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Epizootological role of fleas in the Gorno-Altai natural plague focus (Sailugemsk focus) and numerous data on the flea viability are analyzed and generalized. Information concerning the flea natural infectivity with Yersinia pestis altaica is repressented. Ecological peculiarities of some flea species parasitizing the main host, Mongolian pika Ochotona pallasi, and nature of their interrelations with Y. pestis are investigated. It is shown that the flea taxocenosis provides the permanent all year-round circulation of Y. pestis in the Gorno-Altai natural focus. Certain combinations of structural elements of the flea taxocenosis have a dominant significance in determination the circulation process at different phases of the annual epizootic cycle.

Comparative zoogeographic analysis of outcomes accumulated in the course of long-term investigation the parasitic and free-living arthropods (Acarina: Parasitiformes; Insecta: Siphonaptera) associated with three subspecies of the narrow-skulled vole (Microtus gregalis gregalis, M. g. major, M. g. eversmanni) in various landscape zones and subzones in a flat part of Western Siberia, foothills of the Altai-Sayan mountain system and mountains of Southwest Altai. The obtained data, on the one hand, recover specific features of certain parasite communities that reflect ecological peculiarity of a host species living in conditions of concrete landscapes, on the other hand, — a high degree of similarity between species set of some ectoparasitic and nidicolous arthropod groups from different subspecies of M. gregalis. The systematic list of ectoparasitic and nidicolous arthropods associated with M. gregalis and representing Gamasoidea mites, ticks and fleas is provided.

The first tarsal segment of the taiga tick bears 4 general types of sensilia (except for sensilia forming the Haller's sensory organ on the dorsal surface of the tarsus): tactile mechanoreceptor sensilia of 3 types, contact chemo-mechanoreceptor sensilia of 2 types, and double-walled pore sensilia. One of these types, the chemo-mechanoreceptor upper-pore sensilia, was found only in the Ixodinae, and not found in the examined representatives of the Amblyomminae. This type of sensilia was also found in the palpal receptor organ of the ixodid and argasid ticks.

A morphological description of Sanguinicola armata cercaria is given. This larva belongs to the group of brevifurcate and apharingeal cercariae. Mouth opening is on ventral surface of the anterior organ. The body and branches of the furca bear a membrane. The body / tail ratio is much more than in other cercariae of the family Sanguinicolidae. The gland apparatus of the larva is represented by several types of secretory cells. The penetration glands form a group of 5 pairs of cells on the ventral side. Their ducts form a single bunch directed anterior, go through the anterior organ and open terminally on the proboscis, which is able to turn outside. Other gland cells apparently represent specialized cytones of the tegument. Excretory formula: 2[(1 + 1) + (1 + 1)] = 8. The external surface of the larva bears a small number of sensilla. Some similarities of the families Sanguinicolidae and Schistosomatidae in the cheatotaxy of cercaria have been noticed.

A new species of aseptate gregarine, Sphinctocystis phyllodoces gen. n., sp. n., from the gut of a polychaete Phyllodoce citrina Malmgren, 1865 from White Sea is described. The electron and light microscopic data on trophozoits are presented. Taxonomy of the described species is discussed. Certain ultrastructural characters are included in generic and specific diagnoses. Order Eugregarinida Leger, 1900; suborder Aseptata Chakravarty, 1960; family Lecudinidae Kamm, 1922. Genus Sphinctocystis gen. n. Type species: Sphinctocystis phyllodoces sp. n. Diagnosis. Characters of the family. Free trophozoits elongated, often with several annular constrictions. Anterior end asymmetric, without hooks, not separated from the body, with small apical papilla encircled by smooth area. Epicyte «classical», without additional axial formations at the tops of folds; epicytic folds high, monomorphic in cross sections, finger-shaped, with parallel sidewalls. In the gut of polychaetes. Differential diagnosis. The new genus differs from Lecudina by having asymmetric anterior end, developed smooth area around the apical papilla, and monomorphic epicytic folds looking finger-shaped in cross sections. It also differs from Lankesteria by the absence of additional axial formations at the tops of the epicytic folds. It differs from both named genera by presence of annular constrictions on the trophozoit body. Sphinctocystis phyllodoces sp. n. Diagnosis. Characters of the genus. Free trophozoits elongated, large, up to 617 x 77 µm. The average height of epicytic folds 976 nm, thickness 194 nm; there are 6—8 apical filaments and rippled dense structures per fold. Nucleus spherical (ellipsoid after fixation), 24—52 µm along longest axis, localised in anterior third of the body, carries several karyosomes of various size; 25—30 nm thick fibrils (possible fragments of nucleolonema) may be present in karyolymph. Other stages unknown. Type series. Microscope preparation with 7 trophozoits, Karacci's haematoxylin stained, is kept in the Zoological museum of the Moscow State University (collection number: Z-1). Type host Phyllodoce citrina Malmgren, 1865 (Polychaeta: Phyllodocidae). Localisation. Mid-gut. Type locality. White Sea Biological Station of the Moscow State University, Yeremeyevsky Rapid, Velikaya Salma Strait, Kandalaksha Bay, White Sea.

The hypothesis of the origin and evolution of the hamuli in monogeneans is proposed. It is suggested that the hamuli originated as the adult attachment organs of protomonogeneans inhabited the gills of the first vertebrates. Primarily they were represented by two lateral pairs of large hooks disposed anterior to the larval haptor. The fundamental direction in the evolution of monogeneans was the concentration of ail attachment structures on the growing haptor. It the course of this evolutionary process, the hamuli onchoblasts migrated to the haptor, in which they had reached the position in the hind part of the haptor. The neotenic evolution of the Dactylogyridea and Gyrodactyloidea resulted in the forming new hamuli pairs. The hooks of opposite sides of the haptor are joined in a single complex within each side by the transverse plates (bars). So the presence of 4 hamuli is plesiomorphy for all the monogeneans and the presence of the transverse bars and new hamuli pairs are apomorphy for the Dactylogyridea and Gyrodactyloidea, whose evolution was linked with that of the Teleostei. The origin of the new hamuli pairs and transverse bars in the Dactylogyridea and Gyrodactyloidea appears to be a convergence.

Protective reactions of molluscs are traditionally considered in cell and humoral aspects. The paper analyses original data and recent reference data oncell (phagocytic) reactions of gastropod molluscs. The main attention is paid to peculiarities of hemopoiesis, cell elements of hemolymph, and dynamics and mechanics of encapsulating the parasites.

The effect of invasion of two trematode species, Bunocotyle progenetica (Hemiuridae: Bunocotylinae) and Cryptocotyle cancavum (Heterophiidae) on the host vitality is estimated. The mortality of infected and uninfected individuals of Hydrobia ulvae after different exposure time (3, 6, 9 and 12 hours) under extremely high temperature (+42°C) was compared. The total death of the host was observed after 12 hours of experiment. The death rate of molluscs infected with B. progenetica ranged from 60 to 100% and was significantly higher (p < 0.01) than mortality of individuals infected with C. cancavum and uninfected snails in each case of exposure (3, 6 and 9 hours). At the same time the mortality of individuals infected with C. cancavum was significantly higher (p < 0.05) than the mortality of uninfected snails only after 9 hours of exposure (13.3 and 3.1%, respectively). Difference in pathogen effects B. progenetica and C. cancavum is expressed in different temperature resistance of the host that may be explained by different localisation of redia, their motile activity and manner of feeding. Besides, the invasion by C. cancavum causes the sterilization and accelerated growth of hosts. Low decreasing of the resistance to overheating in snails infected with this trematode could be explained by a compensation effect of this infection. This compensation realizes due to the redistribution of some part of the reproductive energy after the sterilization. An absence of parasitic castration in the case of infection with B. progenetica is a key reason of such considerable declining of their vitality. High lethality of the host appears to be adaptive for the parasite with the monoxenous life cycle.

The influence of the trematode invasion on the accumulation of heavy metals (Cu, Zn, Cd, Pb) by various organs of the mollusc Colletopterum ponderosum have been investigated. An increase of the coefficients of accumulation and bottom biological accumulation, and also changes in the accumulation in pairs of heavy metals were noted in the infected molluscs.