GASTROPOD CHEMORECEPTION

A comparative ultrastructural investigation of the cephalic sensory organs in Opisthobranchia (Mollusca, Gastropoda)

Cephalic sensory organs (CSOs) are specialised structures in the head region of adult Opisthobranchia involved in perception of different stimuli. The gross morphology of these organs differs considerably among taxa. The current study aims at describing the cellular morphology of the CSOs in order to reveal cellular patterns, especially of sensory epithelia, common for opisthobranchs. Transmission electron microscopy was used to characterise the fine structure of the organs and to compare the CSOs of four different opisthobranch species. The cellular composition of the sensory system is conserved among taxa. The epidermal cells in sensory regions are always columnar and ciliated cells are frequently apparent. The sensory cells are primary receptors arranged in subepidermal cell clusters. They extend dendrites which penetrate the epithelium and reach the surface. Some of the dendrites bear cilia, whereas others only build a small protuberance. Processing of sensory information takes place in the peripheral glomeruli of all species. Moreover, few taxa possess additional peripheral ganglia at the base of their CSOs. The results of the present study might support other investigations indicating that the posterior CSOs are primarily involved in distance chemoreception, whereas the anterior CSOs might be used for contact chemoreception and mechanoreception.

Neurogenesis of cephalic sensory organs of Aplysia californica

The opisthobranch gastropod Aplysia californica serves as a model organism in experimental neurobiology because of its simple and well-known nervous system. However, its nervous periphery has been less intensely studied. We have reconstructed the ontogeny of the cephalic sensory organs (labial tentacles, rhinophores, and lip) of planktonic, metamorphic, and juvenile developmental stages. FMRFamide and serotonergic expression patterns have been examined by immunocytochemistry in conjunction with epifluorescence and confocal laser scanning microscopy. We have also applied scanning electron microscopy to analyze the ciliary distribution of these sensory epithelia. Labial tentacles and the lip develop during metamorphosis, whereas rhinophores appear significantly later, in stage 10 juveniles. Our study has revealed immunoreactivity against FMRFamides and serotonin in all major nerves. The common labial nerve develops first, followed by the labial tentacle base nerve, oral nerve, and rhinophoral nerve. We have also identified previously undescribed neuronal pathways and other FMRFamide-like-immunoreactive neuronal elements, such as peripheral ganglia and glomerulus-like structures, and two groups of conspicuous transient FMRFamide-like cell somata. We have further found two distinct populations of FMRFamide-positive cell somata located both subepidermally and in the inner regions of the cephalic sensory organs in juveniles. The latter population partly consists of sensory cells, suggesting an involvement of FMRFamide-like peptides in the modulation of peripheral sensory processes. This study is the first concerning the neurogenesis of cephalic sensory organs in A. californica and may serve as a basis for future studies of neuronal elements in gastropod molluscs.

Habitat associations in gastropod species at East Pacific Rise hydrothermal vents (9 degrees 50'N)

At deep-sea hydrothermal vents on the East Pacific Rise (9 degrees 50'N), distinct megafaunal assemblages are positioned along strong thermal and chemical gradients. We investigated the distribution of gastropod species to determine whether they associate with specific megafaunal zones and to determine the thermal boundaries of their habitats. Gastropods colonized a series of basalt blocks that were placed into three different zones characterized by vestimentiferan tubeworms, bivalves, and suspension-feeders, respectively. Additional gastropods were collected on selected blocks from higher temperature vestimentiferan habitat and from grab samples of alvinellid polychaetes. On the blocks, gastropod species clustered into a "Cool" group (Clypeosectus delectus, Eulepetopsis vitrea, Gorgoleptis spiralis, and Lepetodrilus ovalis) whose species tended to be most abundant in the suspension-feeder zone, and a "Warm" group (Lepetodrilus cristatus, L. elevatus, L. pustulosus, and Cyathermia naticoides) whose species all were significantly more abundant in the vestimentiferan zone than elsewhere. The temperature ranges of Cool species were generally lower than the ranges of Warm ones, although both groups were present at 3 to 6 degrees C; also present was Bathymargarites symplector, which clustered with neither group. Three additional species, Rhynchopelta concentrica, Neomphalus fretterae, and Nodopelta rigneae, co-occurred with Warm-group species on selected blocks from hotter habitats. Although a few species were found only in alvinellid collections, most species were not exclusive to a specific megafaunal zone. We propose that species in the Cool and Warm groups occupy specific microhabitats that are present in more than one zone.

Volatile Foraging Kairomones in the Littoral Zone: Attraction of an Herbivorous Freshwater Gastropod to Algal Odors

Volatile organic compounds (VOCs) produced by algae and cyanobacteria are primarily responsible for odors in fresh waters. Among other functions, VOCs may serve as important infochemicals in biofilms of benthic primary producers. VOCs liberated by benthic, mat-forming cyanobacteria can be used as habitat-finding cues by insects, nematodes, and possibly other organisms. We developed a new gastropod behavioral assay that allows detection of food preference without offering food, thus allowing the distinction between taste, which requires direct contact with the food source, and the detection of odorous infochemicals, which work over distance. We demonstrated that VOCs released from disintegrated cells of a benthic, mat-forming, green alga (Ulothrix fimbriata) are food-finding cues ("foraging kairomones") that attract the herbivorous freshwater snail Radix ovata. A mixture of three C5 lipoxygenase compounds and 2(E),4(E)-heptadienal that mimic the major VOCs released by U. fimbriata attracted the snails, whereas neither the mixture of C5 compounds nor 2(E),4(E)-heptadienal were effective when given alone. This study suggests that VOCs can play a steering role as infochemicals in freshwater benthic habitats, as has been established for many organismic interactions in terrestrial ecosystems.

Odours detected by rhinophores mediate orientation to flow in the nudibranch mollusc, Tritonia diomedea

Tritonia diomedea is a useful neuroethological model system that can contribute to our understanding of the neural control of navigation. Prior work on both sensory and locomotory systems is complemented by recent field experiments, which concluded that these animals primarily use a combination of odours and water flow as guidance cues. We corroborate these field results by showing similar navigation behaviours in a flow tank. Slugs crawled upstream towards both prey and conspecifics, and turned downstream after crawling into a section of the flow tank downstream of a predator. Controls without upstream odour sources crawled apparently randomly. We then tested whether these behaviours depend on odours detected by the rhinophores. Outflow from a header tank was used to generate prey, predator and unscented control odour plumes in the flow tank. Slugs with rhinophores crawled upstream towards a prey odour plume source, turned downstream in a predator odour plume, and showed no reaction to a control plume. Slugs without rhinophores behaved similarly to controls, regardless of odour plume type. Finally, we used extracellular recordings from the rhinophore nerve to demonstrate that isolated rhinophores are chemosensitive. Afferent activity increased significantly more after application of all three odour types than after unscented control applications. Responses were odour specific. We conclude that rhinophores mediate orientation to flow, and suggest that future work should focus on the integration of mechanosensation and chemosensation during navigation in T. diomedea.

Attraction to amino acids by Lymnaea acuminata, the snail host of Fasciola species

Adult Lymnaea acuminata (average length 20-22 mm) were collected locally from lakes and low-lying submerged fields from Gorakhpur. The chemoattraction studies were made in round glass aquaria measuring 30 cm in diameter and filled to a depth of 10 mm with 500 ml dechlorinated tap water. Each aquarium was divided into four concentric zones. At the starting time of the assay 10 snails were placed on the circumference of outermost zone 0. Snail attractant pellets (SAP) were added simultaneously in the center of central zone 3. SAP of different amino acids were prepared at concentrations of 10, 20, 50, 80 and 100 mM/2% agar solution and, subsequently, spread to a uniform thickness of 5 mm. After cooling, SAP were cut in small pieces of 5 mm in diameter. Lymnaea acuminata's attraction to amino acids was studied using different amino acid concentrations in SAP. Pellets containing amino acids with non-polar R groups (proline and tryptophan), a charged polar group (arginine) and uncharged polar R groups (serine, citrulline and asparagine) were tested. The snails were more attracted to the uncharged polar R group amino acid serine than to other groups of amino acids. The preferred amino acid concentration was 80 mM. The attraction of snails to different amino acids was concentration dependent. Snails could discriminate amongst the different amino acids at > or = 50 mM.

Complexities of a simple system: new lessons, old challenges and peripheral questions for the gill withdrawal reflex of Aplysia

The gill withdrawal reflex of Aplysia is generally depicted as a simple behaviour mediated by a simple neural circuit in a simple organism. Such a view has permitted a clear focus upon synapses between relatively small numbers of identified neurones, which are known to participate in the reflex and its plasticity. Ensuing research has provided some of the first and still among the most powerful explanations of the cellular underpinnings of learning and memory. In reality, however, the reflexive withdrawal of the gill and other mantle organs is anything but simple. First, the behaviour itself is complex and varies depending upon the strength of the tactile stimulus and where it is applied. In addition, over 100 central neurones are activated by stimuli, which elicit the withdrawal reflex and likely change their activities during learning (although not all of these cells necessarily contribute to the actual withdrawal response). Moreover, multiple mechanisms are activated at both presynaptic and postsynaptic sites to orchestrate the numerous modifications that underlie observed changes in synaptic efficacy. The picture becomes even more complicated when hundreds of additional peripheral neurones, which are known to participate in various aspects of the response, are also considered. Recent work has shifted attention back to these peripheral cells by suggesting that they might be the previously unidentified light touch receptors that mediate both central and peripheral components of the reflex. While daunting, the complexity of the total circuitry mediating the gill withdrawal reflex may provide yet another important lesson: even in simple systems, memory may not be localized to specific loci, but rather may be an emergent property of physiological mechanisms distributed throughout the entire circuitry.

Feeding clusters and olfaction in the mangrove snail Terebralia palustris (Linnaeus) (Potamididae: Gastropoda)

Large numbers of the snail Terebralia palustris (Linnaeus) (Potamididae; Gastropoda) are frequently observed feeding in a cluster on a single fallen mangrove leaf, yet none are present on leaves nearby. Consequently, we investigated the food-finding ability of T. palustris in a Kenyan mangrove forest using field experiments. We estimated the attractive effect of different cues and analysed the paths (video-recorded) of snails when approaching a food-related odour. This intertidal snail can potentially use both air-borne and water-borne odours to locate food. T. palustris is attracted to conspecifics feeding on leaves, while intact leaves as well as non-feeding snails are not attractive. Moreover, the guiding stimulus appears to be compounds released when the leaves are damaged.T. palustris also seems able to discriminate between different food items; it is more strongly attracted to green mangrove leaves than senescent or fallen ones or mangrove propagules, probably because green leaves release a greater amount of attractive cues.Feeding snails thus recruit more snails to feed on the same leaf. The ecological implications of this behaviour are discussed: a large number of snails on the same leaf counteracts the ability of crabs to remove the leaf being eaten by the snails.

Development of catecholaminergic neurons in the pond snail, Lymnaea stagnalis: II. Postembryonic development of central and peripheral cells

Catecholamines have long been thought to play important roles in different mollusc neural functions. The present study used glyoxylate- and aldehyde-induced histofluorescence to identify central and peripheral catecholaminergic neurons in the snail Lymnaea stagnalis. The majority of these cells were also found to react to antibodies raised against tyrosine hydroxylase. A minority of the catecholaminergic neurons, however, exhibited no such immunoreactivity. The number of central catecholaminergic neurons nearly doubled (from about 45 to about 80 cells) during the first 2-3 days of postembryonic development. Thereafter, catecholaminergic neurons again doubled in number and generally grew by about 100-200% in soma diameter as the snails grew by 1,000% in overall linear measurements. In contrast to the relatively meager addition of central catecholaminergic neurons, several thousand catecholaminergic somata were added to different peripheral tissues during postembryonic development. These small, centrally projecting neurons were particularly concentrated in the lips, esophagus, anterior margin of the foot, and different regions of the male and female reproductive tracts. Chromatographic analyses indicated that dopamine was the major catecholamine present in the central ganglia, foot, and esophagus, although detectable levels of norepinephrine (approximately 20% of dopamine levels) were also found in the ganglia. The total content but not the concentration of dopamine increased within the tissue samples during postembryonic development. The companion study (Voronezhskaya et al. [1999] J. Comp. Neurol. 404:285-296) and the present study furnish a complete description of central and peripheral catecholaminergic neurons from their first appearance in early embryonic development to adulthood.

Sperm allocation in the simultaneously hermaphroditic land snail Arianta arbustorum

To test the idea that individuals of the simultaneously hermaphroditic land snail Arianta arbustorum can control the number of spermatozoa in their spermatophores, we investigated whether they differentially release sperm to virgin or nonvirgin partners with respect to the potential risk of sperm competition in a given mating. The number of sperm transferred ranged from 802 620 to 3 968 800 (X= 2 185 100; N=91), but was related neither to the mating history of the partner nor to copulation duration. This indicates that individuals of A. arbustorum are not able to adjust sperm expenditure to the mating history of the partner. Furthermore, the number of sperm transferred was correlated neither with the size of the donor nor with the size of the recipient. It has been proposed that the sexual conflict between the two genders in simultaneous hermaphrodites could be resolved by gamete trading. Theory predicts that sperm trading should occur in hermaphrodites in which the female role controls fertilization, for example in gastropods with a gametolytic gland and/or sperm storage such as A. arbustorum. To see whether sperm trading occurs, we also examined whether individuals of A. arbustorum adjust the number of sperm they release to the number they receive from their mating partner. There was a high degree of reciprocity in spermatophore transfer: in 45 of the 46 mating pairs investigated both partners delivered a spermatophore that contained spermatozoa. The numbers of sperm transferred by the two mating partners were not correlated, however. This indicates that sperm trading does not occur in this simultaneously hermaphroditic land snail. Copyright 1998 The Association for the Study of Animal Behaviour.

GABA-immunoreactive neurones and interactions of GABA with serotonin and FMRFamide in a peripheral sensory ganglion of the pond snail Lymnaea stagnalis

The osphradium is a putative chemosensory organ of aquatic molluscs. Previously, we identified cells with serotonin (5-HT) and FMRFamide (FMRFa)-like immunoreactivity in the osphradial ganglion of Lymnaea stagnalis. The present investigation has established the presence of cells immunoreactive to gamma-aminobutyric acid (GABA). Some of these cells send processes to the sensory epithelium and are thus considered to be primary sensory neurones. Colocalisation of GABA and FMRFamide-like immunoreactivities was found in some of these and other neurones. The responses of the osphradial output electrical activity to the single and combined application of the above neuroactive substances were examined. 5-HT slightly increased and FMRFa decreased the activity. GABA alone was generally ineffective; however, it had a consistent stimulating effect after pretreatment with 5-HT. In its turn, pretreatment with GABA significantly increased the inhibitory action of FMRFa. Primary sensory neurones giving this kind of responses in the nerve were identified electrophysiologically and morphologically in the osphradial ganglion. Our results indicate that GABA takes part in relay of sensory signals into the central nervous system, and transmitter interactions involving GABA, 5-HT, and FMRFa are considerable for the final output pattern of the osphradial sensory network.

Ultrastructure of the osphradium of the Tertiary relict snail, Campanile symbolicum Iredale (Mollusca, Streptoneura)

The osphradium of Campanile symbolicum Iredale, 1917 is a gill-like, bipectinate sense organ, which is located at the left side of the mantle roof. The mass of the deeply clefted sensory epithelium of the leaflets is built up by sensory cells, which are provided with deeply invaginated aberrant cilia and large cytosomes containing pigment formations. In addition, many free nerve processes are present, bearing a single or few sensory cilia with accessory centrioles. Polyciliary cells are interspersed. A cell type with netlike or concentrically arranged smooth endoplasmic reticulum is commonly found near the central axis of the osphradium . The central zone of each leaflet includes nervous tissue and a complicated muscular grid, with pore cells and fibroblasts also present. Based on the fine-structural data the functional and ecological significance of the osphradium of Campanile symbolicum is discussed. The com bination of herbivory and a lamellar osphradium is rare among the Gastropoda, suggesting that the osphradium of Campanile might also be involved in reproductive biology. Many fine-structural features of the osphradium of Campanile symbolicum are unique among the gastropods and reflect the phylogenetic isolation of this relict snail. The net-like cell type, however, is probably homologous with the so-called Si4 cell in the rem aining caenogastropods, for which a largely different osphradial fine-structure is diagnostic. The affinities of Campanile symbolicum are probably closer to the Caenogastropoda than to the Allogastropoda and Euthyneura. With present knowledge it might be best classified near the base or even as the first clade within the Caenogastropoda.

Gangliogenesis in the prosobranch gastropod Ilyanassa obsoleta

We determined that the larval nervous system of Ilyanassa obsoleta contains paired cerebral, pleural, pedal, buccal and intestinal ganglia and unpaired apical, osphradial, and visceral ganglia. We used a modified form of NADPH diaphorase histochemistry to compare the neuroanatomy of precompetent (including specimens 6, 8, and 12 days after hatching), competent, and metamorphosing larvae with postmetamorphic juveniles. This method highlighted ganglionic neuropils and allowed us to identify individual ganglia at various stages of development, thereby laying a foundation for concurrent histochemical studies. The first ganglia to form were the unpaired apical and osphradial ganglia and the paired cerebral and pedal ganglia. In larvae 6 days after hatching, the neuropil had already appeared in the apical and osphradial ganglia. Neuropil began to be apparent in the cerebral and pedal ganglia 2 days later. At that time, the pleural and buccal ganglia were identifiable and adjacent to the posterior edge of the cerebral ganglia. The ganglia of the visceral loop were concurrently recognizable, although the supraintestinal ganglion developed slightly earlier than the subintestinal and visceral ganglia. By 12 days after hatching all of the major adult ganglia were discernible. The apical ganglion was retained by newly metamorphosed juveniles, but not by juveniles 2 days later. After metamorphosis was complete, the central nervous system (CNS) was consolidated into its juvenile form with ipsilateral cerebral and pleural ganglia being partially fused. The metamorphic translocation of ganglia, which included a caudal relocation of the cerebrals and the migration of the buccals from above the esophagus to a position below it, correlated with the movement of the proboscis to the dorsal part of the head.

Distribution in the central nervous system of Aplysia of afferent fibers arising from cell bodies located in the periphery

The present study using autoradiography to determine the location of the projections of presumptive peripheral afferent neurons into the central nervous system of Aplysia. Selected peripheral tissues (with an emphasis on structures involved in feeding behavior) were exposed to radioactive amino acids, and the distribution of macromolecules transported into the nervous system via afferent fibers was determined by autoradiography. Different regions of the body exhibited different patterns of projections, and, within the neuropil of the cerebral ganglion, there was a loose topographical organization of projections from the head. For some regions of the body, the projections was largely limited to the ganglion from which the nerve enters; for other regions, the projection was very widespread. In some cases (e.g., rhinophore to eye), there was evidence of projections from one peripheral structure to another. Experiments with all peripheral tissues that were studied resulted in extensive labeling of central ganglia, indicating that afferents with peripheral cell bodies may provide a major source of sensory input to the central nervous system and suggesting that many or all of the numerous ultrafine axons visualized via electron microscopy in the nerves of Aplysia may originate from first- or second-order sensory afferents whose cell bodies are located in the periphery.

Detection of FMRFamide-like immunoreactivities in the sea scallop Placopecten magellanicus by immunohistochemistry and western blot analysis

FMRFamide-like immunoreactivity was detected histochemically in the sea scallop Placopecten magellanicus. Most immunoreactivity was concentrated in the cerebral, pedal, and parietovisceral ganglia, particularly in the cortical cell bodies and in their fibers which extend into the central neuropile. Whole-mount immunofluorescence studies were used to localize concentrations of immunoreactive cells on the dorsal and ventral surfaces of each ganglion. Immunoreactivity was also detected in nerves emanating from the ganglia. Strong immunoreactivity was localized in peripheral organs, including the gut and gills of juvenile and adult scallops. Weak immunoreactivity was detected in the gonads, heart, and adductor muscle of the adults. A broad FMRFamide-like immunoreactive band of 2.5-8.2 kDa was detected by Western blotting of acetone extracts of the parietovisceral ganglia. In the presence of protease inhibitors, two FMRFamide-like immunoreactive bands (7.2-8.2 kDa and > 17 kDa) were obtained. Neither of these bands comigrated with the FMRFamide standard. It is concluded that peptides of the FMRFamide family are probably regulators of numerous central and peripheral functions in P. magellanicus.

Developmental analysis reveals labial and subradular ganglia and the primary framework of the nervous system in nudibranch gastropods

Previous ultrastructural observations on late stage larvae of dorid nudibranchs (Gastropoda, Opisthobranchia) revealed two pairs of ganglia within the base of the foot that do not have obvious counterparts in existing descriptions of other gastropod larvae [Chia and Koss (1989). Cell Tiss. Res. 256:17-26.] One of these ganglionic pairs has been implicated in the initiation of settlement preceding metamorphosis [Arkett et al. (1989). Biol. Bull. 176:155-160.] By examining neurogenesis in sequential larval stages, I have found that the pattern of connectives and commissures associated with these enigmatic ganglia is comparable to patterns found in less consolidated adult nervous systems of chitons, monoplacophorans, and archaeogastropods. These comparative data suggest that the two pairs of ganglia in dorid nudibranch larvae are homologues of labial and subradular ganglia. The labial ganglia become incorporated into the cerebral ganglia at metamorphosis. In an attempt to integrate anatomical and developmental observations with behavioral and neurophysiological results, I suggest that receptor cells of the larval labial ganglia may become postmetamorphic primary mechanoreceptors of the oral tube, which have central cell bodies within the "cerebral" ganglia and which help coordinate feeding. Results of this study also address a larger evolutionary issue by questioning the traditional model of an ancestral molluscan nervous system that consists of four longitudinal nerve cords that arise from separate sites along a circumesophageal nerve ring. This pattern results from secondary connections in nudibranchs and possibly other molluscs. The primary condition of a single axon bundle emerging from each cerebral ganglion is more similar to the developing nervous system in polychaete annelids than what has been recognized previously.

The sugar chemoreception niches of Bulinus (physopsis) globosus (Morelet) and Bulinus rohlfsi (Clessin), the two most important snail hosts of Schistosoma haematobium (Bilharz) in west Africa

1. The behavioural responses of two species of freshwater pulmonate snails [Bulinus (P.) globosus and Bulinus rohlfsi] to sugar gradients were investigated by means of diffusion olfactometers. 2. Both snail species proved to be very discriminating in their responses. Of the 17 sugars tested, 35.3%, namely D(-)glucuronic, maltotriose, maltose, cellobiose, D(-)arabinose, D(+)mannose proved to be statistically significant attractants or arrestants to B. rohlfsi. Only 23.5% of these sugars (maltotriose, maltose), D(+) mannose and D(+) xylose were significant attractants or arrestants to B. (P.) globosus. 3. Glucuronic acid was a significant repellent to B. rohlfsi but none of the sugars was a repellent to B. (P.) globosus. 4. The results are compared with those obtained for other snail species and their relevance to the ecology and control of the snails are discussed.

Tracing neural pathways in snail olfaction: from the tip of the tentacles to the brain and beyond

The anatomical organization of the olfactory system of terrestrial snails and slugs is described in this paper, primarily on the basis of experiments using the African snail Achatina fulica. Behavioral studies demonstrate the functional competence of olfaction in mediating food finding, conspecific attraction, and homing. The neural substrate for olfaction is characterized by an extraordinarily large number of neurons relative to the rest of the nervous system, and by the fact that many of them are unusually small. There exist multiple serial and parallel pathways connecting the olfactory organ, located at the tip of the tentacle, with integrative centers in the central nervous system. Our methods of studying these pathways have relied on the selective neural labels horseradish peroxidase and hexamminecobaltous chloride. One afferent pathway contains synaptic glomeruli whose ultrastructure is similar to that of the glomeruli seen in the mammalian olfactory bulb and the insect olfactory lobe. All of the olfactory neuropils, but especially the tentacle ganglion, contain large numbers of morphologically symmetrical chemical synapses. The procerebrum is a unique region of the snail brain that possesses further features analogous with olfactory areas in other animal groups. Olfactory axons from the tentacle terminate in the procerebrum, but the intrinsic neurons do not project outside of it. An output pathway from the procerebrum to the pedal ganglion has been identified and found to consist of inter-ganglionic dendrites. The major challenge for future studies is to elucidate the pattern of connectivity within, rather than between, the various olfactory neuropils.

Fine structure of olfactory epithelia of gastropod molluscs

Among gastropod molluscs the chemical senses are most important for location of distant objects. They are used in food finding, locating mates, avoiding predators, trail following, and homing. Chemoreceptors are commonly associated with the oral area, the tentacles, and the osphradium, which lies in the mantle cavity. Most chemosensory neurons are primary sensory neurons, although secondary sensory cells have been reported in the osphradium of some prosobranch gastropods. Most chemosensory organs contain sensory cells with ciliated sensory endings that are in contact with the external environment. Some sensory endings have only microvilli or have no surface elaborations. Cilia on sensory endings are commonly of the conventional type, but some species have modified cilia; some lack rootlets, some have an abnormal microtubular content, and some have paddle-shaped endings. The perikarya of sensory neurons may be within the sensory epithelium, below it, or in ganglia near the sensory surface. In some groups of gastropods there are peripheral ganglia in the olfactory pathway; in others chemosensory axons appear to pass directly to the CNS. Olfactory epithelia of terrestrial pulmonates have modified brush borders with long branching plasmatic processes and a spongy layer of cytoplasmic tubules which extend from the epithelial cells. Sensory endings of the olfactory receptors are entirely within this spongy layer. Aquatic pulmonates may have a similar spongy layer in their olfactory epithelia, but the cilia of sensory endings, as well as motile cilia of epithelial cells, extend well beyond the spongy layer.

A study of the sugar chemoreception niches of two bulinid snail hosts of schistosomiasis

Components of the sugar chemoreception niches of two host snails of urinary schistosomiasis, namely Bulinus globosus (Morelet) and Bulinus rohlfsi (Clessin), were measured by using a buccal mass olfactometer. Among the polysaccharides tested, amylose was found to be the strongest phagostimulant for adults and juveniles of both snail species. Other phagostimulants identified were maltose and xylose for B. rohlfsi and maltotriose for B. globosus. The a(1-4)-glucosidic linkage and the presence of glucose residues were found to be key factors in the stimulus recognition system of the snails. The possible use of these findings in the design of controlled-release formulations for the selective removal of schistosome host snails is considered. The ecological implications of these studies are also examined.

The biochemical ecology of Biomphalaria glabrata, a snail host of Schistosoma mansoni: short chain carboxylic and amino acids as phagostimulants

1. A buccal mass olfactometer was used to investigate the responses of the fresh-water pulmonate snail Biomphalaria glabrata to carboxylic and amino acids. 2. The snails proved very discriminating as only 6 (14.6%) of the 41 chemical species tested were effective as phagostimulants. These are ranked as follows in order of potency:- butanoate greater than propanoate greater than D-malate greater than 2-hydroxybutanoate = L-tartrate = L-aspartate. 3. The structure-activity relationships of the active compounds, and their significance to the ecology and control of the snails are discussed.

Exposure to novel odors induces opioid-mediated analgesia in the land snail, Cepaea nemoralis

Land snails, Cepaea nemoralis, that were exposed for 1-30 min to a novel odor of either peppermint extract or vegetable juice concentrate displayed an increase in the latency of their nociceptive response to an aversive thermal stimulus (40 degrees C, hot-plate). This "analgesic" response, which entailed the elevation of the fully extended foot in hydrated snails, was evident directly after exposure to the novel chemostimuli and lasted for 15-30 min. This novelty-induced analgesia was blocked by the exogenous opiate antagonist naloxone. Analgesia was not observed in snails that were exposed to the same olfactory cue 4 or 24 h later, but was evident when the alternate novel odor (peppermint or vegetable juice) was presented. However, a significant analgesia was displayed by snails that were reexposed to their initial olfactory stimulus after 48-72 h. These findings indicate that exposure to a novel olfactory stimulus can activate endogenous opioid systems and induce an analgesic response in mollusks.