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.

Vietnam with its limestone ranges that are concentrated mainly in northern, north–central and southern parts, and many different limestone islands running along the coastline, has a phylogenetically-diverse land snails fauna dominated by endemic species. The genus Cyclophorus (Cyclophoridae) in Vietnam is studied. Sixty species and subspecies from various habitats of limestone areas throughout Vietnam are confirmed and recorded. New species and subspecies, Cyclophorus implicatus kanhoensis ssp. nov., from Northwestern Vietnam, and Cyclophorus tamdaoensis sp. nov., from Northern Vietnam are described and discussed. In this paper, the Cyclophorus courbeti var. leucostoma Dautzenberg et Fischer, 1905, is elevated to subspecies.

Unio mongolicus Middendorff, 1851 (Bivalvia: Unionidae) was described from Gorbitza mountain stream (in the east of Trans-Baikal Territory, Russia) and the taxonomy has changed over the past century and a half. A specimen of Middendorffinaia mongolica from the Gorbitza stream collected 160 years later is a match with the type illustration of Unio mongolicus from the type locality. Comparison of the shell morphology of the type specimen of U. mongolicus with the modern specimen collected from this location, combined with our analyses of molecular, anatomical and conchological data of M. cf. mongolica species from rivers of the Upper Amur River Basin and the far eastern Russia, revealed that these taxa all belong to a single polymorphic species. The molecular analysis of the COI gene fragment mtDNA of examined Comparatory Species assigned to Middendorffinaia Moskvicheva and Starobogatov, 1973 confirmed they are only intraspecific forms of M. mongolica. Morphological and genetic distances between M. mongolica and Nodularia douglasiae Griffith et Pidgeon, 1833 confirmed their assignment to two independent genera. Differences of morphologically similar species M. mongolica, N. douglasiae and Unio crassus Philipsson in Retzius, 1788 were separated based on conchological characteristics and anatomical features. History of the taxonomic changes for Unio mongolicus is discussed here.

For the first time, the widely spread Antarctic species Leptochiton antarcticus was found at the Emperor Seamounts in the North Pacific Ocean. In spite of a large distance between the Emperor Seamounts and Antarctica, the found specimen have very similar shell, girdle, radula and gill features to the type material. I propose that L. antarcticus spread to the North Pacific from the Antarctic via a deep-water current near the ocean floor, and perhaps it inhabits the slopes of islands and continents from the South Ocean to the Emperor Seamounts.

Two substitution names are proposed for primary homonyms introduced for land snail of the family Camaenidae in Thach [2018].

The genus-group taxon Elbasania Schileyko et Fehér, 2017 has recently been introduced as a subgenus of Metafruticicola Ihering, 1892 for a species occurring in north-western Greece and Albania. Using mitochondrial and nuclear markers, the phylogenetic relationships of Elbasania within Metafruticicolini (Hygromiidae) are reconstructed. The results of these analyses suggest that Elbasania is more closely related to Hiltrudia Nordsieck, 1993, which has a range adjacent to that of Elbasania from Croatia to northern Albania, than to Metafruticicola. Elbasania shares with Hiltrudia and also Cyrnotheba Germain, 1929 a very characteristic microsculpture of the shell and an overall similar genital system, which however differs among these three taxa with regard to its internal structures, especially those of the penis. Therefore, it is suggested regarding Elbasania as a distinct genus here.

The predatory family Streptaxidae is comprised of five species occurring in Thua Thien Hue Province, central Vietnam. The present paper provides an assessment of the literature data relevant to these species and the description of two new species. Haploptychius bachmaensis sp. nov. has an oblique-heliciform shell with elevated spire. The aperture is subcircular, peristome discontinuous, thick and slightly expanded, apertural dentition with one strong parietal lamella. Perrottetia namdongensis sp. nov. has a rounded and protruded shell periphery. The aperture is subcircular, peristome is discontinuous, thick and expanded, apertural dentition with one large, strong parietal lamella, one small upper palatal lamella, one large palatal lamella, one large basal lamella, and one large strong columellar lamella.

All available data and most of materials on Acicula parcelineata from Ukraine are reviewed. Thirteen areas inhabited by the species are reported for Ukraine, some of which include several known sites. Five of these areas were not reported before. Map of general distribution of A. parcelineata is provided (outside Ukraine based on published data). Habitat preferences of this species are reviewed, it occurs in Ukraine almost exclusively in forests with presence of beech, on altitude 300-1100 m. Conservation status of A. parcelineata in Ukraine is shown to be “Near Threatened” according to IUCN criterions, it is recommended to be included into the next edition of Red Book of Ukraine. Shells’ measurements of A. parcelineata are provided and compared with related Platyla jankowskiana. The later species is discussed; it seems to be possible that it represents large abnormal specimens of A. parcelineata.

In present paper, we report the first findings of two large and conspicuous nudibranch species for Russian waters - Goniodoris nodosa and Flabellina pellucida. Both species show wide distribution in the northeastern Atlantic waters or even in the northwestern Atlantic in the case of F. pellucida. The morphology of both species was studied using anatomical dissections and scanning electron microscopy. Molecular diversity was assessed by sequencing of COI marker and reconstruction of haplotype networks. In the Barents Sea, both species show minor genetic differences from their European relatives and similar morphological characters. Putative explanations of new findings for Russian Arctic are discussed.

Anatomical study of five specimens of an endemic species of the Mauritius Island, Erepta odontina (Morelet 1851) showed that within one population disappearing of additional organs of the reproductive tract (epiphallic caecum, penial caecum, penial appendix, flagellum and retractor of penis) occur in four different combinations. It is significant that the shells of all five specimens were practically identical. It is assumed that the described phenomenon has a mutagenic nature and is regarded as a rare (if not unique) case of reduction (up to complete disappearance) of additional organs within one population.

It is proposed to restore the old name of the genus Terenochiton for Leptochiton norfolcensis (Hedley et Hull, 1912). We also provide a new description for the genus since the old description does not reflect the main features associated with the development of the articulamentum. All valves of the genus have rudimentary unslitted insertion plates. Similar rudimentary insertion plates have evolved independently in different families of the primitive chitons.

An annotated and illustrated check-list of species of the subfamily Sphaeriinae (family Sphaeriidae) of the Kama region (the Middle Urals, Russia), with enumeration of species’ localities, is presented. After examination of own collections and critical assessment of literary data, 8 species from two genera (Musculium and Sphaerium) have been included into the final list. One of these species, Sphaerium asiaticum, is recorded for the first time from the studied area. The ecological characteristics of all species as well as remarks on their distribution within the region are provided. From the zoogeographical point of view, the most sphaeriine species of the studied area belong to the European-Siberian faunistic group.

The recent record of Xeropicta derbentina in the south of the Central Russian Upland (Belgorod Region, Russia) provides the opportunity to study the alien populations structure at the stage of colonization of a new territory. High intra-and interpopulation variability of the X. derbentina shell in different parts of the range is revealed. Based on the allozyme and PCR-ISSR analysis, it was established that native and adventitious populations of X. derbentina had the same level of genetic variability. The potential number of multilocus genotypes calculated for the Belgorod population indicate a further complexity of their genetic structure. The result of the ISSR analysis showed significant genetic differentiation (FST=0.746; p=0.01) of two adventive populations from the Belgorod Region, which indicates their different origin from the natural area.