The development of the serotonergic and FMRF-amidergic nervous system in<Emphasis Type="Italic"> Antalis entalis</Emphasis> (Mollusca,Scaphopoda)

The morphogenesis of serotonin- and FMRF-amide-bearing neuronal elements in the scaphopod Antalis entalis was investigated by means of antibody staining and confocal laser scanning microscopy. Nervous system development starts with the establishment of two initial, flask-like, serotonergic central cells of the larval apical organ. Slightly later, the apical organ contains four serotonergic central cells which are interconnected with two lateral serotonergic cells via lateral nerve projections. At the same time the anlage of the adult FMRF-amide-positive cerebral nervous system starts at the base of the apical organ. Subsequently, the entire neuronal complex migrates behind the prototroch and the six larval serotonergic cells lose transmitter expression prior to metamorphic competence. There are no strictly larval FMRF-amide-positive neuronal structures. The development of major adult FMRF-amide-containing components such as the cerebral system, the visceral loop, and the buccal nerve cords, however, starts before the onset of metamorphosis. The anlage of the putative cerebral system is the only site of adult serotonin expression in Antalis larvae. Establishment of the adult FMRF-amidergic and serotonergic neuronal bauplan proceeds rapidly after metamorphosis. Neurogenesis reflects the general observation that the larval phase and the expression of distinct larval morphological features are less pronounced in Scaphopoda than in Gastropoda or Bivalvia. The degeneration of the entire larval apical organ before metamorphic competence argues against an involvement of this sensory system in scaphopod metamorphosis. The lack of data on the neurogenesis in the aplacophoran taxa prevent a final conclusion regarding the plesiomorphic condition in the Mollusca. Nevertheless, the results presented herein shed doubts on general theories regarding possible functions of larval "apical organs" of Lophotrochozoa or even Metazoa.     

Larval and early post-larval shell of three deep-sea Scaphopoda (Mollusca) from the southwest Atlantic

The larval shells of Antalis circumcincta (Watson, 1879) (order Dentaliida), Pertusiconcha callithrix (Dall, 1889) (order Gadilida) and of an undetermined species of uncertain systematic position are described. The material studied comprises mainly samples from deep waters, collected by expeditions along the southeast coast of Brazil. The larval shell of the three taxa matches other types previously described in the literature. Antalis circumcincta and P. callithrix have four regions (protoconch A, protoconch B, teleoconch A, teleoconch B), but differ in dimensions and sculpture from each other, while the undetermined species has three regions (protoconch A, protoconch B, teleoconch B). A morphometric approach combined with a discriminant analysis also indicates that the three taxa are significantly distinct. This study confirms patterns of larval shells at the taxonomic rank of orders but other supraspecific patterns remain uncertain.     

The Ultrastructure and Functional Morphology of a Captaculum in Graptacme calamus(Mollusca,Scaphopoda)

The structure and function of the captaculum in Graptacme calamuswere studied using light microscopy, videomicrography, and transmission electron microscopy. Graptacme calamusutilizes a longitudinal ciliary band to transport small food particles along the outstretched filament. The head of the captaculum, which bears the alveolar indentation, contains a large ganglion, at least three types of gland cells, circular and longitudinal muscles, and connective tissue. It is likely that gland 1 secretes mucus which is used in the transport of food particles and that gland 2 secretes material which promotes adhesion of the alveolus to the substratum. The function of gland 3 is unknown. Critical observation of the captaculum surface revealed that the cilia of the alveolar region have truncated tips unlike those found on other regions of the captaculum. These alveolar cilia probably aid in the distribution of adhesive substance and in adhesion.     

Fine structure of the kidney and characterization of secretory products inDentalium rectius (Mollusca,Scaphopoda)

Summary The ultrastructure of the scaphopod kidney and secretory product composition is described, for the first time, inDentalium rectius. The kidney epithelium consists of two primarily secretory cell types. The first exhibits extensive vacuolation, and scattered granules are formed within the vacuolar space by a process of surface accretion; the incorporation of glycogen particles in this process is associated with very fine, electronopaque threads which radiate from the granule. The second cell type possesses granules enclosed individually within secretory vesicles, and intermediate stages in their growth are characterized by needle-like crystals on the granule surface. The secretory vesicles in some cases coalesce to form a large central vacuole filled with granules. This cell type possesses an apical membrane with sparse microvilli, which may indicate a secondary reabsorptive capacity. Granules in both cell types show a concentric ring ultrastructure, and are composed primarily of calcium phosphate with a small amount of zinc; there is also an organic component of protein, mucopolysaccharide and a large amount of glycogen. Ultrastructural and histochemical observations indicate a lysosomal origin for the granules, although granules of the second cell type develop intracellularly to a greater extent than those of the first. All granules are extruded into the kidney lumen by a process of merocrine secretion prior to release into the mantle cavity via an externally ciliated, muscular excretory pore.     

Comparative neuroanatomy of Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda (Mollusca) and its phylogenetic implications

The nervous system of invertebrates is considered to be a very conservative organ system and thus can be helpful to elucidate questions of phylogenetic relationships. Up to now, comparative neuroanatomical studies have been mainly focused on arthropods, where in-depth studies on major brain structures are abundant. In contrast, except for Gastropoda and Cephalopoda, the nervous system of representatives of the second largest phylum of invertebrates, the Mollusca, is as yet hardly investigated. We therefore initiated an immunohistochemical survey to contribute new neuroanatomical data for several molluscan taxa, especially the lesser known Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda, focusing on the cellular architecture and distribution of neurotransmitters in the brain. Antisera against the widespread neuroactive substances FMRFamide and serotonin were used to label subsets of neurons. Both antisera were additionally used in combination with acetylated ??-tubulin and the nuclear marker DAPI. This enables us to describe the morphology of the nervous system at a fine resolution and to compare its cellular architecture between different species of one taxon, as well as between different taxa of mollusks. On the basis of these results, the nervous system of caudofoveates seems to be most highly derived within the so-called basal (non-conchiferan) mollusks, and a monophyly of a clade Aplacophora could not be confirmed. In general, the brain as well as the remaining nervous system of the molluscan taxa investigated shows a great variability, suggesting a deep time origin of the diversification of this prominent protostome clade.     

The phylogeny and classification of Scaphopoda (Mollusca): an assessment of current resolution and cladistic reanalysis

The first cladistic analysis of phylogeny in the class Scaphopoda (Steiner 1992a,1996) examined relationships among family and selected sub-family taxa using morphological data. A preferred/ consensus tree of relationships illustrated monophyly of the orders Dentaliida and Gadilida, partial resolution among dentaliid families, and complete resolution among gadilid taxa. However, several alternative replications of the analysis, including use of a revised data matrix, did not produce the reported tree number or level of resolution; in all cases, monophyly of the Dentaliida was not supported by strict consensus of resultant parsimonious trees. Reanalysis, using unordered characters and outgroup rooting, only clearly resolves monophyly of the Gadilida and the sister relationship of the Entalinidae with the remaining gadilid families. These analyses emphasize the need for more comparative data and thorough parsimony analysis in scaphopod cladistic phylogenetics, as relationships in this class are still some way from resolution.     

Polymorphic shell banding in the dog-whelk, Nucella lapillus (Mollusca)

The shell of the Common dog-whelk (Nucella lapillus (L.)) is white and unbanded at most places around the British Isles. However, high frequencies of banding occur on the Buchan coast, around Anglesey and the Menai Straits, on the Cower Peninsula, around the Devon–Somerset border in the Bristol Channel, and especially on the north Cornish coast (reaching a peak between Newquay and Padstow). The frequency of banding is significantly less in older than younger whelks in the same locality, and this change is uncorrelated with the selection against shell shape variation that takes place on exposed shores. It is concluded that banding is a pleiotropic manifestation of physiological variation, and that a study of such variation in different morphs could indicate the importance of different physiological stresses at different stages of the life history of N. lapillus.     

Distribution and Ultrastructure of Ciliated Sensory Receptors in the Posterior Mantle Epithelium of Dentalium rectius (Mollusca,Scaphopoda)

The scaphopod mantle cavity opens posteriorly via the pavilion, a siphon-like extension of the posterior mantle through which the respiratory currents pass. The pavilion was examined for ciliated sensory cells in Dentalium rectius Carpenter, 1865, using scanning and transmission electron micropscopy. Three types of sensory receptor were distinguished on the basis of number, length and ultrastructure of the associated cilia. Receptors with 2–5 cilia of ? 1.7 μm length lined the pavilion edge. A second type, possessing 1–2 cilia, ? 8.2 μm in length, was found throughout the internal and on part of the external surface of the pavilion. The third receptor type consisted of a rigid bundle of 16–40 cilia with a length of ? 14.4 μm, and was present close to the periphery and at the base of the pavilion near the entrance to the mantle cavity. The structure and distribution of these cells are similar to peripheral chemo- and mechanoreceptors which sample respiratory currents and the surrounding environment in other molluscs, but they may assume a greater functional significance in scaphopods due to the absence of an osphradium in this class.     

The burrowing process of Dentalium (Scaphopoda)

Investigations of the burrowing activity of Dentalium , using cine film and electronic recording techniques, have shown it to penetrate the sand in a series of steps, each termed a digging cycle. Cycles involve first, pedal dilation, second, retraction followed by extension and probing of the foot. The epipodial lobes are elevated during pedal dilation and form a secure pedal anchor so that at retraction the shell is drawn down over the foot.
A comparison of the burrowing process in the Scaphopoda with that of the Bivalvia indicates that essentially the same mechanisms and sequence of activities are involved, for in both digging consists of the integration of pedal protraction and retraction with the application of shell and pedal anchors. The principal differences, such as the absence in Dentalium of water jets to loosen the sand and high pressures in the pedal haemocoele, are related to the form of their shell. The strength of the pedal anchor was determined and, relative to the weight of Dentalium , is comparable to that of bivalves. In contrast the probing force was relatively weak since the shell anchor of Dentalium , which holds the shell still during probing, is largely limited to its own weight, whereas that attained by the Bivalvia is principally due to the valves being pressed against the substrate by the opening moment of the ligament.     

The fine structure of the protonephridial system in the land nemertean Pantinonemertes californiensis (Rhynchocoela,Enopla, Hoplonemertini)

Summary The protonephridial terminal organ in the nemertean Pantinonemertes californiensis is composed of two cells that are similar in size and shape and are mirror images of each other. Basally in the organ the two cells combine to form a binucleate cytoplasmic mass. Apically they are intimately joined to form a subcylindrical thin-walled weir apparatus; this part is supported by two opposed cytoplasmic columns running the length of the weir region, one originating from each of the two cells, and by a number of regularly spaced circular bars that arise from the two columns. The ciliary flame consists of 94–114 cilia that originate in the bases of the two cells, and it is surrounded by a palisade of incomplete circlets of long, straight microvilli. The convoluted protonephridial tubule is rich in structures that indicate intensive reabsorption from the primary urine. It is argued that the terminal organs in Pantinonemertes and Geonemertes are fundamentally similar and differ only in the amount of microtubules present in the longitudinal supports.Abbreviations BL basal lamina - CF ciliary flame - CT connective tissue - CV coated vesicle - E endocytotic pit - FM filtration membrane - G Golgi complex - LC longitudinal cytoplasmic column - M mitochondrion - MT microtubules - MV microvilli - N nucleus - NPC nucleus of protonephridial capillary cell - PC protonephridial capillary cell - R rootlets - TB transverse bar - TC terminal cell - WE weir, exterior of fenestrated wall - WI weir, interior of same     

An ultrastructural analysis of Dentalium vulgare (Mollusca, Scaphopoda) gametes with special reference to early events at fertilization

The ultrastructure of Dentalium gametes, and their fate at fertilization, were investigated. Unexpectedly, fixation carried out in 2% osmium tetroxide, at 4 degrees C, although destroying most cytoplasmic organelles, reveals the presence of bundles of "stress fibers" in the microvilli and the fertilization cone of the eggs. Anti-actin labeling supports the view that the bundles are made of actin. Fertilization, as in other molluscs, does not cause any cortical granule exocytosis, at least during the first 15 min following insemination. Ultrastructure of the unreacted and reacted spermatozoon is presented and a tentative model for the morphological interpretation of the acrosomal reaction is proposed. A reevaluation of cytoplasmic heterogeneity, e.g., the so-called cytoplasmic prelocalization, as observed after the germinal vesicle breakdown is also provided as well as an analysis of concomitant surface changes.     

The ultrastructure of the protonephridial system of Götte's larva of Stylochus mediterraneus (Polycladida, Platyhelminthes)

The protonephridial system of Götte's larva of Stylochus mediterraneus was studied by electron microscopy. There is one protonephridium on each side of the body, formed by one terminal and one canal cell. The terminal filtration apparatus is formed by a single cell (the terminal cell) with several globular processes, the largest of which includes the nucleus. Fingers of cytoplasm (leptotriches) from each process penetrate the lumen surrounding the bundle of cilia and fingers from adjacent processes interdigitate to form a pattern of convoluted slits which constitute the weir. The single canal cell is inserted internally to the terminal cell at the top of the weir and encloses the lumen without a junction. Septate junctions are present between the terminal and canal cells. The lumen of the canal cell is smooth-walled for most of its length and cilia arise and terminate at all levels of the terminal and canal cells. Posterior to the larval mouth opening, the canal cell crosses the epithelium and the lumen ramifies to form the excretory opening. The terminal apparatus closely resembles that found in the freshwater planarian Bdellocephala brunnea .     

Ultrastructure of the protonephridial system, of Anoplodiscus cirrusspiralis (Monogenea Monopisthocotylea).

The flame bulb is formed by a terminal cell and a proximal canal cell. The weir consists of interdigitating ribs all of which form one circle, i.e. alternating ribs do not have distinctly 'internal' or 'external' positions. Cytoplasmic cords are absent and all ribs, i.e. those continuous with the proximal canal cell and those continuous with the terminal cell, form external leptotriches. At least some external leptotriches have interconnected branches extending along the flame bulb. Internal leptotriches are not branched and arise from the basal perikaryon of the terminal cell. In the cytoplasmic cylinder at the tip of the flame bulb, structures resembling incomplete septate junctions were seen. However, neither the cytoplasmic cylinder nor the small protonephridial capillaries contain complete septate junctions as found in all other Monogenea Polyopisthocotylea, Monogenea Monopisthocotylea, Trematoda Aspidogastrea and Trematoda Digenea examined to date. In the lack of a septate junction, Anoplodiscus resembles Udonella, Amphilinidea, Gyrocotylidea and Eucestoda. However, the presence in this species of rudimentary septate junctions in the small capillaries and of complete junctions in larger ones indicates that complete junctions have been secondarily lost. Anoplodiscus resembles the Monogenea and Trematoda in the presence of lamellae in the larger protonephridial ducts. For the first time in a monogenean, the ultrastructure of the excretory bladder is described. A nucleated convoluted duct opens through a narrow connecting duct into the bladder, which in turn opens through a narrow connecting duct into the excretory pore lined by tegument. Convoluted duct, connecting ducts and bladder are lined by a lamellated epitheliu.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fluctuation in shell composition in Nautilus (Cephalopoda, Mollusca): evidence from cathodoluminescence

Nautilusr pompilius and N. macromphalus shells show, under cathodoluminescence microscopy. a zoned luminescence of yellow to green for the former and blue to blue-green for the latter. Cathodoluminescence results from fluctuations in the amount of manganese present in the aragonite. There is a relationship between this phenomenon and growth lines. Luminescence intensity increases with ontogeny. Variations in the metabolic activity appear to be linked with the manganese content of the shell. Environmental factors may have an effect by way of their repercussion on the metabolic activity of Nautilus . The Mn²+ amount may be influenced by the physico-chemical pattern of the surrounding water but it appears also to be controlled at the species level.     

A new model for periostracum and shell formation in Unionidae (Bivalvia, Mollusca)

The periostracum in Unionidae consists of two layers. The outer one is secreted within the periostracal groove, while the inner layer is secreted by the epithelium of the outer mantle fold. The periostracum reaches its maximum thickness at the shell edge, where it reflects onto the shell surface. Biomineralization begins within the inner periostracum as fibrous spheruliths, which grow towards the shell interior, coalesce and compete mutually, originating the aragonitic outer prismatic shell layer. Prisms are fibrous polycrystalline aggregates. Internal growth lines indicate that their growth front is limited by the mantle surface. Transition to nacre is gradual. The first nacreous tablets grow by epitaxy onto the distal ends of prism fibres. Later growth proceeds onto previously deposited tablets. Our model involves two alternative stages. During active shell secretion, the mantle edge extends to fill the extrapallial space and the periostracal conveyor belt switches on, with the consequential secretion of periostracum and shell. During periods of inactivity, only the outer periostracum is secreted; this forms folds at the exit of the periostracal groove, leaving high-rank growth lines. Layers of inner periostracum are added occasionally to the shell interior during prolonged periods of inactivity in which the mantle is retracted.     

Structure,crystallography, and morphogenesis of the cryptic shell of the terrestrial slug Limax maximus (Mollusca,gastropoda)

The structure and crystallography of the internal shell of the pulmonate gastropod slug Limax maximus were studied at the levels of light and scanning electron microscopy, revealing patterns of shell ontogeny and morphogenesis. The calcified portion of the slightly convex ovoid shell is composed of a single palisade layer of calcitic crystals. Numerous projections, 100 μm in width at the dorsal tip, are found on the dorsal surface of the shell and coincide with local nucleation sites of primordial calcium salt deposition onto the periostracum. With continued calcification these projections coalesce ventrally, forming the single crystalline shell layer. The organic portion of the shell includes the periostracum and an extensive PAS-staining conchiolin. In EDTA-etched preparations, conchiolin appears as a spongy network of fibers throughout the shell. Both horizontal and vertical components of the conchiolin are present, the former of variable thickness and occurring in an intercrystalline manner, the latter always occurring normal to the horizontal set. Macromorphogenic growth is characterized by three distinct temporal stages. Primary growth occurs radially from the umbonal region. Secondary growth is synonymous with shell thickening. Tertiary growth is characterized by both a lateral component, in which the shell extends beyond the primary growth boundaries, and a ventral component, in which the shell continues to grow in thickness. SEM of the ventral shell surface reveals a pattern of growth at the crystalmatrix interface. Proteinaceous fibers of the conchiolin occur unidirectionally in horizontal rows. Zones of incipient calcitic crystallization onto these hypostracal fiber bundles are contrasted by zones of increasing crystallization until the fibrous template (reduced hypostracum) is completely covered by crystals.     

The anatomy of Addisonia (Mollusca,Gastropoda)

Summary The organization of Addisonia lateralis (Requien, 1848) and A. brophyi McLean, 1985 is described. Addisonia species have a thin, asymmetrical, cup-like shell and a very simple shell muscle. Eyes, oral lappets and epipodial tentacles are absent and the right cephalic tentacle is also used as a copulatory organ. Most characteristic is the enormously developed gill which is enlarged into the right subpallial cavity. It is composed of about 30 leaflets with skeletal rods and its epithelia are uniquely arranged. The heart is large and the single auricle is situated anteriorly left. There are two kidneys: the left is small, while the right forms large coelomic cavities and has no connection with the pericardium or the hermaphroditic genital system. Testis and ovary are separate: both have a simple duct proper (vas deferens, oviduct). They are connected to the copulatory organ by an open seminal groove; a small receptaculum is present. The mouth opening is typically triangular, with no jaws or subradular sense organs. Addisonia possesses tuft-like salivary glands, a radula diverticulum and distinct, tubular oesophageal glands. The oesophagus itself is simple. The radula and the posterior alimentary tract are unique; the stomach is completely reduced and the intestine forms a pseudostomach. The streptoneurous, hyoathroid nervous system has pedal cords with three commissures. The visceral loop is also cord-like. A single (left) osphradium is present and the small statocysts have several statocones.The peculiarities and unique combination of primitive and advanced characters in Addisonia reflect a highly enigmatic organization among the Archaeogastropoda. Possible relationships to other archaeogastropod groups are discussed.