Development of the pellicle and thecal plates following ecdysis in the dinoflagellateGlenodinium foliaceum

Summary The ultrastructure and development of the amphiesma of the dinoflagellateGlenodinium foliaceum was studied using conventional electron microscopy and immunocytochemistry. Ecdysis (shedding of the flagella, the outer two membranes of the cell, and the thecal plates) was induced by centrifugation. The cells were resuspended and the thickening of the pellicle and the development of the new thecal vesicles and plates was studied over a 9 h period. After ecdysis, the thin pellicle which underlay the thecal plates in the motile cells thickens to form a complex structure of four distinct layers: an outer layer of randomly oriented fibrils, a 50 nm layer of fibrils oriented perpendicular to the dense layer, the dense layer which has a trilaminate structure, and a wide inner homogeneous layer. The new thecal vesicles form in these pelliculate cells by the migration of electron translucent amphisomal vesicles over the layer of peripheral microtubules to a position directly under the plasmalemma. The thecal vesicles then flatten and elongate. A discontinuous pellicular layer appears within them. Subsequently, the thecal vesicles widen and are filled with a fibrillogranular substance overlying the pelliculate layer. The thecal plates form on top of this fibrillogranular material. By this time, most cells have escaped from the pellicle and are motile. At first, the outer thecal vesicle membrane is continuous with the inner thecal vesicle membrane at the sutures, but when this connection is broken, the dense pelliculate layers become continuous across the suture as does the inner thecal vesicle membrane. At ecdysis, this membrane becomes the new plasmalemma of the cell. Cells at each stage of pellicle thickening and thecal development were labelled with a polydonal antiserum raised against the 70 kDa epiplasmic protein ofEuglena acus. This antiserum labelled both the thecal plates of the motile cells and the inner homogeneous layer of the pellicle of ecdysed non-motile cells. No other amphiesmal structure was labelled, nor was any intracellular compartment.Abbreviations PBS phosphate-buffered saline - PIPES piperazine-N,N-bis[2-ethane sulfonic acid]     

Taking the plunge. Terminal differentiation in Dictyostelium

During the last stage of Dictyostelium development a motile, cylindrical slug transforms into an immotile, stalked fruiting body and the constituent cells change from amoebae to either refractile spores or vacuolated stalk cells. Analysis of this process using genetics and simple culture techniques is becoming a powerful way of investigating a number of conserved signal transduction processes. A common pathway activating cAMP-dependent protein kinase (PKA) triggers the maturation of spore cells and those stalk cells forming the stalk. It uses a eukaryotic version of the 'bacterial' two-component phospho-relay system to control cAMP breakdown. A second pathway, inhibiting the GSK3 protein kinase, might control the maturation of a distinct set of stalk cells at the base of the fruiting body.     

Early development of Eimeria papillata (Apicomplexa: Eimeriidae) in the mouse

Early development of Eimeria papillata (Apicomplexa) in the mouse was evaluated using Nomarski interference-contrast and brightfield microscopy. Sporozoite-shaped meronts, which were motile and contained a large posterior refractile body and a smaller anterior refractile body, were observed entering and leaving host cells in the jejunum of an experimentally infected mouse at 26 h post inoculation (HPI). However, early developmental stages were not observed in tissue of the duodenum, ileum, cecum and colon. The mean length and width of these meronts (n = 20) were 12.0 microns and 3.7 microns, respectively. Spherical or subspherical meronts containing crescent-shaped merozoites were observed at 36 HPI.     

A novel method for the isolation and study of a magnetotactic bacterium

The magnetococcus, a magnetotactic bacterium, has been grown in a complex simulated natural environment. Sufficiently pure samples of cells were obtained magnetically making axenic cultures unnecessary for many purposes. The magnetococcus is a Gram-negative coccus, 1.6 m in diameter and readily distinguished by highly refractile inclusions and its magnetotactic behavior. This organism is actively motile by means of two bundles of flagella. Electron dense ferromagnetic inclusions were localized between the flagellar bundles. Collections of magnetococci were morphologically homogeneous and negligibly contaminated by extraneous bacteria. DNA extracted from pooled collections of cells was homogeneous by analytical CsCl centrifugation. The guanine-cytosine content was 61.7%. Total iron by percent cellular dry weight was 3.8%. Comparisons with a previously described magnetotactic marine coccus were made.Non-Standard Abbreviations Tris Tris (hydroxymethyl) aminomethane buffer - EDTA Dipotassium ethylenediamine tetraacetic acid - GC Guanosine cytosine     

Early Development of Eimeria Papillata (Apicomplexa: Eimeriidae) In the Mouse

ABSTRACT Early development of Eimeria papillata (Apicomplexa) in the mouse was evaluated using Nomarski interference-contrast and brightfield microscopy. Sporozoite-shaped meronts, which were motile and contained a large posterior refractile body and a smaller anterior refractile body, were observed entering and leaving host cells in the jejunum of an experimentally infected mouse at 26 h post inoculation (HPI). However, early developmental stages were not observed in tissue of the duodenum, ileum, cecum and colon. the mean length and width of these meronts (n = 20) were 12.0 μm and 3.7 μm, respectively. Spherical or subspherical meronts containing crescent-shaped merozoites were observed at 36 HPI.     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

Flocculation in Azospirillum brasilense and Azospirillum lipoferum: exopolysaccharides and cyst formation.

The phenomena of flocculation and floc formation by Azospirillum brasilense Sp7 (ATCC 29145) and Azospirillum lipoferum Sp59b (ATCC 29707) were studied in aerobic liquid cultures. Carbon sources representative of various entry pathways in combination with various nitrogen sources induced flocculation in both species of azospirilla. Noticeably, the combination of fructose and nitrate was the most effective in terms of floc yields. Phase-contrast microscopic observations revealed a transition in cell morphology from freely motile, vibrioid cells to nonmotile, highly refractile encysting forms during the formation of flocs. The nonmotile forms in flocs appeared to be entangled within a fibrillar matrix, and the cells were highly resistant to desiccation. Dried flocs kept for almost 6 months still maintained the highly refractile encysting forms, and their viability was confirmed by pellicle formation and acetylene reduction in semisolid malate medium. Electron microscopic observations of the desiccated flocs revealed the presence of cell forms containing abundant poly beta-hydroxybutyrate granules within a central body and surrounded by a thick layer of exopolysaccharides. The latter were characterized by alkali and acid digestion, crude cellulase hydrolysis, and calcofluor staining. It was concluded that the overproduction of exocellular polymers induces the flocculent growth and is associated with the concomitant transformation of vegetative cells to the desiccation-resistant encysting forms under limiting cultural conditions.     

Fine structure of dense-cored vesicles in glomus cells of rat carotid body after fixation with permanganate or glutaraldehyde

Summary Glomus (Type I) cells of the carotid body of adult rats were studied electron microscopically after fixation with potassium permanganate or with glutaraldehyde and osmium tetroxide. Two permanganate fixation methods (using Krebs-Ringer-glucose, pH 7.0, or acetate buffer, pH 5.0) were compared. Numerous dense-cored vesicles were observed only in about one tenth of the glomus cells when neutral permanganate was used for fixation, although all glomus cells showed such vesicles after fixation with glutaraldehyde and osmium tetroxide. Numerous vesicles with a dense core were observed in about one third of the cells after fixation with acid potassium permanganate. With this fixation, small dense-cored vesicles similar to those in adrenergic nerve terminals were occasionally seen in the cytoplasm of glomus cells. It is tentatively concluded that the amine-storing vesicles of the carotid body are different from those in the small intensely fluorescent (SIF) cells and those in adrenergic nerve terminals.     

Fine structure of dense-cored vesicles in glomus cells of rat carotid body after fixation with permanganate or glutaraldehyde.

Glomus (Type I) cells of the carotid body of adult rats were studied electron microscopically after fixation with potassium permanganate or with glutaraldehyde and osmium tetroxide. Two permanganate fixation methods (using Krebs-Ringer-glucose, pH 7.0, or acetate buffer, pH 5.0) were compared. Numerous dense-cored vesicles were observed only in about one tenth of the glomus cells when neutral permanganate was used for fixation, although all glomus cells showed such vesicles after fixation with glutaraldehyde and osmium tetroxide. Numerous vesicles with a dense core were observed in about one third of the cells after fixation with acid potassium permanganate. With this fixation, small dense-cored vesicles similar to those in adrenergic nerve terminals were occasionally seen in the cytoplasm of glomus cells. It is tentatively concluded that the amine-storing vesicles of the carotid body are different from those in the small intensely fluorescent (SIF) cells and those in adrenergic nerve terminals.     

Light and electron microscopy on the sporulation of the oocysts of Eimeria brunetti. II. Development into the sporocyst and formation of the sporozoite

The later stages of sporulation in oocysts of Eimeria brunetti were examined in samples which had been allowed to sporulate at 27 degrees C for 24, 36 and 48 hours. It was observed that the sporoblasts became ellipsoidal and the nucleus underwent the final division. A nucleus with associated Golgi bodies was not observed at either end of the organism. The cytoplasm was limited by two unit membranes and contained rough endoplasmic reticulum, dense bodies, electron translucent vacuoles and mitochondria. The first evidence of sporozoite formation was the appearance of a dense plaque at either end of the organism. This appeared in the vicinity of the nuclei, and adjacent to the limiting membrane of the soroblast. At this stage the sporocyst wall was still unformed. Then the two sporozoites were formed from opposite ends of the organism by growth of the dense plaques and invaginations of the plasmalemma which thus formed the pellicles of the developing sporozoites. A conoid and subpellicular microtubules were observed at this stage as development continued, a number of vacuoles were found between the nucleus and the conoid. These vacuoles constituted the precursors of the rhoptries and micronemes. At the same stage a large dense body had appeared within the forming sporozoite. As the sporozoite developed, this body, anterior refractile body, is followed by the nucleus and another dense body which formed the posterior refractile body. During this period, the thin sporocyst wall was formed and Stieda and sub-Stieda bodies were now present at one end of the sporocyst. Each mature sporocyst contained two sporozoites.     

Transmission Electron Microscopy of Meront Development of Eimeria vermiformis Ernst,Chobotar and Hammond, 1971 (Apicomplexa,Eucoccidiorida) in the Mouse,Mus musculus1

First and second generation meronts of Eimeria vermiformis developed in epithelial cells of the crypts of Lieberkühn. They were usually between the host cell nucleus and the basement membrane. Sporozoite organelles dedifferentiated with the first generation meront's development except for the refractile body and the apical complex, which persisted. After several nuclear divisions, the apical complex dedifferentiated further until only micronemes remained attached by a duct system to the plasmalemma. The form of the apical complex was highly variable. Sometimes the duct system was absent and the micronemes were attached directly to the plasmalemma or a dense material on it. Crescent body-like material was often present in the parasitophorous vacuole next to the microneme structure. The microneme structure was not present in second generation meronts but evaginations of the plasmalemma, cytoplasmic outpocketings, and cytoplasmic vesicles were associated with the round granular bodies in the parasitophorous vacuoles. During first generation merogenesis, invaginations from the parasitophorous vacuole formed channels into the meront along which merozoites budded. Micropores were often at the ends of these invaginations. These and other micropores of the meront had a dense U-shaped band for a collar while those of the merozoites had a collar with a double band of dense material that connected to the inner membrane. First generation merozoites budded randomly from the meront, resulting in a residual body that was usually in the middle of the parasitophorous vacuole. Second generation merozoites budded in one direction, resulting in a peripheral residual body and merozoites that were parallel in an oblong parasitophorous vacuole.     

Ultrastructural aspects of nervous-system and statocyst morphogenesis during embryonic development of Convoluta psammophila (Turbellaria,Acoela)

The notion that statocysts originated from an infolding of ectoderm lined by ciliated sensory cells has been challenged with evidence of capsule-limited, non-ciliary statocysts in several independent phyla. Statocysts in turbellarians primitively lack cilia and are embedded within or closely adjoined to the cerebral ganglion; they are likely to be derived from nervous tissue. We investigated the development of the simple statocyst in an acoel turbellarian, a statocyst consisting of three cells. Observations of serial TEM sections of embryos at different stages of development support the hypothesis of an inner (non-epithelial) origin of the statocyst. First, a three-cell complex is delimited by a basal lamina; it then undergoes cavitation by swelling, autophagy, and fluid secretion. The statocyst becomes discernible within the precursor ganglion cells while they still contain yolk inclusions. The two outer (parietal) cells, enclosed together by a 10-nm-thick basal lamina, arrange themselves in an ovoid of about 10 µm diameter and surround the inner statolith-forming cell. The statolith is formed later within vacuoles of the statolith-forming cell.     

Über Bau und Funktion der Statocyste bei der Schnecke Aplysia limacina

Zusammenfassung Die Statocyste von Aplysia limacina zeigt in ihrem Bau keine wesentliche Abweichung vom durchschnittlichen Gastropoden-Typ. Besondere statolithfreie Räume oder Sinneshaare, wie sie von Tieren mit echtem Rotationssinn bekannt sind, wurden nicht angetroffen.Frei schwimmende, aus ihrer Normallage gebrachte Aplysien zeigen Lagekorrekturbewegungen, bei denen der Kopfteil führt. Auch fixierte und im Wasser hochgehobene Aplysien zeigen nach Drehung um horizontale Achsen kompensatorische Kopfstellreflexe. Auf Drehung um die Vertikalachse wird nicht reagiert. Einseitige Entstatung (Durchschneidung des N. staticus) ruft keinen, beiderseitige Entstatung einen vollständigen Ausfall der statischen Lagekorrektur- und Reflex-bewegungen hervor; die schwimmende Aplysia vollführt dann Purzelbäume. Taktile Reize vom Untergrund unterstützen die Lageorientierung. Ein orientierender Lichteinfluß machte sich nicht geltend.Nach einseitiger Durchschneidung des Cerebro-Pedal-Konnektivs reagiert eine fixierte Aplysia nur mehr in ipsilateraler Seitenlage mit der kompensatorischen Kopfdrehung zur intakten Seite hin; in kontralateraler Seitenlage wird nicht mehr reagiert. Das Ergebnis der Ausschaltversnche (Tabelle, S. 49) führt zu Schluß-folgerungen über den Verlauf der statischen Reflexbahnen, die in einem Diagramm (Abb. 7, S. 53) zusammengefaßt sind.Diese und andere Befunde werden in Zusammenhang mit den Ergebnissen früherer Autoren diskutiert. Bezüglich des Reizvorganges wird angenommen, daß auch in der Schneckenstatocyste Scherung der Cilien den effektiven, physiologisch adäquaten Reiz darstellt.

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Structure and functioning of the statocyst in the gastropod Aplysia limacina

Summary The statocyst of Aplysia limacina is a rounded vesicle with a diameter of 200–250 . Its wall is composed of two kinds of cells. The outer supporting cells are separate cells in fresh tissue; only under the influence of pressure or fixing agents their walls burst and artificial syncytia are created. The inner sense or giant cells are on their inner surface covered with motile cilia. Each statocyst of Aplysia contains 13 sense cells; their nervous offshoots constitute the statocyst nerve which runs towards the cerebral ganglion. The statolith is a cluster of about 1000 loosely aggregated chalk particles (statoconia). It fills the greater part of the statocyst lumen and is lightly moved by the cilia. Special statolith-free cavities or sense hairs, such as are known from animals with a true rotation sense, were not found in the statocyst of Aplysia.Freely swimming Aplysiae perform correction movements with their head leading, when they are brought out of their normal position in space. Likewise, fixed Aplysiae, when lifted up in the water and rolled or tilted about horizontal axes, show compensatory static head reflexes. Rotation around a vertical axis causes no response. Unilateral section of the statocyst nerve causes neither a loss of the position reflexes nor any asymmetry of posture or movement. Bilateral section of this nerve, however, abolishes all correction movements and compensatory reflexes; swimming animals perform somersaults. Tactile stimuli from the underground support the animal's spatial orientation. An orienting influence of light was not observed.After unilateral section of the cerebro-pedal connective a fixed Aplysia only responds when rolled the towards ipsilateral side (with a compensatory turn of the head towards the contralateral side); when rolled 90° towards the controlateral side no reaction occurs. The results of the elimination experiments (Table, p. 49) lead to the following conclusions: 1) from each statocyst two reflex pathways originate, one of which is activated after a roll around the long axis to the left side and causes a head turn to the right, whereas the other one comes into action after a roll to the right side and causes a head turn to the left; 2) the pathways of both statocysts which turn the head to the left run from the cerebral ganglion through the left cerebro-pedal connective towards the left pedal ganglion; both pathways which turn the head to the right run through the right cerebropedal connective towards the right pedal ganglion (diagram, Fig. 7, p. 53).These and other results are discussed in relation to data of earlier investigations. The course of the static nerve as shown morphologically to occur in other gastropods resembles closely the pathways postulated for Aplysia on physiological grounds. With regard to the process involved in stimulation it is assumed that in the statocyst of gastropods, like in other static organs, a shearing force exerted on the cilia represents the effective, physiologically adequate stimulus. Recent findings about the submicroscopical structure of the cilia in the statocyst of gastropods as well as about the mechanical sensitivity of motile cilia give this assumption strong support.

     

Development of the cell covering in the dinoflagellate Scrippsiella hexapraecingula (Peridiniales, Dinophyceae)

The organization and development of cell coverings in two alternate phases of the life cycle in a marine dinoflagellate, Scrippsiella hexapraecingula Horiguchi et Chihara, were investigated by thin sectioning and freeze‐fracture electron microscopy. In one of these phases, the motile phase, cells have an outermost plasma membrane that is lined with flattened amphiesmal vesicles. Groups of microtubules lie beneath these vesicles. In mature motile cells, thecal plates are completely enclosed in individual amphiesmal vesicles. After settling, the cells enter the second, non‐motile phase. Here, ecdysis occurs, resulting in several steps including formation of the first pellicle layer (PI), fusion of the inner amphiesmal vesicle membranes to form the new plasma membrane, deposition of the second pellicle layer (PM) under PI, and the appearance and fusion of juvenile amphiesmal vesicles to form new territories, which eventually give rise to new thecal plates in the next motile phase. Thus, the pattern in which thecal plates are arranged in motile cells is determined at the time when the amphiesmal vesicles develop into non‐motile cells.     

The effects of 6-hydroxydopamine on the appearance of granulated vesicles in glomus cells of the rat carotid body

The glomus cells of the rat carotid body reveal an intense fluorescence after exposure to paraformaldehyde vapor and contain catecholamines. After initial fixation in glutaraldehyde, many granulated vesicles are seen in the glomus cells. After initial fixation in osmium tetroxide, most of the vesicles are depleted of their dense interiors and granulated vesicles occur infrequently. Administration of 6-hydroxydopamine followed by initial fixation in osmium tetroxide leads to the reappearance of dense interiors in virtually all vesicles. 6-Hydroxydopamine apparently is taken up by the membrane pump of the glomus cell and is incorporated into the amine storage granules, thereby displacing the endogenous monoamines. Osmium tetroxide does not dissolve the 6-hydroxydopamine from the vesicles, as it apparently does for the normal vesicular contents. The 6-hydroxydopamine does not fluoresce, hence 6-hydroxydopamine administration results in a decreased intensity of formaldehyde induced fluorescence in the glomus cells. Administration of reserpine after 6-hydroxydopamine treatment (and subsequent initial fixation in osmium tetroxide) depletes the previously restored dense material from the vesicles of the glomus cells. 6-Hydroxydopamine acts like a monoamine in that it is taken up by the glomus cell, incorporated into the vesicles, and can be depleted from the vesicles by reserpine.     

Comparative morphology of statocysts in the Plathelminthes and the Xenoturbellida

The general fine-structural organization of statocysts in Catenulida, Nemertodermatida, Acoela, Proseriata, Lurus (Dalyellioida), and Xenoturbella are summarized. In lithophorous (statocyst-bearing) members of the Catenulida, the statocysts exhibit a few parietal cells and one or several movable statoliths within a spacious intracapsular cavity. Statocysts in the Nemertodermatida have several parietal cells and two lithocytes, each equipped with one statolith, whereas those of the other acoelomorphan taxon, the Acoela, always have two parietal cells and one movable lithocyte. The statocysts of lithophorous members of the Proseriata represent more sophisticated systems: each has two clusters of accessory cells in addition to several parietal cells and a voluminous lithocyte in which the statolith is movable. In catenulids and proseriates, processes of outer neurons penetrate the capsule of the statocyst, whereas such innervations have not been found in the Nemertodermatida and Acoela. I conclude that the different types of statocysts have evolved independently within the Plathelminthes. Xenoturbella displays an intraepidermal statocyst with many monociliary parietal cells and several mobile cells (lithocytes) within the central cavity of the statocyst. Each of these mobile cells carries a statolith-like structure and one prominent cilium. The statocyst of Xenoturbella does not correspond to any type of plathelminth statocyst.     

Structure and function of the Nautilus statocyst.

The two equilibrium receptor organs (statocysts) of Nautilus are avoid sacks, half-filled with numerous small, free-moving statoconia and half with endolymph. The inner surface of each statocyst is lined with 130,000-150,000 primary sensory hair cells. The hair cells are of two morphological types. Type A hair cells carry 10-15 kinocilia arranged in a single ciliary row; they are present in the ventral half of the statocyst. Type B hair cells carry 8-10 irregularly arranged kinocilia; they are present in the dorsal half of the statocyst. Both type of hair cells are morphologically polarized. To test whether these features allow the Nautilus statocyst to sense angular accelerations, behavioural experiments were performed to measure statocyst-dependent funnel movements during sinusoidal oscillations of restrained Nautilus around a vertical body axis. Such dynamic rotatory stimulation caused horizontal phase-locked movements of the funnel. The funnel movements were either in the same direction (compensatory funnel response), or in the opposite direction (funnel follow response) to that of the applied rotation. Compensatory funnel movements were also seen during optokinetic stimulation (with a black and white stripe pattern) and during stimulations in which optokinetic and statocyst stimulations were combined. These morphological and behavioural findings show that the statocysts of Nautilus, in addition to their function as gravity receptor organs, are able to detect rotatory movements (angular accelerations) without the specialized receptor systems (crista/cupula systems) that are found in the statocysts of coleoid cephalopods. The findings further indicate that both statocyst and visual inputs control compensatory funnel movements.     

Schistosoma mansoni: a new parenchymal cell in cercariae

A new type of cell has been identified in cercariae of Schistosoma mansoni. The perikarya (cell bodies) of these cells were located in the body (midsegment), in an area oral to the acetabulum (ventral sucker). Cytoplasmic processes extending from the perikarya ramified throughout the parenchyma of the anterior organ (oral sucker), body, and tail segments by following the path of the nerve processes from the neuropile. The perikarya of these cells had heterochromatic nuclei and a predominance of particulate material and granules (240-360 nm) in their cytoplasm. Aggregates of granules (240-360 nm) and associated vesicles (34 nm) were scattered throughout the cytoplasmic processes of the cells and formed distinct varicosed areas. These processes often connected to the tegument in the midsegment (body) of the cercariae. The granules and associated vesicles reacted (became electron dense) with fixatives reported to be detectors of biogenic amines: The glutaraldehyde/osmium tetroxide fixation procedure rendered the granules electron dense while the glutaraldehyde/chromate/osmium tetroxide fixation procedure rendered the granules and the associated vesicles electron dense. The chromate solution of the latter procedure was responsible for the electron density of the associated vesicles. The morphology of these cells (their long ramifying cytoplasmic processes) and their reaction to chromium suggests that they are probably biogenic aminergic sensory cells.     

Fine structural study of the statocysts in the veliger larva of the nudibranch,Rostanga pulchra

Summary The two statocysts of the veliger larva of Rostanga pulchra are positioned within the base of the foot. They are spherical, fluid-filled capsule that contain a large, calcareous statolith and several smaller concretions. The epithelium of the statocyst is composed of 10 ciliated sensory cells (hair cells) and 11 accessory cells. The latter group stains darkly and includes 2 microvillous cells, 7 supporting cells, and 2 glial cells. The hair cells stain lightly and each gives rise to an axon; two types can be distinguished. The first type, in which a minimum of 3 cilia are randomly positioned on the apical cell membrane, is restricted to the upper portion of the statocyst. The second type, in which 9 to 11 cilia are arranged in a slightly curved row, is found exclusively around the base of the statocyst. Each statocyst is connected dorso-laterally to the ipsilateral cerebral ganglion by a short static nerve, formed by axons arising from the hair cells. Ganglionic neurons synapse with these axons as the static nerve enters the cerebral ganglion. The lumen of the statocyst is continuous with a blind constricted canal located beneath the static nerve.A diagram showing the structure of the statocyst and its association with the nervous system is presented. Possible functions of the statocyst in relation to larval behavior are discussed.