Using the methods of light and electron microscopy, the morphological properties were studied, and the evolutionary changes in the vitreous body of the camera-like eyes of some species of terrestrial gastropod pulmonate molluscs were traced. It was demonstrated that all the considered properties of the vitreous body of molluscs are typical for terrestrial pulmonates. In the course of evolution, as a result of the land development by mollusks, the properties of their vitreous body have hardly changed.

Three small but abundant in specimens isolated colonies of Helix lutescens have been found in the eastern part of Moscow City in the central part of European Russia for the first time. This is an easternmost known colony of the species. Differences from the related and similar small form of H. pomatia are given and discussed.

Two colonies of an invasive West European land snail Cepaea nemoralis are reported from Kyiv region of Ukraine, as well as a third locality where snails were present in 2019, but no living individuals were found in 2020. One more locality is reported from Odessa City in Southern Ukraine. These localities are the most south-eastern ones for the range of C. nemoralis and are the first known records of this species in Ukraine outside its western regions, with closest previously published confirmed colonies in more than 400 km. Distribution of this species in Eastern Europe is discussed, it is argued that invasion of C. nemoralis is probably driven by the same processes as invasion of the Spanish slug, Arion vulgaris, that invaded Eastern Europe during the last decades.

The importance of trees for an invasive Mediterranean cellar slug, Limacus flavus, was studied within one block of a residential area in Kyiv city (Central Ukraine) from spring of 2020 to spring of 2021. Slugs tended to occur in the old poplars (Populus nigra var. italica). Live animals or their traces were found on the 71 of 320 poplars and on 17 trees of other species in the studied area. Slugs often go up to at least 12 m above the ground on these poplars and feed on the lichens there. Animals breed and spend the daytime inside at least some of these trees under the bark. Slugs were wintering in some of these trees in 2020-2021 and probably also at least in 2019-2020, while previously L. flavus was reported from Eastern Europe only in connection to cellars, basements and greenhouses. Apparently these slugs are occupying additional habitats in response to climate change and their occurrence in the trees may become common in Eastern Europe with the warming of climate. The colouration of L. flavus and closely related L. maculatus is discussed. Various differences of the colouration were suggested in the literature to distinguish the two species, most notably the central light stripe on the back of L. flavus, but this character is absent in most of the studied specimens and, therefore, the overall colouration overlaps in the studied populations of the two species.

The spat of the scallop Flexopecten glaber were collected in cages with the Pacific oyster Crassostrea gigas cultured on a mussel-and-oyster farm (outer roadstead of Sevastopol Bay). For two years they were reared in plastic cages at a depth of 2–3 m. The cages were periodically withdrawn to measure the size and weight parameters of the molluscs: the length (L, mm), height (H, mm), and width (D, mm) of shells and the total live weight (W, g). The correlations among the parameters under study are presented. The changes in the indices of the frontal (D/L) and sagittal (H/L) curvatures, convexity ((H+D)/L) and conditional volume (H×D×L/1000) of the shells in ontogenesis are shown. Upon reaching a shell length of 30–35 mm, the allometry of the volumetric and weight growth of molluscs changed from positive to negative. The largest values of the shell convexity index were registered in the same length range. A conclusion about the interval-type growth of F. glaber in linear size and weight is made. It is suggested that the optimal strategy of shell formation in F. glaber in ontogenesis implies the ripening and the first reproduction of the molluscs occurring at the highest volumetric characteristics of the shell. The relationships for the linear size and weight growth of F. glaber in the first two years of life are obtained. It is concluded that the scallop F. glaber should be considered as a possible element for the diversification of the existing aquaculture of molluscs (mussels and oysters) off the coast of Crimea.

Species of the genus Mastoniaeforis are mainly distributed in the Indo–West Pacific with ten species being recorded in the world. To date this genus was not recorded in Vietnam. Specimens of three species, M. chaperi, M. lifuana and M. speciosa, were collected in two field trips from May, 2019 to October, 2020 in Truong Sa archipelago (Spratly Islands), Khanh Hoa Province, Vietnam. The diagnostic characters, distribution, and illustrations of three Mastoniaeforis species in Vietnam are provided.

The contribution to malacology made by the eminent Russian scientist and traveler, Alexander Theodor von Middendorff (1815–1894) is reviewed. Middendorff’s research is rightly considered the highest achievement of Russian malacology in the 19th century. It is shown, despite the relatively short timespan of his malacological activity, Middendorff could reach substantial progress in the knowledge of the Russian malafocauna, both marine, and continental, and authored more than 15 malacological publications, including a series of fundamental systematic works. Middendorff’s views on taxonomy, variability, and zoogeography of molluscs are discussed, and the research program in malacology, proposed by him, is reviewed as well as the impact of his studies on the further development of malacology in the Russian Empire. The full list of all molluscan taxa described by Middendorff is provided as an “Appendix” to the article.

The species composition and distribution of freshwater gastropods in the western part of the Kola Peninsula and northern Karelia has been studied. The explored region lies in the far north of Europe and covers several landscape zones: from the northern taiga to the tundra. In sum, seventeen species found, for each of them the distribution maps and photographs of intraspecific forms of conchological variability are presented. The taxonomic remarks are also given when appropriate. Gyraulus acronicus (Planorbidae) and Ampullacena balthica (Lymnaeidae) are the most common species in the studied region. Two species (Ampullacena balthica and Galba truncatula) have been recorded in the Barents Sea islands off the Kola Peninsula. A single non-indigenous species, Physella acuta (Physidae) has been registered in the studied territory. Based on the results of a comparison of the faunas of gastropods from various areas within the whole territory studied and other regions of northern Europe and Western Siberia, it has been shown that the most dramatic reduction in the species composition occurs during the transition from taiga to tundra landscapes.

On the territory of Republic of Armenia a new species of land snails has been found. Illustrated description is presented. Relationship between five close related Transcaucasian species of Armenica is discussed. New species differs from closely related A. unicristata in having more large dark shell with well developed axial striation and character of clausilium.

Based on a study of samples made by the author, 70 species of freshwater bivalves belonging to 6 genera and 2 families are recorded for the Taz River basin (north of Western Siberia). An annotated list of bivalves of the Taz basin is provided, with data on species’ range, their findings in Western Siberia and within the studied area. Some information about bionomics and abundance of bivalves are given. 45 species are for the first time registered in this basin. Most studied molluscan communities are characterized by low species richness; the distribution of species by their occurrence was extremely uneven. 22 species are characterized as rare. The highest species richness of bivalves was found in river channels, rivers and brooks; the lowest – in temporary habitats. The core of the fauna is constituted by species with broadest range (cosmopolitan, trans Holarctic, trans Palearctic) as well as by taxa with European Siberian type of distribution. From the taxonomic point of view, the bivalve fauna of the Taz basin is relatively separated from the faunas of other river basins of Western Siberia and is more similar to the fauna of the Lower Yenisean zoogeographic province (sensu Starobogatov [1986]).

TEM and SEM electron microscopy have been used to study osphradia in 6 species of marine Caenogastropoda. The ultrastructural features of mechanoreceptor cells that perform the Littorina osmoreception function in osphradium organs are presented. Mechanoreception is based on a possible change in the volume of cisterns of microvilli of supporting cells, which can be transmitted by the cilia of nearby mechanoreceptor cells. These cells obviously, have mechanosensory channels on the apical surface. It has been first discovered in predatory molluscs actively searching for food, that single receptor cells with a mobile sensilla consisting of several cilium were joined together. They are located along the groove zone and follow the direction and force of the movement of water along the osphradium lamellae.

A new species Acrotoma (Acrotomina) mallabica sp. nov. from the Western Caucasus is described. The morpho-anatomical relations of the new taxon with closely related species are discussed.

The invasion of the predatory gastropod mollusk Rapana venosa, known as veined rapa whelk, in the Black Sea and its outspread in many regions of the World Ocean have led to complex structural changes in the communities of bivalve mollusks. When capturing a prey, the adult rapa whelk excretes a biotoxin from the hypobranchial gland that causes severe paralysis of the musculature of bivalve mollusks. Under experimental conditions, it was established that the biotoxin, after the short-term exposure of the fertilized eggs of the mussels Mytilus galloprovincialis to it, has a mutagenic effect, causing chromosomal aberrations in the embryos. The acentric groups of chromosomes, single and multiple chromosome bridges, chromosome lags and polyploidy indicate transformation or destruction of threads of the achromatin spindle in the mitosis anaphase. The chromosome fragmentation and emergence of ring chromosomes in the metaphase and anaphase of mitosis of mussel embryos are the consequence of the partial destruction of chromatin. The underdeveloped shell, anomalous development of D-veligers’ velum, and the changed form of the ‘eye spots’ and hepatopancreas of the larvae at the veliconcha stage result from chromosomal mutations during the embryo development. The mechanism of the rapa whelk biotoxin action on the chromosomal apparatus of mussel embryos is discussed.

The extant polyplacophoran tail valve is traditionally considered as a unity despite its clear separation into two distinct regions, which were in relation to the delimiting point – the mucro – termed the antemucronal area for the front part and the postmucronal area for the hindermost region. However, earlier conceptions do exist, which consider the postmucronal area as semiplate, with the antemucronal area as modified “intermediate” plate.

To test the usefulness of the terms “antemucronal” and “postmucronal” in their traditionally sense, three independent attempts were made to demonstrate existing differences between the mucro-separated areas.

Leptochiton rugatus was histologically examined allowing the confirmation of a cardial complex-antemucronal relation. Valve morphology of the brood of Radsia nigrovirescens not only confirms a tegmental development prior the building of the articulamentum but shows that the postmucronal area develops to its final shape before the antemucronal area appears. For the first time it is demonstrated that the antemucronal area of Schizoplax brandtii shows a delayed splitting of the relevant area, characteristic for the conditions found in the intermediate valves of this species only. That leads to the assumption that the underlying valve build processes are of the same nature as in the intermediate valves. Additionally, literature data on valve characters were compiled that show a stronger relationship (61%) of the antemucronal area to the central area of intermediate valves rather than to the merged postmucronal area. In the light of this evaluation it is suggested that the term “tail valve” should be used to demonstrate the merged condition of two independent zones only, the antemucronal area and the postmucronal area.

The monotypic land snail genus Campolaemus, known only from Saint Helena Island, has been classified in the family Hypselostomatidae, a family occurring in the eastern Palaearctic and in the Oriental region. Due to biogeographical reasons and morphological traits, especially the arrangement and morphology of apertural barriers, Campolaemus is moved to the family Streptaxidae, which is a pantropic family, being highly diverse in tropical Africa.

The paper describes a new species of the genus Thermochiton, T. papuaensis sp. nov., found in deep waters off Papua New Guinea. This species differs from T. undocostatus primarily in the dorsal scales, the marginal spicules, sculpture of the jugal area and the shape of the central teeth of radula. There are apparent similarities between the species of the genus Thermochiton, Connexochiton platynomenus, C. kaasi and Ischnochiton crassus. The last species is proposed to be transferred to Connexochiton. Owing to the friable, rusty brown deposits that densely cover the shell and girdle of both specimens of T. papuai, the latter probably lives in areas of high chemical activity.

The phenotypic composition of the introduced colonies of Cepaea nemoralis in five settlements of Ivano-Frankivsk and Lviv regions of Ukraine, studied from May to July 2019, is described. The largest colonies were found in Bohorodchany (Ivano-Frankivsk region) and Lviv. In most cases, there was a pronounced dominance (from 40% or more) of the individuals with unbanded and five-banded shells – pink, less often yellow. Among the snails with five-banded shells, phenotypes with the fusion of two or more bands prevailed, among which in Lviv the phenotype (12)3(45) most often occurred, and in Bohorodchany (123)(45). Molluscs with brown shells were found in only one of the studied colonies. In Bohorodchany a differentiation of the phenotypic structure of the C. nemoralis colonies separated by a highway, was noticed. The available data on the present distribution in Western Ukraine of two introduced species of the genus Cepaea: C. nemoralis and C. hortensis are analyzed.

A new deep-water polyplacophoran, Stenosemus nitens sp. nov., from Papua New Guinean waters is described. It differs from other species of the genus Stenosemus by having a sculptureless, smooth and shiny surface of the tegmentum, smooth dorsal spicules with only a few small transverse wrinkles on top, and unusual bends of the front margin of the intermediate valves. Within the genus the new species belongs to a limited group of coloured chitons.

Cellular composition and morphology of components of photoreceptor apparatus of the retinae of camera eyes of some species of terrestrial gastropod pulmonate molluscs were studied and its evolutionary transformations were traced. It was demonstrated that all examined characteristics of photoreceptor apparatus of the most investigated species were normal for terrestrial pulmonates, while those of one species were special. Evolutionary transformations of photoreceptor apparatus of terrestrial pulmonate molluscs due to habitat shift were quite significant and concerned of its cellular composition and morphology of photoreceptor cells.

The morphological variability of the bivalve mollusc Flexopecten glaber was studied based on the analysis of 306 specimens from the Kazach’ya Bay (Sevastopol, Crimea) of the Black Sea. The sampling of mollusks was carried out with the help of skuba equipment in June–September of 2017–2018. For F. glaber, the age and size characteristics are given and the main trends in their ratios with the shell growth are considered. For the first time, quantitative indicators of thickness of the shell valves, their weight and height of the radial ribs are presented and discussed. A wide intraspecific polymorphism of the shell is shown in a relatively narrow range of habitat conditions. The obtained data expand the understanding of the morphological variability of F. glaber in the Black Sea and provide new information on the functional morphology of the species.