The localization of aluminium in the digestive gland of the terrestrial snail Helix aspersa

The terrestrial snail Helix aspersa was exposed to food containing elevated levels of aluminium for up to 33 days and the digestive gland examined by light and electron microscopy and X-ray microanalysis. Four types of cell are found in the digestive gland, (digestive, excretory, calcium and thin) although aluminium was only found in the excretory cells. The aluminium was localised in the 'yellow' or excretory granules that are a characteristic feature of the excretory cells. Aluminium was only found in the granules of snails fed aluminium but there was no difference in the appearance of granules from control or aluminium-fed snails. The granules were large (up to 20 mum in diameter), irregularly shaped and electron-dense. Sulphur, phosphorus and calcium were detected in granules from all snails. The presence of sulphur may indicate protein residues. The amount of aluminium and phosphorus in the granules increased over the experimental period but the number of granules did not change. Levels of aluminium in the granules decreased when the snails were given control food. The role of the excretory granules in the localisation, detoxification and excretion of aluminium is discussed.     

Ultrastructure of maxillary gland of Antrobathynella stammeri (Syncarida,Malacostraca)

The maxillary gland of the highly adapted stygobiont species, Antrobathynella stammeri (Bathynellacea, Syncarida), consists of an end sac, an excretory tubule, and a terminal duct. No valve was found. The excretory tubule forms a loop extending back into the fourth thoracic segment. The end sac is composed of five typical podocytes. Ultrastructurally and functionally, two cell types characterize, respectively, proximal and distal sections of the excretory tubule. Epithelial cells are covered with extremely broad (up to 0.4 μm) microvilli. A basal labyrinth was not seen. Therefore, it is unlikely that the maxillary gland is able to produce a hypoosmotic urine necessary for freshwater animals. Tubule cells can be surrounded by parenchymal cells that produce numerous vesicles, suggesting possible physiological interactions between tubule cells and parenchyma. The ectodermal terminal duct is lined with cuticle and is differentiated into a pulsatile body consisting of two interconnected ampules. The first functions as a bladder. The second ampule, under muscular control, excretes the urine. Pulsatile body, looping tubule, and broad microvilli appear to be distinctive features of the bathynellacid excretory organ. © 1996 Wiley-Liss, Inc.     

The fine structure of the mesonephros of the lamprey,Entosphenus japonicus Martens

Summary The fine structure of the mesonephric kidney of the lamprey, Entosphenus japonicus Martens, has been investigated with the electron microscope and discussed from the viewpoint of comparative morphology of the mesonephros.The structure of the capillary wall of the glomerulus essentially coincides with that of higher vertebrates, though its basement membrane is remarkably thick (300–400 m) because of a dense accumulation of fibrillar material between the endothelium and the basal lamina of epithelial cell. No obvious fenestration of the endothelial cell has been observed in the glomerulus or capillaries in any part of this organ.The kidney tubule is divided into three segments: 1. neck segment composed of ciliated cells with numerous mitochondria and glycogen particles, 2. proximal tubule composed of brush bordered cells provided with extensive pinocytotic vesicles and lysosomal granules in the apical cytoplasm and with lamellar membranes in the basal, and 3. distal tubule characterized by cells which, with their abundant mitochondria and branched tubular endoplasmic reticulum (about 500 Å diameter) with a central core, closely resemble the chloride cells in the gill filament of some teleosts. The possibility that the lamellar membranes in the proximal tubule cells correspond to basal infoldings is discussed.The extensive development of the tubular reticulum and of the mitochondria in the distal tubule cells is believed to reflect the active absorption of urine chloride in the urinary tubule of lamprey mesonephric kidney evidenced by physiologists. The proximal tubule is suggested to take a part also in the urinary transport of water and ions, as the lamellar membranes found in the cells of this portion likely correspond to the basal infoldings in more advanced forms of the kidney.The epithelial cells of the ureteric duct are characterized by granules suggesting a mucous secretion. No fine structure implying an absorptive activity in this duct has been observed.     

Increase in number and size of kidney concretions as a result of PCP exposure in the freshwater snail Planorbarius corneus (Gastropoda, Pulmonata)

Molluscan kidneys are able to excrete solids in the urine in the form of concretions. It is thought that increased formation of these concretions occur under pollutant, environmentally or reproductive induced stress. This study examined the formation of concretions in the kidney of the freshwater snail Planorbarius corneus L. experimentally exposed to pentachlorophenol (PCP). Light microscopic histopathological analysis of the PCP-exposed P. corneus revealed significantly enhanced production of the kidney concretions when compared to the kidneys of control individuals. Measurements of the number of kidney concretions, the apparent area of the concretions, and the epithelial area filled with concretions indicated an increase in the number and size of concretions in all treated snails. Lipofuscin content of excretory cell concretions was detected.     

THE FUNCTIONAL ANATOMY OF THE GUT OF THE PROSOBRANCH GASTROPOD POMACEA CANALICULATA AND OF SOME OTHER PILIDS

The structure and functioning of the gut of Pomacea canaliculata (D'Orb.) has been investigated using living and preserved material. Anatomical studies were also carried out on preserved specimens of Pila globosa, Turbinicola saxea and Lanistes ovum bangweolicus .
The gut of pilids is specialised for a macrophagous diet, usually of aquatic angiosperms. The mid-oesophagus is a crop for storage, and the stomach has a large triturating gizzard developed from the gastric shield area. This is the site of extra-cellular digestion; there is no intra-cellular digestion in any part of the gut. The ducts of the digestive gland open into a special region of the stomach, the vestibule, which is histologically similar to them. The style sac begins the compacting of the faeces, which is completed in tho intestine. There is no evidence that absorption ocrurs in the epithelium of tho stomach or intestine; soluble products of digestion are carried into the digestive gland, which is the main site of absorption. Its activity is supplemented by amoebocytes entering the lumen of the style sac and intestine. Two types of cell occur in the gland, one type producing digestive enzymes and absorbing soluble products of digestion, the other type being excretory in function. There is no sign of phagocytosis in either. The excretory activity of the kidney is further supplemented by an anal gland.     

Variations in the retinal designs of pulmonate snails (Mollusca, Gastropoda): squaring phylogenetic background and ecophysiological needs (I)

Abstract. The eyes of aquatic pulmonates differ from those of terrestrial pulmonates; the latter, in species such as Cepaea nemoralis and Trichia hispida , possess conventional, cup-shaped retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis , and Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions ("pits"). The pits are separated by an internal ridge, called the "crest", and on account of their pigmentation can be seen in vivo . The dominant cellular components of the retinae of terrestrial as well as aquatic snails are pigmented cells and microvillar photoreceptors, the latter occurring in two morphologically distinct types (I and II). Aquatic snails with preferences for shallow water possess eyes with both type I and type II photoreceptive cells, but Pl. corneus , an inhabitant of deeper water, only has type-I receptors, supporting an earlier finding that type I cells represent dim- and type II cells bright-light receptors. On the basis of histological and optical comparisons, we conclude that the eyes of L. stagnalis and R. peregra , species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes of P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors.     

Cadmium stress stimulates tissue turnover in Helix pomatia: increasing cell proliferation from metal tolerance to exhaustion in molluscan midgut gland

In terrestrial pulmonate snails, cadmium (Cd) uptake leads to the induction of a Cd-specific metallothionein isoform (Cd-MT) that protects against adverse interactions of this toxic metal ion. Increasing concentrations of Cd cause increased individual mortality possibly linked to pathological alterations in the snail midgut gland. Histological, immuno-histochemical, and electron-microscopic methods in combination with tissue metal analyses and quantification of MT induction parameters were applied to the midgut gland of Cd-exposed Roman snails (Helix pomatia). Conspicuous concentration-dependent alterations occurred in this organ, including the metal-induced increase of Cd-MT concentration and manifestation of Cd-MT mRNA precipitations in all midgut gland cell types. The most evident alteration was an increase of cellular turnover reflected by enhanced cell proliferation. Intensified vesiculation of endoplasmic reticulum was noted in basophilic cells and an increasing formation of lipofuscin granules in excretory cells. At the highest Cd concentrations, mitochondrial membranes were disrupted in basophilic cells, and lipofuscin granules were released from excretory cells into the midgut gland tubular system. Some of these alterations (e.g., increased cell proliferation rate, vesiculation of endoplasmic reticulum) detected at low Cd concentrations were interpreted as adaptive response processes enhancing the tolerance of exposed individuals to metal stress. Cellular alterations at higher Cd concentrations (e.g., mitochondrial structural damage) clearly represented ongoing irreversible cellular disruption. Combined evaluation of cellular biomarkers and MT saturation levels indicated that the transition from stress resistance to depletion of resistance capacity occurred above a threshold of 0.8 µmol Cd/g dry weight in the midgut gland of H. pomatia. At these Cd concentrations, Cd-MT was saturated with Cd²⁺ ions, whereas at the cellular level, structural alterations turned into pathological deterioration.     

Induction of metallothionein in rat tissues following subchronic exposure to mercury shown by radioimmunoassay

Although the analysis of metallothionein (MT) by radioimmunoassay (RIA) is not a common technique, its use is preferred over other methods since it offers the advantages of sensitivity and specificity. In this paper we present data on the basal levels of MT in rat tissues and physiological fluids of female Sprague-Dawley rats. The mean basal MT concentrations of the following organs and fluids were determined by RIA to be: liver (9.8 μg/g), kidney (68 μ/g), brain (0.8 μg/g), spleen (1.0 μg/g), heart (5.4 μg/g), plasma (11 ng/ml), and urine (200–300 μg/g creatinine). Following subcutaneous exposure to inorganic mercury (0.2 μmol/kg/d, 5 d a week for up to 4 wk), the metal accumulated primarily in the kidney. There was also a simultaneous accumulation of zinc in the liver and of zinc and copper in the kidney. Induction of MT did take place in liver, kidney, brain, and spleen. No increases in the MT contents of blood and urine were noted. The excess zinc and copper in the kidney of exposed animals were found to be associated predominantly with MT. No overt signs of mercury toxicity were noted in these animals and the incidence of proteinurea was nil. The data are discussed with reference to methods of MT determination in animal tissues and in relation to mercury metabolism and toxicity.     

Differences in dispersal- and colonization-related traits between taxa from the freshwater and the terrestrial realm

The aquatic and terrestrial realms differ in many physical properties that not only require specific physiological adaptations but also cause differences in dispersal options. We thus expect that life-history traits related to dispersal and colonization are under selection pressure because freshwater habitats are more isolated and thus more difficult to reach. We compared traits from European databases of three taxonomic groups along the passive–active dispersal gradient: plants (Plantes), snails (Mollusca: Gastropoda: Prosobranchia et Pulmonata) and hoverflies (Diptera: Syrphidae), all of which have both terrestrial and freshwater species (plants and snails) or early life stages (hoverflies). Aquatic taxa seem to be more successful long-distance dispersers than are terrestrial taxa. Our analysis also revealed lower numbers of seeds or eggs produced in the aquatic habitats. However, aquatic taxa often allocate resources to offspring guarding (vegetative propagules in plants, egg capsules in snails) and breeding-site selection (syrphids). Colonization of the aquatic realm is reinforced by increases in life span (plants), clonal spread (plants), shorter generation times (snails), selfing ability (marginal effect in pulmonate snails) or paedogenesis (two incidences in hoverflies, needs further studies). Probably, the variety of strategies reflects the different evolutionary backgrounds that elicit different combinations of trade-offs, but all traits also might increase invasibility of species.     

Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana

Multiple associations of fossil snails with dinosaur coprolites demonstrate that snails and dinosaurs not only shared ancient habitats but were trophically linked via dinosaur dung. Over 130 fossil snails representing at least seven different taxa have been found on or within herbivorous dinosaur coprolites from the Upper Cretaceous Two Medicine Formation of Montana. The terrestrial snail Megomphix is the most common taxon, but three other terrestrial taxa (Prograngerella, Hendersonia and Polygyrella) and three aquatic snails (Lioplacodes, ?Viviparus and a physid) also occur in coprolites. At least 46% of the shells in the faeces are whole or nearly so, indicating that most (if not all) of the snails were not ingested by dinosaurs, but were post‐depositional visitors to the dung pats. The sizeable, moist and microbially enriched dinosaur faeces would have provided both food and roosting sites for the ancient snails, and the large number of snail–coprolite associations reflect recurring, opportunistic exploitation of dung. The terrestrial taxa in the coprolites suggest that this Late Cretaceous locality included sufficiently moist detrital or vegetative cover for snails when dinosaur dung was not present. The aquatic snails probably entered the faeces during flood events. Dinosaur dung would have provided an abundant but patchy influx of resources that was probably seasonally available in the ancient environment.     

Variation in foot size and shape in some British land snails and its functional significance

Live weight, and the length, breadth and surface area of the extended foot sole were measured in 24 species of British terrestrial snails. Allometric relationships between these variables are described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/weight relationships deviate from isometric expectation for several species; this may be attributed to density changes, foot-shape variation or the physical constraint imposed by shell aperture size.
The rate of increase of foot sole area with live weight among species is greater than expected under isometry, indicating that larger species partially compensate for the increase in foot loading (weight per unit foot area). The rate of whorl expansion of the shell is related to the deviations from the interspecific foot area/weight relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot length:foot width) is negatively related to live weight and foot loading.
The findings are discussed in relation to the known behaviour and microdistribution patterns of the species.     

An evaluation of the interactions between freshwater pulmonate snail hosts of human schistosomes and macrophytes

An account is given of a laboratory investigation designed to evaluate the extent to which the freshwater pulmonate snail Biomphalaria glabrata (Say) can utilize various species of aquatic plants, mainly macrophytes, when presented in the following forms over different time scales: normal plants; dried plant material; homogenized plant material in calcium alginate matrices; water-soluble filtrates of plant homogenates in the medium. The following propositions, derived from the theory of phased coevolution of components of the module consisting of the epiphytic bacteria, algae, snails and macrophytes, are evaluated on the basis of the present results and others including those obtained in this laboratory. That as the snails had become specialized to exploit surface communities of epiphytic algae, decaying plant material and dissolved organic matter (DOM) early in their evolutionary history they would continue to exploit these resources when they later become associated with aquatic macrophytes. That pulmonate snails would tend to be feeding generalists capable of adapting to food of varying chemical composition, given sufficient time, provided it was sufficiently small or flaccid. That although macrophytes and snails show a strong positive relationship, the living macrophyte tissue would be little used by the snails. That the hard outer envelope, inherited from their terrestrial ancestors, would remain as the major defence mechanism of aquatic macrophytes against attack by snails and other aquatic invertebrates. That aquatic macrophytes would invest little in the nutrient deficiency strategy to reduce attack by invertebrates such as snails. That truly aquatic submerged macrophytes would not possess secondary plant compounds (SPC) that would be molluscicidal. Emergent parts of subaquatic or aquatic plants might be expected to be better sources of SPC with molluscicidal factors than submerged aquatic plants. Species of epiphytic or planktonic algae might be better sources of SPC with molluscicidal effects than aquatic macrophytes. That the strategies developed by pulmonate snails for obtaining their energy supplies would not be conducive to rapid speciation. The analysis of the present and other related results supports these propositions. Predictions based on the theory of mutualism involving the snails, macrophytes and other components of the module also receive some support from an analysis of the present results. The additional empirical work that could be undertaken to test this theory is briefly discussed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Genomic comparison of chitinolytic enzyme systems from terrestrial and aquatic bacteria

Chitin degradation ability is known for many aquatic and terrestrial bacterial species. However, differences in the composition of chitin resources between aquatic (mainly exoskeletons of crustaceans) and terrestrial (mainly fungal cell walls) habitats may have resulted in adaptation of chitinolytic enzyme systems to the prevalent resources. We screened publicly available terrestrial and aquatic chitinase‐containing bacterial genomes for possible differences in the composition of their chitinolytic enzyme systems. The results show significant differences between terrestrial and aquatic bacterial genomes in the modular composition of chitinases (i.e. presence of different types of carbohydrate binding modules). Terrestrial Actinobacteria appear to be best adapted to use a wide variety of chitin resources as they have the highest number of chitinase genes, the highest diversity of associated carbohydrate‐binding modules and the highest number of CBM33‐type lytic polysaccharide monooxygenases. A ctinobacteria do also have the highest fraction of genomes containing β‐1, 3‐glucanases, enzymes that may reinforce the potential for degrading fungal cell walls. The fraction of bacterial chitinase‐containing genomes encoding polyketide synthases was much higher for terrestrial bacteria than for aquatic ones supporting the idea that the combined production of antibiotics and cell‐wall degrading chitinases can be an important strategy in antagonistic interactions with fungi.     

Olfactory metamorphosis in the Coastal Giant Salamander (Dicamptodon tenebrosus)

This study examined the gross morphology and ultrastructure of the olfactory organ of larvae, neotenic adults, and terrestrial adults of the Coastal Giant Salamander (Dicamptodon tenebrosus). The olfactory organ of all aquatic animals (larvae and neotenes) is similar in structure, forming a tube extending from the external naris to the choana. A nonsensory vestibule leads into the main olfactory cavity. The epithelium of the main olfactory cavity is thrown into a series of transverse valleys and ridges, with at least six dorsal and nine ventral valleys lined with olfactory epithelium, and separated by ridges of respiratory epithelium. The ridges enlarge with growth, forming large flaps extending into the lumen in neotenes. The vomeronasal organ is a diverticulum off the ventrolateral side of the main olfactory cavity. In terrestrial animals, by contrast, the vestibule has been lost. The main olfactory cavity has become much broader and dorsoventrally compressed. The prominent transverse ridges are lost, although small diagonal ridges of respiratory epithelium are found in the lateral region of the ventral olfactory epithelium. The posterior and posteromedial wall of the main olfactory cavity is composed of respiratory epithelium, in contrast to the olfactory epithelium found here in aquatic forms. The vomeronasal organ remains similar to that in large larvae, but is now connected to the mouth by a groove that extends back through the choana onto the palate. Bowman's glands are present in the main olfactory cavity at all stages, but are most abundant and best developed in terrestrial adults. They are lacking in the lateral olfactory epithelium of the main olfactory cavity. At the ultrastructural level, in aquatic animals receptor cells of the main olfactory cavity can have cilia, short microvilli, a mix of the two, or long microvilli. Supporting cells are of two types: secretory supporting cells with small, electron-dense secretory granules, and ciliated supporting cells. Receptor cells of the vomeronasal organ are exclusively microvillar, but supporting cells are secretory or ciliated, as in the main olfactory cavity. After metamorphosis two distinct types of sensory epithelium occur in the main olfactory cavity. The predominant epithelium, covering most of the roof and the medial part of the floor, is characterized by supporting cells with large, electron-lucent vesicles. The epithelium on the lateral floor of the main olfactory cavity, by contrast, resembles that of aquatic animals. Both types have both microvillar and ciliated receptor cells. No important changes are noted in cell types of the vomeronasal organ after metamorphosis. A literature survey suggests that some features of the metamorphic changes described here are characteristic of all salamanders, while others appear unique to D. tenebrosus.     

Biological control of Salvinia by the snail Pila globosa Swainson

The recent spread of Salvinia molesta , a free floating weed in tropical and sub-tropical regions has attracted attention the world over. In India, many states face serious problems on account of Salvinia infestation. Several methods of controlling the weed have been tried, but with limited success. Biological control by snails, especially Pila globosa , a common tropical snail found in Kerala and other parts of India is suggested. It was found to feed voraciously on Salvinia , but not on paddy which forms a major crop in Kerala and other parts of India. The snail has efficient adaptations for a tropical environment in aquatic, semi-aquatic and even terrestrial habitats and its flesh is edible. Further studies on the performance of the snails in the field are, however, required.     

Fine structure and cytochemistry of the endothelial cells and rodlet cells in the bulbus arteriosus in species of Cichlidae (Teleostei)

The ultrastructure of endothelial cells and rodlet cells in the bulbus arteriosus of specimens representing six genera of Cichlidae is described. The former are very closely packed by membrane–bound and mainly electron–dense inclusion bodies (0.3–0.7μm).
In Apistogramma ramirezi I observed numerous subendothelial rodlet cells throughout the entire length of the bulbus arteriosus. These cells penetrate the endothelium and connect to the latter by desmosomes and tight junctions. The luminal part of the cell contains numerous vesicles and tubules (width 50–100 nm), whereas the basal part is occupied by a number of membrane–bound, club–like inclusions (length ≤ 5 μm). Between these two layers there occurs a layer of small, elongated mitochondria. Peripherally, these cells consist of a filamentous wall, except in the apical area.
The endothelial and rodlet cell inclusion bodies do not react with phosphotungstic acid (pH 1) or Sudan black B stain. The endothelial cells react strongly with periodic acid–Schiff (PAS) stain, whereas the rodlet cells are only moderately coloured by this stain.
The present results are discussed and compared with those reported previously for endothelial/ endocardial cells and rodlet cells in bony fish.     

Lack of renal effects of DOCA, ACTH, spironolactone, and angiotensin II in Squalus acanthias

Angiotensin II was infused intravenously in spiny dogfish sharks (Squalus acanthias). There were no significant effects on arterial blood pressure, glomerular filtration rate, urine flow, or Na excretion either in comparison with pre- and postinfusion values or in comparison with values measured in a control group of fish given elasmobranch saline intravenously. In other dogfish, glomerular filtration rate, urine flow, and Na and K excretory rates were measured for 3 days following implantation of desoxycorticosterone (DOCA), adrenocorticotropin (ACTH), or spironolactone; a control group was given no drug. There were no significant differences between these four groups of fish with respect to any of the measured parameters. These results suggest that the dogfish kidney is not a target organ for several substances known to affect renal function, either directly or indirectly, in other animals.     

Inhibition of the development of the reproductive tract in parasitized snails

Summary

Myoblasts, muscle cells with the capacity to divide, have been detected in “Anlagen” of the male copulation organ of Lymnaea stagnalis. They only occur in the apical part of the penis. Here they could be found throughout life. Mitotic activity of these cells can be demonstrated by using an antiserum to a S-phase specific cell cycle marker, PCNA [see, e.g., Baserga (1991)]. The number/percentage of PCNA positive myoblasts is a good parameter for growth of this male copulation organ and hence also for inhibition of its growth and development as occurs in parasitized snails. In transplantation experiments, “Anlagen” of the copulation organ were used from snails 7–9 weeks after being parasitized as they can be excised in this stage and transplanted into either parasitized or nonparasitized snails. These experiments have indicated that humoral, parasitic excretory/secretory factors can be responsible for the inhibition of growth and differentiation of the copulation organ in parasitized snails as reflected by a relatively low number of PCNA positive myoblasts compared to the controls. Data obtained in in vitro experiments showed a significant decrease of the number of myoblasts in “Anlagen” cultured in the presence of parasitic E/S products. The fact that no significant effect was found on the relative low number of PCNA positive myoblasts is discussed. The effect of parasitic E/S products on these myoblasts appeared to be exerted in a direct way, not mediated by CNS-derived factors or by factors from cells in the connective tissue sheath around the CNS. Although it appears possible to use transplantation and/or in vitro culturing of these “Anlagen” as a bioassay for identification of the parasitic factor(s) responsible for the inhibitory effects on myoblasts, the methods are very laborious and do not seem very appropriate for testing many fractions of E/S products.     

Pulmonate phylogeny based on 28S rRNA gene sequences: a framework for discussing habitat transitions and character transformation

Pulmonate snails occupy a wide range of marine, estuarine, freshwater and terrestrial environments. Non-terrestrial forms are supposed to be basal in pulmonate evolution but the group's phylogeny is not well resolved either morphologically or on the basis of available DNA sequence data. The lack of a robust phylogeny makes it difficult to understand character polarization and habitat transformation in pulmonates. We have investigated pulmonate relationships using 27 new sequences of 28S rRNA from pulmonates and outgroups, augmented with data from GenBank. The complete alignments comprised about 3.8kb. Maximum parsimony, maximum likelihood and Bayesian analyses of alignments generated under different assumptions are reported. Complete alignments appear to have a degree of substitution saturation so where there is conflict between hypothesised relationships more weight is given to analyses where regions of random similarity are excluded and which are not affected by this complication. Monophyly of the five main pulmonate groups was robustly supported in almost all analyses. The marine group Amphiboloidea and the freshwater Glacidorbidae are the most basal. The remaining pulmonates (Siphonariidae, Hygrophila and Eupulmonata) form a moderately-supported monophyletic group in all analyses bar one probably affected by saturation of substitutions. Siphonariidae, a predominantly marine and intertidal family, and Eupulmonata (mainly terrestrial with marine, estuarine and freshwater species) form a strongly supported clade that is the sister group to Hygrophila (freshwater). Multiple colonizations of freshwater and terrestrial habitats by pulmonate snails are suggested. No analyses strongly support the possibility of habitat reversions. The colonizations of freshwater by Hygrophila and of land by Stylommatophora were apparently phylogenetically independent although it cannot yet be excluded that there were transient terrestrial phases in the history of the former group or freshwater phases in the latter.