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A new species of aseptate gregarine, Difficilina cerebratuli gen. et sp. n. (order Eugregarinida Leger, 1900; suborder Aseptata Chakravarty, 1960; family Lecudinidae Kamm, 1922) from the gut of the White Sea nemertean Cerebratulus barentsi Burger, 1895, has been described. The electron and light microscopic data on trophozoites are presented. Their general morphology resembles the representatives of the genus Lecudina, but the features of the epicyte ultrastructure are different from Lecudina and similar to those of the Lankesteria spp. Taxonomy of the described species is discussed. Certain ultrastructural characters are included in its generic and specific diagnoses. Genus Difficilina gen. n. Type species: Difficilina cerebratuli sp. n. Diagnosis. Characters of the family. Free trophozoites elongated, anterior end rounded, without hooks or exfoliations, not separated from the rest of the body, with well-developed terminal smooth area. The epicytic folds undulating vertically, in cross sections — mono-morphic, finger-shaped, with strongly developed cell-coat, with additional electron-dense axial structure ("middle axis") at the tops; number of rippled dense structurtes and apical filaments 3, the furthers are thick and slightly flattened in diameter. Other stages unknown. In testinal parasites of nemerteans. Differential diagnosis. The new genus differs from Lecudina by presence of smooth area at the apical pole of the body and the epicyte structure: vertically undulating monomorphic finger-shaped (in cross section) epicytic folds, oligomerization of the rippled dense structures and apical filaments, and development of the "middle axis". It also differs from Lankesteria by the shape of the body, vertical undulation of the folds, and non-tunicate host. Difficilina cerebratuli sp. n. Diagnosis. Characters of the genus. Free trophozoites slightly bent, up to 250 x 70 µт. Anterior end with less granular cytoplasm; with feebly marked apical papilla encircled by the smooth area. Posterior end pointed. The average height of the epicytic folds is 280 nm, thickness — 160 nm, average distance between folds is 130 nm. Endoplasm contains a number of accumulations of the fibrillar matter. Nucleus spherical 14—26 µт in diameter, localized in the middle or in the fore third of the body, carries 1—2 karyosomes of various size. Other stages unknown. Type series. Microscopical slide with 7 trophozoites (Bouin, Delafield's haematoxylin) is kept in the Zoological museum of the Moscow State University (collection number: Z-2). In addition, two syntypes are observed with ТЕМ and two other syntypes — with SEM. Type host. Cerebratulus barentsi Burger, 1895 (Nemertini: Cerebratulidae). Localization. Intestine (?). Type locality. White Sea Biological Station of the Moscow State University, Yeremeyevsky Rapid, Velikaya Salma Strait, Kandalaksha Bay, White Sea.

Microphallus pseudopygmaeus belongs to the "pygmaeus" microphallids, a group of closely related species with homotypic two-host life cycle. This cycle involves one intermediate host (littoral or sublittoral gastropods, mostly of the genus Littorina), and bird-invading metacercariae develop within daughter sporocysts. In spite of the fact that the name Microphallus pseudopygmaeus is widespread in special literature, the original description of this species (Galaktionov, 1980) was performed in contravention of the International Code of Zoological Nomenclature. This article is aimed to correct the above defect. The maritae of Microphallus pseudopygmaeus were obtained from the common eider ducklings which were infected experimentally with metacercariae of Microphallus sp. I Galaktionov, 1980. These metacercariae were extracted from the naturally infected mollusks Littorina saxatilis collected at the Barents Sea coast. The valid description of the species M. pseudopygmaeus, its comparison with other representatives of the "pygmaeus" microphallids and synonymy are provided. Detailed analysis of digeneans from marine and coastal birds preserved in the collections of the Zoological Institute RAS (St. Petersburg, Russia) and the Natural History Museum (London, UK) showed that in some cases M. pseudopygmaeus was misidentified as M. pygmaeus. It has been shown that M. pseudopygmaeus has an amphiboreal-arctic distribution and the spectrum of its first intermediate hosts is unequally wide for digeneans. It includes 15 species of north-Atlantic and north-Pacific mollusks belonging to different families and even orders of Prosobranchia. At the same time, the range of final hosts of M. pseudopygmaeus is limited by benthophagous marine ducks, first of all the common eider.

A new nematode species of the genus Severianoia (Schwenk, 1926) Travassos, 1929, S. blapticola sp. n., is described from the hind gut of cockroach Blaptica dubia (Serville, 1839) from laboratory culture. The main morphological feature of S. blapticola sp. n. is the presence of columnar cuticular structures of different height on the ventral side of male posterior end. In the gut of B. dubia these nematodes coexist with another thelastomatid genus, Cranifera Kloss, 1960 [C. cranifera (Chitwood, 1932) Kloss, 1960]. Males of C. cranifera are characterized by the presence of cloacal projection and columnar structures on both dorsal and ventral sides of male posterior end.

Morphological variability in the cestode Proteocephalus tetrastomus parasitizing smelt in Lake Ladoga (Baltic Sea basin) and Lake Goluboye (Pechora River basin) was studied. Two forms of P. tetrastomus differing in the proglottid structure, and two variations of the ovary were discriminated. Slightly craspedote individuals with a dumbbell-like ovary were included in the first group. The second group includes distinctly craspedote individuals. It is heterogeneous by the ovary shape and comprises cestodes with two variants of the latter, dumbbell-like and triangular. Both forms are present in the Lake Ladoga. The slightly craspedote form dominates, whereas the distinctly craspedote form is rare. In Lake Goluboye, P. tetrastomus is represented by the only form, distinctly craspedote with a triangular ovary. Differences in the morphometric parameters of the forms, as well as geographic and habitat distinctions, were determined. As compared to other studied cestode species of the genus Proteocephalus (P. torulosus, P. longicollis, P. percae), the smelt parasite P. tetrastomus has fewer polymorphic traits and their variations, and a much more restricted range of variability in the measurable, counting and relative characters. Analysis of literature (Willemse, 1969; Scholz and Hanzelova, 1998; Scholz et al., 2004) and original data suggest that the species is host-specific and relatively stable. It can be concluded, that P. tetrastomus is a polymorphic, complex-structured species.

Data on species composition, hosts, localities, and occurrence of myxosporidean in four host species of the scorpionfish genus Sebastes from different areas of North Atlantic and Barents Sea are presented. Thirteen myxosporidean species are recorded.

For the first time substantiated was the role of the phenomenon of Yersinia pestis biofilm — extracellular matrix envelope (EME) — formation as the basis that determines the nature of plague agent interaction with flea organism. Implication of vector's proventriculus in the process of biofilm formation was demonstrated. Ultrastucture of plague microbe conglomerates in flea proventriculus and midgut was analysed and uniform mechanism of their formation was elucidated. The role of Yersinia pestis biofilm in plague agent preservation in the gut of ectoparasites and soil of rodents burrows was discussed. PCR analysis confirmed the presence of plague microbe in plague infected corpses and fleas excrements stored at +8-10°C for 7 years 9 months.

Data of long-term investigations (1998—2004) on the abundance of the taiga tick larvae and nymphs in the cut-over lands of different age in the middle taiga subzone of Karelia (62°04'S; 33°55'W) are presented. The investigation was carried out on three model cut-over lands of different age: I) "young" cut-over land; age of cut-over in the beginning of investigation is 7 years; Betula—Deschampsia cespitosa—Agrostis tenuis; 2) "middle" cut-over land; age of cut-over is 12 years; Salix—Deschampsia cespitosa—Agrostis tenuis; 3) "old" cut-over land; age of cut-over 25 years; Alnus incana—Rubus idaeus—grass. The number of ticks was estimated by using common parasitological indices: prevalence, abundance, and index of feeding intensity (the tick abundance multiply by the number of small mammals per hundred traps-nights). In the beginning of investigation the "young" cut-over land was a typical meadow association. The lowest tick abundance was recorded here. That was a result of unfavorable abiotic conditions and low number of small mammals in the beginning of summer. "Middle" cut-over land is characterized by the highest number of the tick larvae, which is the evidence for high number of the hosts of tick imago. "Old" cut-over land has the optimum conditions for development of taiga ticks. High abundance of the ticks (larvae and nymphs) was recorded during the whole period of investigations. The number of preimaginal ticks is shown to be much higher in cut-over lands as compared with that in mixed and coniferous forests, due to the higher number of small mammals.

Summary is absent.

Summary is absent.