Ultrastructural studies on the ciliated receptors of the long tentacles of the giant scallop,Placopecten magellanicus (Gmelin)

Summary The long tentacles of the Giant scallop Placopecten magellanicus (Gmelin) have been examined with light, scanning, and transmission electron microscopy. Three types of ciliated cells have been observed, one of which is located in specialised papillae born on the distal third of the tentacle. There are two separate cell types within the papillae. Type I cells are non-ciliated supporting cells, which form a capsule within which are found the Type II cells. These cells bear up to five cilia at their apices, and it is suggested that these are the receptor cells of the organ. No function has yet been determined for the receptors, but is suggested that they might be mechanoreceptors. A third cell type, Type III cells, occur at the base of the papillae. These cells bear many cilia and also macrocilia. Another ciliated cell type occurs on the proximal two thirds of the tentacle. These cells bear many cilia that are thought to be motile and not sensory.This research was supported by National Research Council of Canada Operating Grant No. A-6444 to Dr. V.C. Barber. Additional support came from the Department of Biology and School of Graduate Studies, Memorial University. Contribution No. 249 from the Marine Sciences Research Laboratory, Memorial University of Newfoundland     

Structure of the organs of attachment of brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) (Echinodermata: Asteroidea)

The structure of the brachiolar arms and adhesive disk of the brachiolaria larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) was determined from light microscopy and from scanning and transmission electron microscopy. The structure of these organs was very similar in both species.The brachiolar arms are comprised of a stem region terminating in a crown of adhesive papillae which are made up of a variety of secretory cell types. Principal among these are elongated cells producing very electron-dense secretory particles, which are released at the free cell surface attached to cilia. Secretory particles appear to be important in temporary attachment of the brachiolar arms to the substratum. Ciliary sense cells, possibly used in the recognition of specific substrata are located at the tip of adhesive papillae.The adhesive disk is comprised of large cells packed with secretory droplets and elongated intracellular fibres. In the attached adhesive disk, secretory droplets are lost, having formed the cement that attaches the disk to the substratum. It appears that adhesive papillae lateral to the adhesive disk hold the disk in position close to the substratum during secretion and hardening of the cement. The intracellular fibres are the principal anchoring structures running from microvilli (locked into the attachment cement) on the surface of the disk to the underlying connective tissue of the attachment stalk.     

The ultrastructure of the tentacles of eleven species of dendrochirote holothurians studied with special reference to the surface coats and papillae

Summary The tentacles of eleven species of dendrochirote holothurians have been studied. The water vascular system, deep fibre system, ectoneural nerve ring and superficial fibre system are described and are similar to those of other holothurian tentacles. A conspicuous fuzzy coat covers the entire tentacular surface except for the tips of cilia. On the basis of its structure it is thought to be an attenuated glycocalyx. Its function is discussed in relation to anti-fouling and surface adhesion. The two surface coats underlying the fuzzy coat are termed the cuticle. Bacteria are found both within the surface coats and in the sub-cuticular space. Primary fixatives lacking osmium give poor preservation of the surface coats. The adhesive papillae of the apices of the tentacles contain elements of support cells and two other cells named Type-1 and Type-2 papillar cells. The secretions of Type-1 papillar cells are dense-cored vesicles and may contain a proteinaceous adhesive. The vesicles fuse with the cuticle and release their products which are apparently disseminated along the fuzzy coat filaments. The secretions of Type-2 papillar cells may have a neurosecretory function. The different models of food capture by dendrochirote tentacles are discussed as are duo-glandular adhesive systems in relation to dendrochirote tentacles.     

Adhesive Papillae of Phallusia mamillata Larvae: Morphology and Innervation

The swimming larvae of most solitary ascidians belonging to the Ascidiidae family bear three anterior, simple conic adhesive papillae. They secrete adhesive substances that are used to effect transitory settlement at the beginning of the metamorphosis.The adhesive papillae of newly hatched Phallusia mamillata larvae examined by the SEM are covered by the tunic. When the larvae are about to settle, the tunic becomes fenestrated over the central part of the papilla and bulb-ended microvilli protrude through the holes. These papillae have two types of elongated cells: many peripheral cells and few larger central cells with microvilli and bundles of microtubules oriented along the major axis of the cells.We have done immunofluorescence experiments with an anti-beta-tubulin monoclonal antibody (clone 2-28-33) reacting with axonal microtubules. Only the central cells of the papillae were stained and the axons appeared to arise from the proximal ends of these cells. These axons form a long nerve that reaches the brain vesicle. Branches of the same nerve appear to connect to the basal ends of the peripheral cells. By confocal laser microscopy we were able to follow the course of the papillary nerve. The two nerves connecting the dorsal papillae fuse together into a single nerve that runs posteriorly. The nerve connecting the ventral papilla runs posteriorly for a long tract before fusing with the nerve of the dorsal papillae just near the brain.The reported observations raise the hypothesis that the central cells of the adhesive papillae might be primary sensory neurons and that they may have chemosensory function.     

STRUCTURE OF THE CEPHALIC TENTACLES OF SOME SPECIES OF PROSOBRANCH LIMPET (PATELLIDAE AND FISSURELLIDAE)

The morphology and ultrastructure of the cephalic tentaclesand eye (optic vesicle) of some patellid and a fissurellid limpetspecies are described. The epithelium of the tentacle bearsciliated cells which have neural connections suggesting a sensoryfunction. In patellid limpets the density of these cells variesbetween species, with the greatest density (18 ciliated tufts.100 µm^–2) being recorded in the territorial limpet,Patella cochlear. The surface of the tentacle of a fissurellidlimpet, Fissurella mutabilis, is papillate, which contrastswith the smooth surface of a patellid tentacle. Ciliated tuftsare borne at the tip of most papillae. Movement of the tentacleis controlled by longitudinal muscle and a radial muscle-collagentissue association, which function as a muscular hydrostat.The eye of a patellid consists of a simple open vesicle anda retina which is comprised of one type of cell. By contrastthe optic vesicle of a fissurellid contains a lens, enclosedby a cornea and a retina which is composed of two types of cell,pigmented and photoreceptive. The ultrastructural features ofthe cells resemble those described for other molluscan eyes. (Received 5 June 1989; accepted 16 August 1989)     

Adhesive papillae on the brachiolar arms of brachiolaria larvae in two starfishes, Asterina pectinifera and Asterias amurensis, are sensors for metamorphic inducing factor(s)

It has been hypothesized by Barker that starfish brachiolaria larvae initiate metamorphosis by sensing of metamorphic inducing factor(s) with neural cells within the adhesive papillae on their brachiolar arms. We present evidence supporting Barker's hypothesis using brachiolaria larvae of the two species, Asterina pectinifera and Asterias amurensis. Brachiolaria larvae of these two species underwent metamorphosis in response to pebbles from aquaria in which adults were kept. Time-lapse analysis of A. pectinifera indicated that the pebbles were explored with adhesive papillae prior to establishment of a stable attachment for metamorphosis. Microsurgical dissections, which removed adhesive papillae, resulted in failure of the brachiolaria larvae to respond to the pebbles, but other organs such as the lateral ganglia, the oral ganglion, the adhesive disk or the adult rudiment were not required. Immunohistochemical analysis with a neuron-specific monoclonal antibody and transmission electron microscopy revealed that the adhesive papillae contained neural cells that project their processes towards the external surface of the adhesive papillae and they therefore qualify as sensory neural cells.     

The fine structure of the buccal tentacles of Holothuria forskali (Echinodermata,Holothuroidea)

Summary Ultrastructural study of the buccal tentacles of Holothuria forskali revealed that each tentacle bears numerous apical papillae. Each papilla consists of several differentiated sensory buds.The epidermis of the buds is composed of three cell types, i.e. mucus cells, ciliated cells, and glandular vesicular cells (GV cells). The GV cells have apical microvilli; they contain bundles of cross striated fibrillae associated with microtubules. Ciliated cells have a short non-motile cilium. Bud epidermal cells intimately contact an epineural nervous plate which is located slightly above the basement membrane of the epidermis. The epineural plate of each bud connects with the hyponeural nerve plexus of the tentacle. This nerve plexus consists of an axonic meshwork surrounded in places by sheath cells. The buccal tentacles have well-developed mesothelial muscles. Direct innervation of these muscles by the hyponeural nerve plexus was not seen.It is suggested that the buccal tentacles of H. forskali are sensory organs. They would recognize the organically richest areas of the sediment surface through the chemosensitive abilities of their apical buds. Tentacles presumably trap particles by wedging them between their buds and papillae.     

Tentacle structure and feeding processes in life stages of the commercial sea cucumber Parastichopus californicus (Stimpson)

Tentacle structure and function in the pentacula larva, juvenile, and adult life stages of Parastichopus californiens (Stimpson) were examined via light and electron microscopy. Food particle adherance to the tentacle surface is mediated by an adhesive material in the case of the pentacula larva and additionally by mechanical entrapment in juvenile and adult animals. Mechanical entrapment is of secondary importance to adhesion during feeding.     

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.     

Immunohistochemical analysis of adhesive papillae of Clavelina lepadiformis (Müller, 1776) and Clavelina phlegraea (Salfi, 1929) (Tunicata,Ascidiacea)

Almost all ascidian larvae bear three mucus secreting and sensory organs, the adhesive papillae, at the anterior end of the trunk, which play an important role during the settlement phase. The morphology and the cellular composition of these organs varies greatly in the different species. The larvae of the Clavelina genus bear simple bulbous papillae, which are considered to have only a secretory function. We analysed the adhesive papillae of two species belonging to this genus, C. lepadiformis and C. phlegraea, by histological sections and by immunolocalisation of β-tubulin and serotonin, in order to better clarify the cellular composition of these organs. We demonstrated that they contain at least two types of neurons: central neurons, bearing microvilli, and peripheral ciliated neurons. Peripheral neurons of C. lepadiformis contain serotonin. We suggest that these two neurons play different roles during settlement: the central ones may be chemo- or mechanoreceptors that sense the substratum, and the peripheral ones may be involved in the mechanism that triggers metamorphosis.Key words: Settlement, neurotransmitter, serotonin, β-tubulin, papillary nerves, metamorphosis.Ascidians (phylum Chordata; subphylum Tunicata) are sessile filter-feeding organisms that can be found in all benthic marine environments and develop through a swimming tadpole larva. Larvae of colonial ascidians have a short planktonic life that can vary from minutes to hours (Burighel and Cloney, 1997). Prior to metamorphosis, the larva attaches to the substratum by means of peculiar organs of ectodermic origin, located in the anterior region of the trunk. These organs, known as adhesive papillae, secrete sticky substances and effect primary adhesion of the larva to the substrate (Cloney, 1977). They have an important role in the initiation of settlement and metamorphosis and there is evidence that, at least in some species, they participate in substrate selection (Torrence and Cloney, 1983; Svane and Young, 1989; Groppelli et al., 2003). In many species, they are organised in a triangular field, whereas in others they are aligned along the mid-sagittal plane of the trunk. Adhesive papillae have been classified into two types: eversible papillae, typical of some colonial species, composed by several cell types and rapidly changing shape as they touch the substrate, and non-eversible papillae, typical of solitary species, which do not change shape after settlement (Burighel and Cloney, 1997).With few exceptions, all adhesive papillae are formed by elongated secretory and sensory cells, which are recognised as primary neurons (Cloney, 1977, 1979). It has been proposed that sensory cells may detect the chemical and physical characteristics of the substratum at potential sites for settlement and metamorphosis (Young and Braithwaite, 1980; Groppelli et al., 2003).The presence of primary neurons in the papillae has been reported in the larvae of several species such as Distaplia occidentalis, Diplosoma macdonaldi, Phallusia mammillata, Ciona intestinalis and Ascidia malaca (Cloney, 1977;Torrence and Cloney, 1983; Sotgia et al., 1998; Takamura, 1998; Gianguzza et al., 1999).These neurons have axons that join together to form the papillary nerves that enter the central nervous system at the level of the sensory vesicle (Imai and Meinertzhagen, 2007).Recently, different neurotransmitters have been localised in the sensory neurons of the papillae of different species. The presence of GABAergic neurons has been reported in the papillae of Ciona savygni (Brown et al., 2005) and of Ciona intestinalis (Zega et al., 2008), while serotonergic neurons have been localised in the papillae of Phallusia mammillata (Pennati et al., 2001) and Botrylloides leachi (Pennati et al., 2007). Moreover, it has been demonstrated that serotonin plays a role in the mechanism triggering metamorphosis in ascidians (Zega et al., 2005).After attachment, all papillae retract to draw the larva closer to the substratum. In the colonial ascidian Distaplia occidentalis, the process of retraction is reversibly inhibited by cytochalasin B, suggesting that microfilaments are involved in this process (Cloney, 1979).Clavelina lepadiformis is a colonial species, whose larvae bear non-eversible simple bulbous papillae.They have been described as being formed only by columnar glandular cells, whose secreted material is responsible for the sticky properties of the organ. These papillae do not contain sensory cells and were considered the simplest among those studied (Turon, 1991).In this work, the morphology of the adhesive papillae of C. lepadiformis and C. phlegraea was further investigated by histological analysis and immunolabelling techniques in order to clarify the actual cellular composition and function of these organs.     

Light- and Scanning-Electron-Microscopic Analyses of the Plica glossoepiglottica lateralis of the neonate and adultPan troglodytes troglodytes — Blumenbach (1799)

A unique type of papillae is evident in the plicae glossoepiglotticae laterales of the tongue ofPan troglodytes troglodytes. Areas of occurrence show flat lobes of varying length with elongated miniature processes on their free tips. The function of these papillae is presumably the convection of liquid food into the pharynx (along with the lateral folds). Furthermore, it is assumed that the papillae are sensory organs which relay taste, temperature, pain, and pressure, similar to the papillae filiformes of the tongue of other mammalia (Kunze 1969). Free nerve fibres and nerve endings were found in both the epithelium and the connective-tissue. Nerve structures resembling the “Organs of Meissner” were also discovered in the subepithelial connective-tissue. Goblet cells are found in the surface layers in the epithelium of the papillae of the newborn. These are absent in the papillae of the adultPan troglodytes troglodytes. The secretion of the goblet cells functions as a mechanical and chemical protectant for the papillae.     

Larval adhesive organs and metamorphosis in ascidians

Summary The larva of Distaplia occidentalis bears three cup-shaped adhesive papillae, each with a prominent axial protrusion. At the onset of metamorphosis these organs rapidly evert through fenestrations in the cuticular layers of tunic exposing hyaline caps of adhesive. Additional adhesive material is secreted from collocytes during eversion. The stickiness of the papillae facilitates attachment to a variety of substrates.Each papilla is composed of more than 900 cells; six different types were identified. The wall of the cup contains about 260 myoepithelial cells with long attenuated processes. These extend from the rim of the cup to the base in the parietal (inner) layer. The apices of the myoepithelial cells are held in place by 11 pairs of specialized anchor cells bearing long bulbous microvilli. When the myoepithelial cells contract they force the axial protrusion forward and transform the papilla into a hyperboloidal configuration. The papilla is innervated by small motor fibers, but sensory fibers were not detected. The adhesive papillae of Distaplia are discussed in relationship to nine other recognizable types of papillae in the ascidians.     

An osmoregulatory syncytium and associated cells in a freshwater mosquito

In embryological terms the anal papillae are the product of eversion of the hindgut tissues. The rectum and the anal papillae have the same origin and have a marked structural similarity. The insect hindgut is very labile being able to produce salt transporting or ‘chloride cells’ from any of the tissues of which it is composed.The hindgut consists of four distinct regions: the ileum and part of the anal canal have a mechanical function, the rectum and the posterior anal canal contain transporting cells. Two new cell types, ‘interstitial’ and ‘tertiary’ are reported in the rectum.The structure of the anal papillae changes with increased salinity. Changes in the plasma membranes alter the surface area for transport. Changes in the number of mitochondria are not accompanied by changes in oxygen consumption. If mitochondria are the site of oxidative metabolism then their number docs not control the level of oxygen consumption.In Aedes aegypti the papillary epithelium appears to be a syncytium. Across the lumen of the papillae there are cellular sheets supporting the tracheoles. At the base of the papillae there is a cellular transition zone; circular muscles in this region may be used to occlude the papillae. The control of salt transport may be hormonal.     

The attachment complex of brachiolaria larvae of the sea star <Emphasis Type="Italic">Asterias rubens</Emphasis> (Echinodermata): an ultrastructural and immunocytochemical study

The attachment complex of brachiolaria larvae of the asteroid Asterias rubens comprises three brachiolar arms and an adhesive disc located on the preoral lobe. The former are used in temporary attachment and sensory testing of the substratum, whereas the latter is used for permanent fixation to the substratum at the onset of metamorphosis. Brachiolar arms are hollow structures consisting of an extensible stem tipped by a crown of dome-like ciliated papillae. The papilla epidermis is composed of secretory cells (type A, B and C cells), non-secretory ciliated cells, neurosecretory-like cells and support cells. Type A and B secretory cells fill a large part of the papilla epidermis and are always closely associated. They presumably form a duo-gland adhesive system in which type A and B cells are respectively adhesive and de-adhesive in function. The adhesive disc is an epidermal structure mainly composed of secretory cells and support cells. Secretory cells produce the cement, which anchor the metamorphic larva to the substratum until the podia are developed. The relatedness between the composition of the adhesive material in the brachiolaria attachment complex and in the podia of adults was investigated by immunocytochemistry using antibodies raised against podial adhesive secretions of A. rubens. Type A secretory cells were the only immunolabelled cells indicating that their temporary adhesive shares common epitopes with the one of podia. The attachment pattern displayed by the individuals of A. rubens during the perimetamorphic period—temporary, permanent, temporary—is unique among marine non-vertebrate Metazoa.     

Ciliary filter-feeding structures in adult and larval gymnolaemate bryozoans

Abstract. Ciliary filter-feeding structures of gymnolaemate bryozoans—adults of Flustrellidra hispida and Alcyonidium gelatinosum , larvae of Membranipora sp.—were studied with SEM. In F. hispida and A. gelatinosum , the distal part of each tentacle has a straight row of stiff laterofrontal cilia which carry out "ciliary sieving" to capture suspended food particles that are subsequently transported downward towards the mouth by tentacle flicking; both structure and function resemble those of stenolaemate tentacles. The proximal part of the tentacle and of the ciliary ridge of a cyphonautes larva have strikingly similar structures, except that the laterofrontal cells are monociliate in the adults and biciliate in the larvae. The laterofrontal cells of the tentacles are arranged in a zigzag row and their cilia form two parallel rows, a frontal and a lateral row. The latter probably forms the sieve of stiff filter cilia in front of the water-pumping lateral cilia, whereas the frontal row appears to be held close to the frontal ciliary band of the tentacle. The biciliate laterofrontal cells of the cyphonautes larva have the cilia arranged in similar rows. The detailed morphological similarities between the ciliary bands of adult and larval filtering structures suggest that the feeding mechanisms are similar, contrary to what has been previously thought.     

Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida.

Sensory and ganglion cells in the tentacle epidermis of the sea anemone Aiptasia pallida were traced in serial transmission electron micrographs to their synaptic contacts on other cells. Sensory cell synapses were found on spirocytes, muscle cells, and ganglion cells. Ganglion cells, in turn, synapsed on sensory cells, spirocytes, muscle cells, and other neurons and formed en passant axo-axonal synapses. Axonal synapses on nematocytes and gland cells were not traced to their cells of origin, i.e., identified sensory or ganglion cells. Direct synaptic contacts of sensory cells with spirocytes and sensory cells with muscle cells suggest a local two-cell pathway for spirocyst discharge and muscle cell contraction, whereas interjection of a ganglion cell between the sensory and effector cells creates a local three-cell pathway. The network of ganglion cells and their processes allows for a through-conduction system that is interconnected by chemical synapses. Although the sea anemone nervous system is more complex than that of Hydra, it has similar two-cell and three-cell effector pathways that may function in local responses to tentacle contact with food.     

The fine structure and function of the tentacle in Tokophrya infusionum

The feeding apparatus of Suctoria consists of long, thin, stiff tubes called tentacles. When a swimming prey attaches to the tip of the tentacle a number of events follow in rapid succession. The tentacle broadens, a stream of tiny granules starts to move upward at its periphery to the tip, the prey becomes immobilized and shortly thereafter the cytoplasm of the still living prey begins to flow through the center of the tentacle to the body of the predator. An electron microscope study of the tentacle in Tokophrya infusionum, a protozoan of the subclass Suctoria, has disclosed a number of structural details which help to clarify some of the mechanisms involved in this unusual way of feeding. Each tentacle is composed of two concentric tubes. The lumen of the inner tube is surrounded by 49 tubular fibrils most probably of contractile nature. In the inner tube the cytoplasm of the prey is present during feeding, and in the outer tube are small dense bodies. It was found that the dense bodies originate in the cytoplasm of Tokophrya. They have an elongate, missile-like appearance, pointed at one end, rounded at the other, and are composed of several distinct segments. At the tip of the tentacle they penetrate the plasma membrane, with their pointed ends sticking out. It is assumed that the missile-like bodies play a major role in the feeding process. Their composite structure suggests that they might contain a number of enzymes which most probably are responsible for the various events preceding the actual food intake.     

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.     

Ultrastructure of the anal papillae of a salt-water mosquito larva, Aedes campestris

Anal papillae from fourth instar larvae of the salt-water mosquito Aedes campestris maintained in normal (hyperosmotic) lake water are made up of a single epithelial layer. The only type of cell present in this layer shows several ultrastructural features characteristic of transporting tissues. The apical plasma membrane (facing the external medium) is infolded into a series of lamellae which bear a particulate coat on the cytoplasmic surface. The basal plasma membrane (facing the haemolymph) is also highly infolded to form a system of interconnecting channels throughout the cell. These channels may be considerably dilated. Dense mitochondria are abundant in anal papillae cells and tracheoles frequently penetrate deeply into the cells. No qualitative or statistically significant quantitative differences are observed in the ultrastructure of anal papillae taken from A. campestris larvae maintained in diluted (hyposmotic) lake water. It is suggested that in both hyperosmotic and hyposmotic external media the anal papillae are actively engaged in ion transport and that adaptive changes in transport rates which have been demonstrated physiologically may require only minor structural modifications such as permeability changes or activation of enzyme systems already present in the cell membranes.     

Ordered progression of nematogenesis from stem cells through differentiation stages in the tentacle bulb of Clytia hemisphaerica (Hydrozoa, Cnidaria)

Nematogenesis, the production of stinging cells (nematocytes) in Cnidaria, can be considered as a model neurogenic process. Most molecular data concern the freshwater polyp Hydra, in which nematocyte production is scattered throughout the body column ectoderm, the mature cells then migrating to the tentacles. We have characterized tentacular nematogenesis in the Clytia hemisphaerica hydromedusa and found it to be confined to the ectoderm of the tentacle bulb, a specialized swelling at the tentacle base. Analysis by a variety of light and electron microscope techniques revealed that while cellular aspects of nematogenesis are similar to Hydra, the spatio-temporal characteristics are markedly more ordered. The tentacle bulb nematogenic ectoderm (TBE) was found to be polarized, with a clear progression of successive nematoblast stages from a proximal zone (comprising a majority of undifferentiated cells) to the distal end where the tentacle starts. Pulse-chase labelling experiments demonstrated a continuous displacement of differentiating nematoblasts towards the tentacle tip, and that nematogenesis proceeds more rapidly in Clytia than in Hydra. Compact expression domains of orthologues of known nematogenesis-associated genes (Piwi, dickkopf-3, minicollagens and NOWA) were correspondingly staggered along the TBE. These distinct characteristics make the Clytia TBE a promising experimental system for understanding the mechanisms regulating nematogenesis.