacle crown carries a ciliarysieving system with multiciliate cells and the body does notshow archimery, neither during ontogeny nor during budding,so the tentacles cannot be characterized as mesosomal. The entoproctshave tentacles without coelomic canals and with a downstream-collectingciliary system like that of trochophore larvae and adult rotifersand serpulid and sabellid annelids. Planktotrophic phoronidand brachiopod larvae develop tentacles at an early stage, buttheir ciliary system resembles those of echinoderm and enteropneustlarvae. Ectoproct larvae are generally non-feeding, but theplanktotrophic cyphonautes larvae of certain gymnolaemates havea ciliary band resembling that of the adult tentacles. The entoproctshave typical trochophore larvae and many feed with downstream-collectingciliary bands. Phoronids and brachiopods are thus morphologicallyon the deuterostome line, probably as the sister group of the"Neorenalia" or Deuterostomia sensu stricto. The entoproctsare clearly spiralians, although their more precise positionhas not been determined. The position of the ectoprocts is uncertain,but nothing in their morphology indicates deuterostome affinities."Lophophorata" is thus a polyphyletic assemblage and the wordshould disappear from the zoological vocabulary, just as "Vermes"disappeared many years ago.