Organization of statocysts in the Otoplanidae (Plathelminthes): an ultrastructural analysis with implications for the phylogeny of the Proseriata

Summary The statocyst of otoplanids is enveloped by a bipartite capsule which consists of two different extracellular matrices. This capsule encircles three different types of aciliary cells: several peripherally located flattened parietal cells, one central statolith forming cell (lithocyte) and two clusters of accessory cells. Intracapsular lumina exist which are different from extracapsular intercellular spaces. The accessory cells most probably represent those structures that are mainly involved in nervous conduction. These cells extend cytoplasmatic processes towards different peripheral regions of the statocyst where processes of outer nerve cells penetrate the capsule. The statocyst does not seem to represent a more evolved equilibrium receptor system but may function as a relatively simple aciliary sense organ suitable for positive geotactic behaviour. The otoplanid statocyst corresponds to statocysts in other lithophorous proseriates but not to statocysts in other taxa of the free-living Plathelminthes. The monophyly of a taxon Lithophora within the Proseriata is corroborated by this autapomorphic characteristic.Abbreviations ac accessory cell(s) - c capsule of the statocyst - ce cerebrum - ci cephalic intestine - co capsule opening - cp cell process(es) of accessory cell(s) and cell(s) containing filaments - ecm extracellular matrix - fc cell(s) containing filaments - ic intercellular spaces within the capsule - mc muscle cell(s) - n lobed nucleus of the lithocyte - nac nucleus (nuclei) of accessory cell(s) - nc nerve cell(s) - npc nucleus (nuclei) of parietal cell(s) - pc parietal cell(s) - s statolith - sc statolith cell (lithocyte)     

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.     

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.     

A Fine Structural Analysis of the Statocyst in Turbellaria Acoela

Ferrero, E. (Istituto di Biologia Generale, Universith di Pisa, Pisa, Italy.) A fine structural analysis of the statocyst in Turbellaria Acoela. Zool. Scr. 2 (1): 5–16, 1973.—The fine structure of the statocyst components in the acoelan Convoluta psammophila is described, namely: capsule, parietal cells, lithocyte, and the statolith. The absence of ciliary structures, the highly developed endoplasmic reticulum of the lithocyte and the layered texture of the statolith are remarkable. A functional interpretation of the muscles inserted on the statocyst and of the nerve bundle running nearby is suggested. The morphological similarities and differences between the acoelan statocyst and the statocyst of lower and higher invertebrate phyla are discussed.     

An Ultrastructural Account of Otoplanid Turbellaria Neuroanatomy I. The cerebral ganglion and the peripheral nerve net

The otoplanid nervous system investigated in Otoplana truncaspina Lanfranchi, 1969 and Parotoplanella heterorhabditica Lanfranchi, 1969 consits of: (a) an ellipsoidal cerebral ganglion located between the gut and the cephalic intestine and invested by a fibrillar collagen-like capsule 0.3 μm thick; (b) anterior extracapsular ganglion cell clusters; (c) a peripheral nerve plexus locally thickened at the level of the epithelial sensory and glandular areas, with extensive synaptic connections. At least two neuron types can be identified within the ganglion: (a) an inner layer close to the central neuropile of the 1st type of neurons, showing a vesicular cytoplasm rich in RER and Golgi complexes processing both round, clear, 25–45 nm in diameter, and dense cored vesicles, 50–80 nm in diameter; (b) an outer layer of the 2nd type of neurons, adjoining the capsule and filled with uniformly dense vesicles, 60–90 nm in diameter. Synaptic endings in the neuropile are provided with clear vesicles and dense cored vesicles or uniformly dense vesicles. The presynaptic side has paramembranous projections channelling the vesicles to the active zone; omega-like profiles are also observed. Thin banded muscle fibres run within the brain. A comparison is drawn with the other turbellarian neuron types described in the literature, to suggest their possible function. The functional implications of the synaptic ultrastructure are discussed.     

Fine structure of the statocyst sensilla of the mysid shrimp Neomysis integer (Leach, 1814) (Crustacea,Mysidacea)

The fine structure of the statocyst sensilla of Neomysis integer was investigated. The statocyst contains about 35 sensilla, which are composed of two bipolar sensory cells, nine enveloping cells, and a seta. The sensory cells consist of an axon, a perikaryon, and a dendrite. The dendrite contains a proximal segment with a ciliary rootlet and at least one basal body, and a distal segment with a ciliary axoneme (9 × 2 + 0) at its base. The distal segment extends along the peripheral wall of the seta and is in close contact with the wall of the hair shaft. The enveloping cells surround the proximal and distal segments of the dendrite. The innermost enveloping cell contains a scolopale rod. It surrounds the receptor lymph cavity and secretes flocculent material into this cavity. From the tip of the cell a dendritic sheath, which encloses the distal segment of the dendrite, emerges. A peculiar feature of the second enveloping cell is the presence of a scolopale-like rod, which is more slender and less pronounced than in the first enveloping cell. The seta consists of three parts: a socket, a tubular midpart, and a gutter-like apical part, the tip of which penetrates into the statolith. The seta shows over its full length a bilaterally symmetrical axis that is coplanar with the plane in which the seta is bent toward the statolith. The structure of the seta and the position of the distal segments provide morphological evidence for directional sensitivity of the sensilla and for the magnitude of shear on the setal wall being an adequate stimulus.     

Ultrastruktur der Statocyste von Ototyphlonemertes pallida (Keferstein, 1862) (Nemertini)

Zusammenfassung Ototyphlonemertes pallida (Keferstein, 1862) hat zwei Statocysten, die unmittelbar hinter den Dorsalganglien auf den verlängerten Ventralganglien liegen. Jede Statocyste besteht aus einer Statolithenkammerzelle, mehreren Nervenzellen und einer Anzahl Hüllzellen und ist von einer dicken Basalmembran umgeben. Die Statolithenkammerzelle umschließt in der Regel drei Statolithenkammern, die von einer doppelten Membran umgeben sind und untereinander in Verbindung stehen. Sie enthalten je einen frei beweglichen Statolithen. Cilien und Ciliarstrukturen fehlen. Auf der Dorsalseite der Statocyste liegen mehrere stark verästelte Nervenzellen, die einen gemeinsamen Strang bilden. In der Nähe der Statolithenkammerzelle spalten sie sich auf und bilden pro Kammer eine oder mehrere synaptische Platten mit elektrischen Synapsen. Die Statolithenkammerzelle wird von zahlreichen Hüllzellen umgeben, die durch Desmosomen fest verbunden und zusätzlich in der ventralen Hälfte der Statocyste an den Außenseiten stark miteinander verzahnt sind. Die Hüllzellen unterscheiden sich im Aufbau deutlich von den beiden anderen Zelltypen und sind nicht an der Reizperzeption oder Reizleitung beteiligt. Zu den cilienlosen Statocysten bei Coelenteraten, Turbellarien, Holothurien, Xenoturbella und Tunicaten-Larven bestehen keine engeren morphologischen Beziehungen. Die Statocyste von O. pallida stellt eine Bildung sui generis innerhalb der Nemertinen dar.

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Ultrastructure of the Statocyst of Ototyphlonemertes pallida (Keferstein, 1862) (Nemertini)

Summary Ototyphlonemertes pallida (Keferstein, 1862) has two statocysts, which are situated just behind the dorsal ganglions on the elongations of the ventral ganglions. Each statocyst consists of one statolith chamber cell, some nerve cells and a number of covering cells and is surrounded by a thick basement membrane. Usually the statolith chamber cell encloses three statolith chambers, which are intercommunicated and surrounded by a double membrane. Each chamber contains a single mobile statolith. Cilia and ciliary structures are lacking. Within the dorsal part of the statocyst some very ramified nerve cells are situated, which form a nerve fibre. In the vicinity of the statolith chamber cell the nerve cells split up into synaptical plates with electric synapses; there are one or several synaptical plates at the level of each chamber. The statolith chamber cell is surrounded by numerous covering cells, which are connected by desmosomes and additionally linked together at the outside in the ventral part of the statocyst. With regard to their structure the covering cells differ greatly from the other cell types, and they do not participate in impulse perception and impulse conduction. There do not exist any closer morphological relations to the statocysts lacking cilia in Coelenterates, Turbellaria, Holothuria, Xenoturbella and Tunicata. The statocyst of O. pallida represents an indigenous structure within the Nemerteans.

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Abkürzungen bm Basalmembran - d Dendrit - de Desmosom - dg Dorsalganglion - dm doppelte Membran der Statolithenkammer - ds deltaförmige Strukturen - ep Epidermis - es elektrische Synapse - hms Hautmuskelschlauch - hz Hüllzelle - k Zellkern - m Mitochondrium - ma abgewandeltes Mitochondrium - mu Muskulatur - n Nervenstrang - nv Neurosekretvesikel - nz Nervenzelle - rs Rüsselscheide - sc Statocyste - sk Statolithenkammer - skz Statolithenkammerzelle - sp synaptische Platte - st Statolith - v Vakuole - vg Ventralganglion     

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.     

The Structure and Function of Müller Vesicles in Loxodid Ciliates1

ABSTRACT. The Müller vesicle is a characteristic organelle of loxodid ciliates. Its structure and development have been investigated using light microscopy and TEM. The organelle consists of a membrane-covered mineral body (the statolith), a vacuole, and various structures derived from the overlying kinety. There is strong evidence that the vesicle functions as a gravity sensor: a) its structure and relative dimensions fulfil the minimum requirements of a functional statocyst; b) its structure bears a close resemblance to the statocysts of some higher animals; c) re-orientation of the cell with respect to gravity produces a gravity-induced displacement of the mineral body, and d) geotaxis in Loxodes can be demonstrated experimentally. The transduction of the signal probably takes place at the level of the two kinetosomes of the organelle, one of which is in close contact with the cell membrane, while the other is connected to the statolith by a fairly rigid stalk containing a bundle of microtubules.     

Ultrastructural aspects of the 'statocyst' of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

The "statocyst" in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body ("statolith"), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.     

Ultrastructure of the Statocysts in the Apodous Sea Cucumber Leptosynapta inhaerens (Holothuroidea, Echinodermata)

The burrowing sea cucumber Leptosynapta inhaerens possesses five pairs of statocysts, one pair on either side of each radial nerve cord where it arises from the circumoral nerve ring. The nerve cords exhibit only ectoneural components at the level of the statocysts. A sinus-like epineural canal lies superjacent to each cord. This canal is lined by a robust monociliated neuroepithelium which lacks any special support cells. Beneath the neuroepithelium, the somata of the ectoneural neurons form a perikaryal layer whereas the axons are located within the proximal parts of the cords. Glial cells have not been found. Each statocyst is a hollow sense organ. Its central cavity is lined by a monolayer of monociliated parietal cells. Axons of these parietal cells extend towards the statocyst nerve which connects each statocyst with the ectoneural pathways of the cord. A single lithocyte floats within each central statocyst cavity. This unciliated cell contains a voluminous vacuole with the statolith and several smaller vacuoles. It is concluded that statocysts do not belong to the basic organization of the Holothuroidea but have been evolved within this group. The statement, that the statocysts of apodous sea cucumbers and that of the enigmatic Xenoturbella bocki are homologous organs, is rejected.     

A transmission electron microscopic investigation of the sensory vesicle in the brain of Oikopleura dioica (Appendicularia)

Summary The structure of three cell types in the sensory vesicle is described: (1) The statocyte, with its intracellular statolith, is attached to the medial wall of the vesicle via delicate shaft cells. (2) Cells along the dorsal, ventral and lateral walls which contact the surface of the statocyte with long, slender cilia. These cells are presumed to be primary sensory cells. (3) Presumed secretory cells, along the rostral and dorsal walls, may have a dual function: (a) secretion of the vesicle fluid, and (b) stabilization of the wall by turgor created in characteristic intercellular cavities. The sensory vesicle in Oikopleura contains undoubtedly typical statocyst components adequate for a free-swimming animal, whereas the ascidian system is suggested to be a device that responds to gravitational stimuli and, together with temporary photoreceptors, aids the larva in finding optimal settling conditions.     

The structure of some cephalopod statoliths

Summary The statoliths of Sepia officinalis, Octopus vulgaris, Alloteuthis subulata and Taonius megalops have a smooth outline, but an irregular shape. They have projections and indentations. The statoliths from a pair of statocysts are usually quite similar in size and shape, and the general pattern is probably maintained throughout the size range of the species. Statoliths from large animals are marginally larger than those from smaller ones. The statolith usually occupies only a small part of the cavity of the statocyst, and it is situated in the anterior part of the statocyst. They are joined to the macula by hairs extending from it. These hairs are very delicate and easily broken during preparation of the specimens. The hairs are much longer and narrower than the receptor cilia of the macula. The receptor cilia are enclosed within holes in the tangled hairlike anchoring fibrils.The statolith is made up of crystalline subunits, the statoconia. The crystals vary in size, they are usually elongated, hexagonal with pointed ends. The statolith consists of a closely packed mass of these crystals, sometimes they are irregularly arranged, where in others they are stacked with their long axes parallel. In Sepia officinalis and Taonius megalops, the crystals are arranged in regular shaped packets and these packets of crystals are stacked together. These larger subunits are not always arranged in a regular way, and their major axes can be organised in several different ways. The size and outline of these large subunits do vary in different parts of the statolith.The external surface of the statolith is macroscopically smooth. Over some parts there is a surface layer covering the rod-like crystals that make up the major bulk of the stone. In other regions, the surface is rough at a microscopic level, the roughness is produced by the exposed ends of the filamentous crystals. The crystals are composed of calcium carbonate in the form of aragonite.I wish to thank Professor J.Z. Young, FRS, for considerable help, advice and encouragement throughout this study. Dr. A. Boyde generously allowed me to use his scanning electron microscope and gave freely of his expertise and time. Dr. J. Fitch kindly gave me some fossil statoliths and Dr. J. Elliott examined them with his x-ray diffraction apparatus. Dr. Marion Nixon helped me to collect and prepare the specimens. Mrs. E. Bailey, Miss P. Stephens and Mr. R. Moss provided the expert technical assistance     

Structure of recent and fossil mysid statoliths (Crustacea,Mysidacea)

Statoliths of 61 Recent species representing all subfamilies of Mysidae were studied with special emphasis on internal structure. In addition 5 samples of fossil statoliths from Miocene deposits were examined. Species of Boreomysinae and Rhopalophthalminae show simple roughly spherical organic statoliths, with setae originating from the sensory cushion and anchored in the statolith with distal branches extending shortly below the surface. All other subfamilies possess mineralized statoliths of greater structural complexity, with differentiation in core and mantle, where each part may consist of up to three layers. Habitus is hemispherical to discoidal. External gross structures are dorsal tegmen, ventral fundus, and the ambitus forming the outer toroidal to semi-toroidal circumference. Setae penetrate the mantle through mineralic canals and insert on the surface of the core. As suggested by congeneric species of Schistomysis, there is no principal structural difference between statoliths mineralized with fluorite compared to vaterite. However, vaterite statoliths tend to be more often of moruloid appearance and are exceptional by showing a central conical hole (the hilum) or a central cavity in certain forms. These structures are typical of fossil calcite statoliths. In vaterite and fluorite statoliths, the mantle shows radially arranged (= spherulitic) crystal aggregates. Such arrangements are badly preserved in fossil calcite statoliths. In large extant statoliths, concentric structures, mainly in the form of superficial striation and/or concentric microstrata, are visible in coexistence with radial aggregates. Stratification is possibly due to stratified deposition of the nonmineralized gland product, while the spherulitic structure is indicative of subsequent radial growth of crystal aggregates. The structure of accessory fluorite statoliths in the statocyst of Mesopodopsis slabberi leads to the hypothesis that mantle material is formed by secretions of the caudal statocyst gland. After demineralization of fluorite, vaterite and calcite statoliths, an organic template remains showing most essential morphological features of the statolith. From this we conclude that the structure of the statolith is (almost) entirely matrix mediated. © 1993 Wiley-Liss, Inc.     

Light- and electron-microscopic analysis of the statocyst of the American crayfishOrconectes limosus (Crustacea,Decapoda)

Summary The statocyst ofOrconectes limosus contains static hairs arranged in four groups. All the hairs are the same in basic structure; they differ only in length and diameter and in their positions with respect to the other hairs in the group and to the statolith. In terms of functional morphology, each static hair is part of a unit consisting of an acellular lever string, three receptor cells, a scolopale cell, sheath cells, and enveloping cells. The lever string comprises two components in a characteristic longitudinal arrangement. The structure of the receptor cells resembles that of the arthropod chemo- and mechanoreceptors studied previously. The cilium and the postciliary section lie within two receptor cavities, formed by the scolopale cell and the sheath cells; the two cavities communicate with one another. The receptor cells are fixed in position by various structures. Proximally they form desmosomes with the scolopale cell, medially they are joined by filaments to the inner wall of cavity 1, and distally they are retained by a constriction between the two cavities. Two possible stimulus-mediating mechanisms are discussed: pressure changes in the receptor cavities and shearing of the base of the cilia with respect to the preciliary region. The lever string is part of the cuticle and hence is shed during molting. Nevertheless, the statocyst remains functional during this process because new structural units are formed below the old cuticle prior to ecdysis.Abbreviations a axon - b bulb - bb basal body - c cilium - cu cuticle - d dendrite - de desmosome - dm dense material - ec enveloping cell - f fulcrum - h hair - hs hair shaft - ir inner row of hair group - l lingula - ls lever string - m mitochondrion - n nucleus - or outer row of hair group - pcd postciliary dilation - R1 receptor cavity 1 - R2 receptor cavity 2 - rc receptor cell - ro rootlet - s sheath cell - sc scolopale cell - st statoconium - t tooth     

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.

     

Carbonic anhydrase is required for statoconia homeostasis in organ cultures of statocysts from Aplysia californica

A novel organ culture system has been developed to study the regulation of statoconia production in the gravity sensing organ in Aplysia californica. Statocysts were cultured in Leibovitz (L15) medium supplemented with salts and Aplysia haemolymph for four days at 17°C. The viability of the system was evaluated by examining four parameters: statocyst morphology, the activity of the mechanosensory cilia in the statocyst, production of new statoconia during culture and change in statoconia volume after culture. There were no morphological differences in statocysts before and after culture when ciliary beating was maintained. There was a 29% increase in the number of statoconia after four days in culture. Mean statocyst, statolith and statoconia volumes were not affected by culture conditions. The presence of carbonic anhydrase in the statocysts was shown using immunohistochemistry. When statocysts were cultured in the presence of 4.0 × 10^–4 M acetazolamide to inhibit the enzyme activity, there was a decrease in statoconia production and statoconia volume, indicating a role for this enzyme in statoconia homeostasis, potentially via pH regulation. These studies are the first to report a novel system for the culture of statocysts and show that carbonic anhydrase is involved in the regulation of statoconia volume and production.     

The statocyst of Vampyroteuthis infernalis (Mollusca: Cephalopoda)

The statocyst shows a remarkable combination of features of decapods and octopods confirming that Vampyroteuthis is a relic somewhere near the ancestor of both groups. The lining of the statocyst separates from the outer wall, forming an inner sac, filled with endolymph, surrounded by perilymph. This is the condition found in octopods, never in decapods. The macula is partly divided into a macula princeps and macula neglecta, as in decapods but never in octopods. There are numerous statoconia, but no large statolith has been seen. The crista has four parts as in decapods, but they are not sharply separated. There are numerous small anticristae, with the general form found in decapods, differentiated into pegs and hooks.
The wall of the inner sac contains numerous hair cells. These hairs protrude between the epithelial cells. The bases of the cells are drawn out into fine processes, presumably some dendritic and some axonal. There is thus a plexus of nerve fibres all over the wall, communicating with the crista nerve.
There is a very large posterior sac of unknown function, lying behind the crista. It contains only one large anticrista and the opening of Kölliker's canal, which is very large.     

Morphology of descending statocyst interneurons in the crayfish Procambarus clarkii Girard

Summary The morphological features of descending interneurons that responded to the artificial bending of statolith hairs were assessed with intracellular recording and staining techniques. Seven statocyst interneurons were identified on the basis of their structure and response characteristics and designated as interneurons S1 to S7. All seven identified interneurons project to the optic lobe, where the optic nerve also projects, and to the dorsal part of the tritocerebrum, where the eyestalk motoneurons originate. All except interneuron S6 also extend their major branches to other neuropilar regions. S2 projects to the dorsal part of the deutocerebrum, where the statocyst nerve terminates, and S3 to the dorsal part of deutocerebrum and the antennal lobe. Four other interneurons (S1, S4, S5, S7) also extend their branches to the parolfactory lobe to which the statocyst nerve projects as well as to the deutocerebrum and antennal lobe. The extensive dendritic projections of S1–S7 suggest that they are complex multimodal interneurons rather than simple relay interneurons, receiving at least visual and statocyst sensory information. The function of the antennal lobe branches, however, has yet to be determined since the functional role of antennal input in equilibrium control is unknown.     

Some aspects of organization and histochemistry of the embryo sac ofScilla sibirica sato

Summary The present investigation deals with some of the organizational and histochemical aspects of the embryo sac ofScilla sibirica. Both the synergids and egg cell are invested by PAS-positive complete walls. The filiform apparatus comprises an elaborate system of fibrillar projections, showing extensive ramifications. The micropylar region of the embryo sac wall from where the filiform apparatus originates is composed of three distinct layers. On a histochemical basis it may be surmised that, unlike the egg cell, the synergids are metabolically very active. Two kinds of wall ingrowths (i) massive and highly branched very much akin to the filiform apparatus, and (ii) small tuberculate wall projections, are unique to the antipodal cells of S.sibirica. Small tuberculate projections have also been observed along the wall of the central cell adjacent to the nutrient-rich nucellar cells. The antipodals and the central cell show the presence of starch grains and abundant total proteins. All the cell types in the embryo sac ofS. sibirica are structurally so organized as to meet the requirements of its nutrition during pre- and postfertilization development. The presence of abundant PAS-positive granular substance in the cells of nucellar epidermis probably establishes a gradient which assists in the pollen tube growth.