Foregut Development and Metamorphosis in a Pyramidellid Gastropod: Modularity and Constraint within a Complex Life Cycle

Pyramidellids are tiny ectoparasitic gastropods with highly derived feeding structures for piercing and sucking. We attempted to resolve homology controversies about unique pyramidellid feeding structures by examining foregut development through larval and metamorphic stages, using sections for light and electron microscopy. We anticipated that, like many marine invertebrate larvae, post-metamorphic structures would differentiate extensively in late larvae to speed metamorphic transition. Previous studies of gastropods suggested that development of juvenile feeding structures in larvae was facilitated by foregut subdivision into dorsal and ventral developmental modules, and spatial uncoupling of these modules may have facilitated adaptive radiation in neogastropods. Observations of Odostomia tenuisculpta suggested that the stylet may be derived from cuticle-secreting buccal epithelium surrounding the proximal end of the salivary duct, whereas the stylet sheath could be either a derived jaw or a radular tooth. The anterior half of the remarkable buccal pump of these euthyneuran gastropods develops from the larval esophagus, which is unorthodox compared to caenogastropods, where extensive post-metamorphic specialization of a dorsal module component has not been previously described. The introvert tube develops from pouches of the distal larval esophagus and may actually be an eversible oral tube rather than an acrembolic proboscis. Minimal differentiation of presumptive juvenile foregut structures occurred during the larval stage of O. tenuisculpta, when compared to other gastropods. The stylet, stylet sheath, and buccal pump may be incompatible with functioning of the larval esophagus; thus, an explosive period of morphogenesis is necessary at metamorphosis. Although dorsal and ventral modules were recognizable during the development of O. tenuisculpta, we failed to find evidence that this modularity facilitated the extreme evolutionary remodeling of post-metamorphic feeding structures.

The gastropod foregut — evolution viewed through a developmental lens

Comparative data on the developing gastropod foregut suggest that this multicomponent feeding complex consists of two developmental modules. Modularity is revealed by delayed development of the buccal cavity and radular sac (“ventral module”) relative to the dorsal food channel (“dorsal module”) in gastropods with feeding larvae compared with those that may have never had a feeding larval stage. If nonfeeding larvae like those of extant patellogastropods and vetigastropods are ancestral for gastropods, then the uncoupling and heterochronic offset of dorsal and ventral foregut modules allowed the post-metamorphic dorsal food channel to be co-opted as a simple but functional esophagus for feeding larvae. Furthermore, by reducing energy cost per ovum, the heterochronic offset may have given mothers the evolutionary option of increasing fecundity or investing in protective egg encapsulation material. A second developmental innovation was spatial separation of the dorsal and ventral foregut modules, as illustrated by distal foregut development in buccinid neogastropods and venom gland development in cone snails. Spatial uncoupling may have enhanced the evolvability of gastropod foreguts by allowing phenotypic variants of ventral module components to be selected within post-metamorphic ecological settings, without needing to be first tested for compatibility with larval feeding. Finally, we describe a case in which foregut modularity has helped facilitate a highly derived life history in which encapsulated embryos ingest nurse eggs.

Muscle and nerve net organization in stalked jellyfish (Medusozoa: Staurozoa)

Staurozoan cnidarians display an unusual combination of polyp and medusa characteristics and their morphology may be informative about the evolutionary origin of medusae. We studied neuromuscular morphology of two staurozoans, Haliclystus 'sanjuanensis' and Manania handi, using whole mount immunohistochemistry with antibodies against FMRFamide and α-tubulin to label neurons and phalloidin to label muscles. All muscles appeared to lack striations. Longitudinal interradial muscles are probable homologues of stalk muscles in scyphopolyps, but in adult staurozoans they are elaborated to inwardly flex marginal lobes of the calyx during prey capture; these muscles are pennate in M. handi. Manubrial perradial muscles, like the manubrium itself, are an innovation shared with pelagic medusae and manubrial interradial muscles are shared with scyphozoan ephyra. Marginal muscles of M. handi displayed occasional synchronous contraction reminiscent of a medusa swim pulse, but contractions were not repetitive. The nerve net in both species showed regional variation in density and orientation of neurons. Some areas labeled predominantly by α-tubulin antibodies (exumbrellar epidermis), other areas labeled exclusively by FMRFamide antibodies (dense plexus of neurites surrounding the base of secondary tentacles, neuronal concentration at the base of transformed primary tentacles; gastrodermal nerve net), but most areas showed a mix of neurons labeled by these two antibodies and frequent co-labeling of neurons. Transformed primary tentacles had a concentration of FMRFamide-immunoreactive neurons at their base that was associated with a pigment spot in M. handi; this is consistent with their homology with rhopalia of medusae, which are also derived from primary tentacles. The muscular system of these staurozoans embodies characteristics of both scyphopolyps and pelagic medusae. However, their nerve net is more polyp-like, although marginal concentrations of the net associated with primary and secondary tentacles may facilitate the richer behavioral repertoire of staurozoans relative to polyps of other medusozoans. J. Morphol. 278:29-49, 2017. ©© 2016 Wiley Periodicals,Inc.

The other gastropod larvae: larval morphogenesis in a marine neritimorph

Two of the three major gastropod clades with feeding larvae are sister groups and larval morphogenesis for members of these clades, the Caenogastropoda and Heterobranchia, has been well studied. The third clade, the Neritimorpha, has an unstable phylogenetic position and little is known about development of their planktotrophic larvae. Information about larval morphology of neritimorphs and resolution of their controversial phylogenetic placement is critically important for understanding evolution of larval feeding within the Gastropoda. We describe larval morphogenesis to metamorphic competence for laboratory-reared larvae of Nerita melanotragus (Smith, 1884) (Neritimorpha: Neritidae). Preliminary observations suggest that prehatch larvae are capable of delayed hatching, possibly by entering a diapause state. Our description of larval morphogenesis, as based on tissue sections for light and transmission electron microscopy, scanning electron microscopy, three-dimensional-reconstructions of sectioned tissue, and labeling of muscles with fluorphore-tagged phalloidin, revealed four features that are unprecedented among both feeding and nonfeeding gastropod larvae. Larvae of N. melanotragus have muscles on the left and right side that both meet current criteria of a larval retractor muscle; shell-anchored muscles with oblique striations that project inside the visceral nerve loop to insert mainly on the velar lobes. They also have left and right digestive glands of similar size and a left and right hypobranchial gland. A larval "heart" is absent, but water circulation through the mantle cavity may be facilitated by large circular orifices, lined by patches of motile cilia, leading in and out of the mantle cavity. Comparison of larval traits among all three groups of gastropods with feeding larvae indicates that larvae of N. melanotragus have many unique characteristics, but they show more similarities to caenogastropod than to heterobranch larvae. These results are a significant step toward the goal of identifying primitive versus derived larval traits among feeding gastropod larvae.

Metamorphic remodeling of a planktotrophic larva to produce the predatory feeding system of a cone snail (Mollusca, Neogastropoda)

I used histological sections and 3D reconstructions to document development through metamorphosis of the foregut and proboscis in the conoidean neogastropod Conus lividus. A goal was to determine how highly derived features of the post-metamorphic feeding system of this gastropod predator develop without interfering with larval structures for microherbivory. A second goal was to compare foregut development in this conoidean with previous observations on foregut development in the buccinoidean neogastropod Nassarius mendicus. These two neogastropods both have a feeding larval stage, but they show major differences in post-metamorphic foregut morphology. Basic events in development of the proboscis and proboscis sheath in C. lividus and N. mendicus were similar. However, the elongate buccal tube of C. lividus forms during metamorphosis as a composite of apical epidermal tissue that grows inward and ventral foregut tissue that extends outward. The larval mouth is not carried through metamorphosis. Comparative observations on foregut development in caenogastropods, which now include data on C. lividus, suggest that the foregut incorporates dorsal and ventral modules having different ontogenetic and functional fates. This developmental modularity may have facilitated evolutionary diversification of the post-metamorphic foregut. Foregut diversification in predatory gastropods may have been further fast-tracked by developmental uncoupling of larval and post-metamorphic mouths.

Developmental modularity and phenotypic novelty within a biphasic life cycle: morphogenesis of a cone snail venom gland

The venom gland of predatory cone snails (Conus spp.), which secretes neurotoxic peptides that rapidly immobilize prey, is a proposed key innovation for facilitating the extraordinary feeding behaviour of these gastropod molluscs. Nevertheless, the unusual morphology of this gland has generated controversy about its evolutionary origin and possible homologues in other gastropods. I cultured feeding larvae of Conus lividus and cut serial histological sections through the developing foregut during larval and metamorphic stages to examine the development of the venom gland. Results support the hypothesis of homology between the venom gland and the mid-oesophageal gland of other gastropods. They also suggest that the mid-region of the gastropod foregut, like the anterior region, is divisible into dorsal and ventral developmental modules that have different morphological, functional and ontogenetic fates. In larvae of C. lividus, the ventral module of the middle foregut transformed into the anatomically novel venom gland of the post-metamorphic stage by rapidly pinching-off from the main dorsal channel of the mid-oesophagus, an epithelial remodelling process that may be similar to other cases where epithelial tubes and vesicles arise from a pre-existing epithelial sheet. The developmental remodelling mechanism could have facilitated an abrupt evolutionary transition to the derived morphology of this important gastropod feeding innovation.

Embryonic toxin expression in the cone snail Conus victoriae: primed to kill or divergent function?

Predatory marine cone snails (genus Conus) utilize complex venoms mainly composed of small peptide toxins that target voltage- and ligand-gated ion channels in their prey. Although the venoms of a number of cone snail species have been intensively profiled and functionally characterized, nothing is known about the initiation of venom expression at an early developmental stage. Here, we report on the expression of venom mRNA in embryos of Conus victoriae and the identification of novel α- and O-conotoxin sequences. Embryonic toxin mRNA expression is initiated well before differentiation of the venom gland, the organ of venom biosynthesis. Structural and functional studies revealed that the embryonic α-conotoxins exhibit the same basic three-dimensional structure as the most abundant adult toxin but significantly differ in their neurological targets. Based on these findings, we postulate that the venom repertoire of cone snails undergoes ontogenetic changes most likely reflecting differences in the biotic interactions of these animals with their prey, predators, or competitors. To our knowledge, this is the first study to show toxin mRNA transcripts in embryos, a finding that extends our understanding of the early onset of venom expression in animals and may suggest alternative functions of peptide toxins during development.

Molluscan larvae: Pelagic juveniles or slowly metamorphosing larvae?

Asking the right questions about evolution of development, larval morphology, and life history requires knowledge of ancestral state. Two hypotheses dominate current opinion about the ancestral life cycle of bilaterians: the "larva-first" and the "intercalation" hypotheses. Until recently, the larva-first hypothesis was preeminent. This proposes that the original indirect life cycle of bilaterians included a planktotrophic larva followed by a benthic adult. Phylogenetic evidence suggests that a planktotrophic larva is plesiomorphic for echinoderms. A preponderance of developmental studies on echinoderms may have fostered a tendency to extrapolate conclusions about echinoderm development to other clades, particularly the concept that larval and juvenile/adult bodies are mostly separate entities. However, some of the recent reconstructions of bilaterian phylogeny suggest that nonfeeding larvae may have been ancestral for bilaterians, and these may have been intercalated into a life cycle that was originally direct. I review comparative data on molluscan development that suggests the trochophore-like stage is little more than a gastrula with transient structures (prototroch and apical sensory organ) to allow a temporary planktonic phase during development. Most lineage founder cells of molluscan embryos generate progeny that develop through the veliger stage into structures of the juvenile, which becomes benthic when the prototroch and apical sensory organ are lost. In light of this, the model of separate larval and juvenile bodies with the latter developing from nests of multipotent cells within the larva is inappropriate for molluscs. The intercalation hypothesis may be a better model for interpreting development of molluscs and other lophotrochozoans.

Shrinking to fit: fluid jettison from a haemocoelic hydrostatic skeleton during defensive withdrawals of a gastropod larva

Although most of the basic animal body plans are supported by hydrostatic skeletons consisting of fluid maintained at constant volume, studies on how animals have solved biomechanical scaling dilemmas during evolution of large body size have emphasized cases where skeletons are formed by rigid solids. Larvae of gastropod molluscs swim using ciliated velar lobes supported by a constant volume hydrostatic skeleton. Defensive behaviour involves rapid withdrawal of the velar lobes and foot into a protective biomineralized shell. Some gastropod larvae grow to giant size and the velar lobes enlarge allometrically, but the lobes and foot of many can still withdraw completely into the mineral-stiffened shell. I dyed internal fluid of a large gastropod larva with fluorescein to show that fluid supporting the extended velar lobes is expelled from discrete release sites during defensive withdrawals. Scanning electron microscopy suggested that release sites are distinctive papillae on the upper velar epidermis. Ultrathin sections revealed that branched tracks of microvilli-free membrane on the surface of these papillae were formed by very thin epithelial cells, which may rupture and re-anneal during and after defensive withdrawals. Behaviours facilitated by fluid discharge from a haemocoelic (non-coelomic) body compartment have been rarely reported among aquatic invertebrates, but may be more widespread than currently recognized.

Early differentiating neuron in larval abalone (Haliotis kamtschatkana) reveals the relationship between ontogenetic torsion and crossing of the pleurovisceral nerve cords

Crossing of the pleurovisceral nerve cords in gastropods has supported the view that gastropods evolved by 180 degrees rotation between the ventral and dorsal body regions. Indeed, a rotation of this type occurs as a dramatic morphogenetic movement ("ontogenetic torsion") during the development of basal gastropods. According to a long-standing hypothesis, ontogenetic torsion in basal gastropods preserves an ancient developmental aberration that generated the contorted gastropod body plan. It follows from this reasoning that crossing of the pleurovisceral nerve cords during gastropod development should be mechanically coupled to ontogenetic torsion. The predicted mechanical coupling can now be examined because of the discovery of an early differentiating neuron in Haliotis kamtschatkana (Vetigastropoda) that expresses 5-hydroxytryptamine-like immunoreactivity. The neuron appeared to delineate the trajectory of the pleurovisceral nerve cords beginning before ontogenetic torsion. Before torsion, the neuronal soma is embedded in mantle epithelium at the ventral midline and two neurites extend anteriorly toward the apical sensory organ. Contrary to expectation, the two neurites of this cell did not cross-over during ontogenetic torsion because the soma of this mantle neuron shifted in the same direction as the rotating head and foot. Full crossing of the pleurovisceral nerve cords occurred gradually during later development as the mantle cavity deepened and expanded leftward. These results are consistent with a generalization emerging from comparative studies indicating a conserved developmental stage for gastropods in which the mantle cavity is localized to one side, despite a fully "post-torsional" orientation for other body components. Developmental morphology before this stage is much more variable among different gastropod clades.

Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan

More than a century of speculation about the evolutionary origin of the contorted gastropod body plan has been inspired by adult anatomy and by long-standing developmental observations. The result has been a concept of gastropod torsion that I call the "rotation hypothesis." Under the rotation hypothesis, gastropods originated when all components of the visceropallium (shell, mantle, mantle cavity with contained structures, and viscera) rotated by 180° relative to the head and foot. This evolutionary rotation is echoed during early development of patellogastropods and vetigastropods and occurs to some extent during development of more derived clades. However, comparative developmental data on ontogenetic torsion are minimal and I argue that the rotation hypothesis is a tautological argument. More recent studies on representatives from 3 major clades of gastropods suggest that the highly conserved aspect of gastropod development is not synchronous rotation of all components of the visceropallium relative to the head and foot but rather a state of anatomical organization in which the developing mantle cavity is on the right but the shell coil is posterior (endogastric orientation). This conserved state of developmental anatomy has inspired an alternative hypothesis for the evolutionary origin of the gastropod body plan, the "asymmetry hypothesis." Under the asymmetry hypothesis, the gastropod mantle cavity originated from 1 side only of a bilateral set of mantle cavities. The asymmetry hypothesis does not require a saltation event to explain the origin of gastropods, nor does it require that the ancient molluscan precursor of gastropods carried the shell coil over the head (exogastric orientation).

Anti-tubulin labeling reveals ampullary neuron ciliary bundles in opisthobranch larvae and a new putative neural structure associated with the apical ganglion

This investigation examines tubulin labeling associated with the apical ganglion in a variety of planktotrophic and lecithotrophic opisthobranch larvae. Emphasis is on the ampullary neurons, in which ciliary bundles within the ampulla are strongly labeled. The larvae of all but one species have five ampullary neurons and their associated ciliary bundles. The anaspid Phyllaplysia taylori, a species with direct development and an encapsulated veliger stage, has only four ampullary neurons. The cilia-containing ampulla extends to the pretrochal surface via a long, narrow canal that opens to the external environment through a very small pore (0.1 microm diameter). Cilia within the canal were never observed to project beyond the opening of the apical pore. The ampullary canals extend toward and are grouped with the ciliary tuft cells and remain in this location as planktotrophic larvae feed and grow. If, as has been reported, the ciliary tuft is motile, the pores may be continually bathed in fresh seawater. Such an arrangement would increase sensitivity to environmental chemical stimuli if the suggested chemosensory function of these neurons is correct. In general, ciliary bundles of newly hatched veligers are smaller in planktotrophic larvae than in lecithotrophic larvae. In planktotrophic larvae of Melibe leonina, the ciliary bundles increase in length and width as the veligers feed and grow. This may be related to an increase in sensitivity for whatever sensory function these neurons fulfill. An unexpected tubulin-labeled structure, tentatively called the apical nerve, was also found to be associated with the apical ganglion. This putative nerve extends from the region of the visceral organs to a position either within or adjacent to the apical ganglion. One function of the apical nerve might be to convey the stimulus resulting from metamorphic induction to the visceral organs.

Development of foregut and proboscis in the buccinid neogastropodNassarius mendicus: Evolutionary opportunity exploited by a developmental module

This article extends previous descriptions of foregut development and metamorphosis in neogastropods by providing data on the buccinid Nassarius mendicus, a species with a feeding larva. Histological sections showed that, like many other gastropods, the postmetamorphic buccal cavity and radular sac of N. mendicus differentiate during the larval stage from a ventral outpocketing of the distal larval esophagus. However, in N. mendicus the outpocketing also gives rise to the entire anterior esophagus and valve of Leiblein, suggesting that both these structures may be evolutionary derivatives of the gastropod buccal cavity. Scanning electron microscopy and three-dimensional reconstructions of section profiles revealed that the distal larval esophagus and larval mouth are completely destroyed at metamorphosis. The postmetamorphic mouth is formed as a new orifice. Furthermore, epithelia covering the proboscis and proboscis sac arise from preexisting epidermal epithelium of the larval head, an interpretation that contradicts an earlier suggestion on the origin of these epithelial elements in neogastropods with a feeding larval stage. These results, when compared to foregut development in other gastropods, lead me to propose that the gastropod buccal cavity and buccal mass is a developmental module. Canalized development of this module may have been important to the "evolvability" of the complex gastropod foregut, because it allowed a silent developmental novelty to arise (secondary formation of the postmetamorphic mouth) without disrupting development of the whole module. Nevertheless, this silent novelty might have subsequently facilitated dramatic evolutionary change by allowing the elaborate foregut structure of predatory, postmetamorphic neogastropods to arise in late stage larvae without compromising larval feeding.

Larval development and metamorphic transformation of the feeding system in the kleptoparasitic snailTrichotropis cancellata(Mollusca, Caenogastropoda)

Trichotropis cancellata Hinds, 1849 has a planktonic larval stage that feeds on microalgae and a benthic stage that feeds both by ctenidial suspension feeding and by stealing food ("kleptoparasitism") from several species of suspension-feeding, tube-dwelling polychaete worms. We used scanning electron microscopy, histological sections, and observations on live animals to document the sequence and timing of morphogenetic events during larval and metamorphic development of T. cancellata. These data were compared with other accounts of gastropod development to test for differences in the timing of developmental events among feeding larvae of two major gastropod clades: the caeno gastropods and heterobranchs. In T. cancellata, as in feeding larvae of previously studied caenogastropods, components of the post-metamorphic body plan differentiate at an earlier stage of larval development (relative to times of hatching and ability to undergo metamorphosis) than in feeding heterobranch larvae. Metamorphosis of T. cancellata was induced by polychaete hosts of this snail's kleptoparasitic benthic stage, and young juveniles of T. cancellata could steal food from these polychaetes within a day after snail metamorphosis began. Rapid onset of kleptoparasitic feeding following metamorphosis of T. cancellata was permitted by development of a specialized feeding structure, the pseudo proboscis, during the larval stage. This novel embellishment of larval development was likely preceded during evolution by selective larval induction by polychaete hosts.

Gastropod ontogenetic torsion: developmental remnants of an ancient evolutionary change in body plan

A dramatic morphogenetic movement ('ontogenetic torsion') during the development of gastropods has been proposed as a recapitulation of the original developmental departure that established the novel gastropod body plan. Nevertheless, speculative literature about ontogenetic torsion and its evolutionary significance has far outstripped empirical observations and recent results suggest that the developmental process may be somewhat different than the traditional description. I used scanning electron microscopy, immunohistochemistry, phalloidin labeling, and histological sections to monitor displacements of five components of the visceropallium with respect to axial coordinates of the cephalopodium in developing embryos of the caenogastropod, Trichotropis cancellata. Embryos of this species achieve a transient stage of anatomical organization that also arises during development of a vetigastropod (Haliotis kamtschatkana), although morphogenetic processes that generate this stage are different in these two species. At the stage of similarity, the embryonic shell has achieved its definitive orientation with respect to the cephalopodium, but the developing mantle cavity, sensory osphradium, and anus are confined to the right side. I also show that this stage of anatomical organization is recognizable during the development of other gastropods, which collectively represent three major gastropod clades. I propose that ontogenetic torsion should be viewed as a conserved stage of anatomical organization during development, rather than a conserved process of 180 degrees rotation between the visceropallium and cephalopodium. The results lead to the suggestion that the mantle cavity of extant gastropods evolved by enlargement of the right side of the mantle cavity in a monoplacophoran-like ancestor. Under this interpretation, there is no need for a hypothetical pre-gastropod with a mantle cavity that was restricted to the posterior end.

Larval and metamorphic development of the foregut and proboscis in the caenogastropod Marsenina (Lamellaria) stearnsii

The specialized, postmetamorphic feeding structures of predatory caenogastropods evolved by changes to an ancestral caenogastropod developmental program that generated a planktotrophic larval stage followed by a herbivorous postmetamorphic stage. As part of a program of comparative studies aimed at reconstructing these developmental changes, I studied the development of the postmetamorphic feeding system of Marsenina stearnsii using histological sections for light microscopy and scanning and transmission electron microscopy. The feeding system of this species has two very different designs during ontogeny. The larval system uses ciliary effectors to capture and ingest microalgae, whereas the juvenile/adult system includes a proboscis, jaws, and radular apparatus for predation on ascidian zooids. The postmetamorphic foregut begins to develop during the early larval phase, but the anlagen does not interfere with larval feeding because it develops as an increasingly elaborate outpocketing from the ventral wall of the larval esophagus. At metamorphosis, an opening is created in the anterior tip of the prospective, postmetamorphic buccal cavity and the margins of this opening anneal with the metamorphically remodeled lips of the larval mouth. This process exposes the jaws, which differentiate within the buccal cavity prior to metamorphosis. As a working hypothesis, I suggest that rupture of the buccal cavity to the outside at metamorphosis was selected as a mechanism to allow precocious development of jaws in species where jaws enhanced feeding performance by young juveniles. The larval esophagus of M. stearnsii appears to be completely destroyed at metamorphosis. Larval esophageal cells have distinctive apical characteristics (cilia, blebbed microvilli, stacks of lamellae within the glycocalyx) and no cells having this signature persist through metamorphosis. Development of the proboscis and proboscis sac, which begins prior to metamorphosis, conforms to previous descriptions of pleurembolic proboscis development, although an acrembolic proboscis has been ascribed to members of the Lamellaroidea.

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.

Apical sensory organ in larvae of the patellogastropod Tectura scutum

The apical sensory organ in veliger larvae of a patellogastropod, a basal clade of gastropod molluscs, was studied using ultrastructural and immunohistochemical techniques. Immediately before veligers of Tectura scutum undergo ontogenetic torsion, the apical sensory organ consists of three large cells that generate a very long apical ciliary tuft, two cells that generate a bilateral pair of shorter ciliary tufts, and a neural ganglion (apical ganglion). Putative sensory neurons forming the ganglion give rise to dendrites that extend to the apical surface of the larva and to basal neurites that contribute to a neuropil. The ganglion includes only one ampullary neuron, a distinctive neuronal type found in the apical ganglion of other gastropod veligers. Serotonin immunoreactivity is expressed by a medial and two lateral neurons, all having an apical dendrite, and also by neurites within the neuropil and by peripheral neurites that run beneath the ciliated prototrochal cells that power larval swimming. The three cells generating the long apical ciliary tuft are lost soon after ontogenetic torsion, and the medial serotonergic cell stops expressing serotonin antigenicity in late-stage veligers. The lateral ciliary tuft cells of T. scutum may be homologs of lateral ciliary tuft cells in planktotrophic opisthobranch veligers. A tripartite arrangement of sensory dendrites, as described previously for veligers of other gastropod clades, can be recognized in T. scutum after loss of the apical ciliary tuft cells.

Comparative study of the apical ganglion in planktotrophic caenogastropod larvae: ultrastructure and immunoreactivity to serotonin

Previous research suggests that a major role of the apical ganglion (also called the apical or cephalic sensory organ) in gastropod larvae is detection and integration of sensory information and relay of motor signals to effectors in the velum. However, the relative impact of ancestry versus velum size and life history on characteristics of the apical ganglion is unresolved. We address this issue by contributing data on the apical ganglion and overlying epidermis in planktotrophic larvae of four caenogastropod species (Euspira [Polinices] lewisii, Lacuna vincta, Trichotropis cancellata, and Amphissa versicolor) derived from light microscopy, scanning and transmission electron microscopy, and immunohistochemical localization of serotonin-like antigenicity. Ultrastructure of the apical ganglion is similar in these caenogastropods, and the basic plan corresponds to previous descriptions of the apical ganglion in planktotrophic opisthobranch larvae (subgroup of Heterobranchia). The only identified structural feature that is unique to all these caenogastropods, relative to opisthobranchs, is modified ciliary axonemes for the ampullary cells, a distinctive type of sensory neuron. Like opisthobranch larvae, caenogastropod larvae have serotonin-immunoreactive neurons within the apical ganglion; the number ranges from three to six, but a lateral pair of serotonergic, nonsensory neurons is common to all species. The pattern of serotonergic neurons in E. lewisii, which develops large, subdivided velar lobes, is the same as that of opisthobranch larvae, which have a relatively small, unelaborated velum. These and other data suggest that common ancestry is a major determinant of overall structural design for the apical ganglion in caenogastropods and heterobranchs, which are sister groups within the Gastropoda. Velum size and life history strategy may account for some, but not all, cases of interspecific differences in the serotonergic component.

Development and evolution of adult feeding structures in Caenogastropods: overcoming larval functional constraints

Comparative study of the developing foregut in three species of caenogastropods, including an herbivorous grazer (Lacuna vincta) and two carnivores (Euspira [Polinices] lewisii and Nassarius mendicus), suggests how the specialized adult foregut of a carnivorous neogastropod evolved within a life cycle having a planktotrophic larva. Postmetamorphic feeding structures (buccal cavity and radular sac) in all three species achieve advanced differentiation in the larval stage, permitting juvenile feeding at 3 days postmetamorphosis. Recent phylogenetic hypotheses for the Gastropoda predict that foregut developmental patterns in E. lewisii and N. mendicus are derived, relative to that of L. vincta. In hatching larvae of these three, the anlage of postmetamorphic feeding structures is a small patch of nonciliated cells embedded in the ventral wall of the larval foregut and the patch soon forms an outpocketing. During subsequent morphogenesis, Euspira lewisii and N. mendicus share a developmental novelty that involves semi-isolation of the developing, postmetamorphic buccal cavity and radular sac from the larval foregut and formation of a new, definitive mouth at metamorphosis. Nassarius mendicus, a neogastropod, embellishes this novelty by adding the entire anterior esophagus and valve of Leiblein (de novo structures) to the semi-isolated buccal cavity. Therefore, a valve and long stretch of muscular anterior esophagus, which are necessary for feeding with a pleurembolic proboscis, are preformed in the larval stage of this neogastropod without interfering with larval feeding. The inferred evolutionary events leading to postmetamorphic feeding specialization in N. mendicus are invisible in adults; they require reconstruction from comparative developmental analysis.

Development of serotonin-like immunoreactivity in the embryos and larvae of nudibranch mollusks with emphasis on the structure and possible function of the apical sensory organ

This investigation provides a light and electron microscopic examination of the development of serotonin-like immunoreactivity and structure of the apical sensory organ (ASO) in embryos and/or larvae of four nudibranch species: Berghia verrucicornis, Phestilla sibogae, Melibe leonina, and Tritonia diomedea. Serotonin-like immunoreactivity is first expressed in somata, dendrites, and axons of a group of five distinct neurons within the ASO. These neurons extend axons into an apical neuropil, a structure that is situated centrally and immediately dorsal to the cerebral commissure. Three of these neurons possess sensory dendrites that extend through the pretrochal epithelium, each supporting two cilia at their distal ends. Later development of serotonin-like immunoreactivity includes 1) axons from the apical neuropil that extend into each of the velar lobes; 2) neuron perikarya in the cerebral and pedal ganglia; 3) axons that extend through the cerebral commissure, cerebral-pedal connectives, pedal commissure, and possibly the visceral loop connective; and 4) axons extending from each pedal ganglion into the larval foot. Ultrastructurally, the ASO can be seen to be composed of three lobes and an apical neuropil that is separately delineated from the cerebral commissure. Four cell types are present within the ASO: ciliary tuft cells, type I and type II parampullary neurons, and ampullary neurons. Immunofluorescence and 3,3' diaminobenzidine tetrahydrochloride (DAB) labeling verify that the serotonergic neurons of the ASO are type I and type II parampullary neurons. The ampullary and type I parampullary neurons possess dendrites that extend through the pretrochal epithelium. These dendrites are partitioned into three bundles, one on either side of the ciliary tuft cells and a third bundle penetrating the pretrochal epithelium centrally between the ciliary tuft cells. One serotonergic type I parampullary neuron is associated with each of these bundles. Two ampullary neurons are associated with each of the lateral dendritic bundles, while the central bundle includes only one. Ultrastructural analyses of serotonergic axonal innervation arising from the ASO agree with those determined from fluorescently labeled material. The structure of the ASO and its associated serotonergic axons suggest that the serotonergic component of this structure senses environmental stimuli affecting velar function, possibly the contractility of muscle fibers in the velar lobes. Similarities and differences among the ASOs of embryos and larvae from various invertebrate phyla may provide useful data that will assist in the reconstruction of phylogenetic relationships.

Larval Shell Muscles in the Abalone Haliotis kamtschatkana

I used light and electron microscopy to investigate shell-attached muscles in larvae of Haliotis kamtschatkana Jonas, 1845, because an early description of these muscles in H. tuberculata by Crofts (1937, 1955) has featured prominently in theories about gastropod evolution. Larval shell muscles in H. kamtschatkana can be grouped into two categories. The first category consists of the larval retractor muscle (LRM) and the accessory larval retractor muscle (ACC); these are striated muscles in which myofilaments begin differentiating before the head and foot rotate relative to the protoconch (this rotation is known as ontogenetic torsion). Collectively, these muscles ultimately insert on tissues within the larval head and mantle, but the ACC and mantle fibers of the LRM degenerate as metamorphic competence is achieved. The second category consists of two nonstriated pedal muscles that differentiate after cephalopodial rotation. The left pedal muscle is anchored on the back of the protoconch, to the left of the shell-attachment plaque for the LRM. It projects into the foot primarily, but also gives rise to muscle slips extending into the mantle fold. The right pedal muscle is anchored on a calcareous septum secreted along the visceral rim of the protoconch. The new data force a reconsideration of the ancestral homologues of larval shell muscles in abalone, because Crofts may have misidentified the accessory larval retractor muscle as a precursor of one of the later pedal muscles.

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.

New Interpretation of a Nudibranch Central Nervous System Based on Ultrastructural Analysis of Neurodevelopment in Melibe leonina. II. Pedal, Pleural, and Labial Ganglia

Electron microscopical analysis of semi-serial sections through larval stages of the dendronotid nudi-branch Melibe leonina (Gould, 1852) revealed paired placodes of neurogenic ectoderm at the base of the foot. The location of these laterocephalic placodes corresponds to descriptions of the ectodermal site generating pleural neurons in prosobranchs. In Melibe, there are two sites of neuronal ingression within each laterocephalic placode. Neurons ingressing from one of these sites join the cerebral ganglia, and their initial axons extend into the cerebrobuccal connectives or run distally along the esophagus. I identify these neurons as homologues of labial ganglia neurons in archeogastropods. However, neurons derived from the second ingression site within each laterocephalic placode join the pedal ganglia. Pedal ganglia are present in hatching veligers and are linked to the cerebral ganglia by cerebropedal connectives associated with the statocyst nerves. A second connective between each cerebral and pedal ganglia appears at the onset of neuronal ingression from the laterocephalic placodes. Peripheral axons branching from this second pair of connectives are associated with laterocephalic neurons that ingress to the pedal ganglia. I argue that these are pleural neurons, meaning that the pleural ganglia in Melibe are uncoupled from the visceral loop.

New Interpretation of a Nudibranch Central Nervous System Based on Ultrastructural Analysis of Neurodevelopment in Melibe leonina. I. Cerebral and Visceral Loop Ganglia

Development of the `cerebropleural' ganglia in the dendronotid nudibranch Melibe leonina (Gould, 1852) was examined by electron microscopy of semi-serial sections through larval stages. Although comparative neuroanatomical studies suggest that the paired cerebropleurals of nudibranchs are formed by fusion of cerebral and pleural ganglia, plus all other ancestral ganglia of the visceral loop, my study indicates that the pleural ganglia are not part of these compound ganglionic masses. In Melibe larvae, the cerebral, optic, and rhinophoral ganglia, arise from pre-trochal cephalopedal ectoderm. At hatching stage, the visceral loop extends from the two cerebral ganglia, is non-ganglionated, and forms a complete circuit beneath the esophagus. Ganglia that subsequently develop along the visceral loop, which were identified as subintestinal, visceral, supraintestinal, and possibly right parietal ganglia, arise from placodes of visceropallial ectoderm that show torsional displacements. In addition, a cluster of neurons, presumed to be osphradial, lies close to the rim of the right mantle fold. Detorsion of the visceral loop is accomplished by migration of subintestinal neurons along the visceral loop fiber tract, not by visceral loop shortening. Localized elongation of a different segment of this fiber tract during metamorphosis displaces the visceral ganglion to the left, where it fuses with subintestinal and left cerebral ganglia.

Acute recurrent gingivitis. A clinical entity

A typical case of a previously undescribed form of gingivitis which is self-limiting and recurrent is discussed. The signs and symptoms include acute pain and swelling of interdental papillae with regional lymphadenopathy. As the disease progresses the localized swelling quickly spreads to the adjacent marginal and papillary gingiva. Ulceration is infrequent and without pseudomembrane formation or interdental cratering. We believe this to be a unique form of gingivitis and suggest the name acute recurrent gingivitis (ARG).

Blastomycosis with oral lesions. Report of two cases

The recent literature suggests that blastomycosis (previously known as North American blastomycosis) with oral lesions may be more common than is generally realized. This report discusses two cases of blastomycosis in which oral lesions played a critical role in the ultimate diagnosis of the disease. The potential importance or oral lesions in early diagnosis and the necessity of culturing the organisms for definitive diagnosis are emphasized.

Cancellous bone marrow grafts in irradiated dog and monkey mandibles

A pilot study was conducted to investigate the osteogenic potential of autogenous cancellous marrow grafts placed in irradiated tissue. Five dogs and three monkeys received either 4,100 or 6,520 rads of cobalt-60 irradiation during a 5- or 8-week period. Postirradiation discontinuity mandibular defects were created surgically and restored with autogenous cancellous bone marrow grafts held in position by a titanium mesh basket. Bony union occurred in all the mandibles. Intraoral ulceration was common in the dogs, and extraoral wound dehiscence was found in the monkeys. Hematopoietic marrow was present in the graft sites of both species 6 months to 1 year after surgery.

The oral melanotic macule

A clinical and histologic study of eighty oral melanotic lesions which do not readily fit into recognized categories of melanotic lesions was conducted. These lesions tend to occur in the fifth decade of life and are most frequently seen on the gingiva, with the buccal mucosa and palate the next most frequent sites. The lesions are usually single, smaller than 1 cm., but they may also occur as multiple lesions. There is no sex bias, and there seems to be a number of etiologic factors. In a few cases long-term follow-up was possible. There is no indication of a tendency toward recurrence or development of malignant lesions. Histologically, none of the lesions shows atypia. Melanin pigmentation tends to be present in significant amounts in the basal-cell layer and less often in the lamina propria. There are no outstanding histologic differences among the eighty specimens. It is suggested that the term oral melanotic macule be used for these lesions, unless a specific cause can be confirmed by clinical data. Although these lesions should not be considered premalignant, it would be prudent to remove them for histologic confirmation of clinical impressions.

Aspergillosis of the maxillary sinus. Review of the literature and report of a case

Aspergillosis of the paranasal sinuses is an infrequently reported disease which can occur in two forms: (1) a noninvasive form that can clinically mimic nonspecific chronic sinusitis and (2) an invasive form that can simulate malignant disease of the sinuses. The disease occurs without known predisposing systemic disease. Primary treatment consists of surgical eradication of infected tissue. The question of whether concomitant antifungal chemotherapy should be used in the noninvasive form of aspergillosis has not been definitely resolved. Presented is a review of the literature, the report of a case, and a discussion of possible pathogenic mechanisms.

The labial melanotic macule

Fifty-five cases of a melanotic lesion of the lips which is well known, but not well described, are reported. The lesions characteristically occur on the lower lips of young adults. Males and females are equally affected. These lesions may be ephelides, postinflammatory melanoses, or unique lesions for which there is no exact cutaneous counterpart. We suggest the term labial melanotic macule as a descriptive one which would encompass the three different constituent entities. On the basis of follow-up information obtained and the histopathologic character of the lesions, this entity is benign and does not, in our opinion, have any malignant potential.