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The study was carried out on Himasthla elongata cercariae shed by infected Littorina littorea snails. The infected periwinkles were collected from the settlement with the low prevalence of H. elongata. As shown earlier with the use of AFLP (Amplified Fragment Length Polymorphisms) method, rediae groups in all the infected periwinkles of this settlement arise from the infection of a mollusc with a single miracidium. Therefore, the cercariae shed by an infected mollusc have the same genotype or, in other words, represent a clone. The LT₅₀ (the time during which 50 % of cercariae perish in the experimental dish) were measured experimentally for cercariae Himasthla elongata belong to different clones. The investigated parameter demonstrated a high level of interclonal variability. Two groups of cercarial clones were identified; one of them was characterized by the high level of intraclonal variability in LT₅₀ and the second, by the low one. It is assumed that the observed heterogeneity may be stipulated by different degrees of mitotic recombinations during formation of different cercarial clones.

The ultrastructural features of the vitelline follicles, uterus and vagina of the amphilinid cestode Amphilina foliacea from the body cavity of Acipenser ruthenus of the Volga basin are described. Some new distinguishing characters are revealed, including the presence of a single type of the cellular component in the vitelline follicles with sarcoplasmic processes filling the space around and within vitellocytes. The uterus of this species is recognized by the presence of the syncytial epithelial lining with underlying secretory peri-karia and different kinds of luminal projections. The most distal part of the uterus, situated closely to the uterine pore, is characterized by nuclei occupying the syncytial uterine epithelium. Slightly further proximally, they are located basally and then transformed into sunken perikaria. The vaginal epithelium is characterized by the syncytial structure with underlying epithelial perikaria in its distal region, with intraepithelial location of the nuclei slightly further proximally and the absence of the nuclei in its proximal region (the seminal receptacle). The syncytial cytoplasm of the vagina contains numerous vesicles and possesses microvilli-like surface structures. The morphological aspects of the female reproductive system of A. foliacea are compared with those of other Cestoda and Neodermata.

The secretory cells were found in the subtegument of the sporocysts Leucochloridium paradoxum by histological assay. Pigment granules are formed by these cells. The movement of granules from secretory cells to the tegument external layer was observed. These pigment granules provide the yellow color of sporocysts broodsacs and the brown color of protuberant spots in the terminal part of broodsacs. It was shown that the pigment granules did not contain proteins, nucleotides, lipids and carbohydrates. The positive result was received while staining on bile pigments. The question on the nature of the green pigment remains open. The paletot on the surface of sporocyst formed by spreading hemocytes was observed. This structure was not described before in brachylaemid parthenites.

Morphological variability of immature Proteocephalus torulosus was studied for the first time. On the basis of scolex shape, three phenotypes were distinguished: clavate, rounded, and lanceolate. The clavate phenotype was found to be dominant, the rounded one is small in number, and the lanceolate one is rare. The wide ranges of morphometric variability were established. The clavate phenotype differs from the rounded one by its higher variability. The high morphologic variability of immature P. torulosus is comparable to that in adult mature parasites from Altai osman, the Central Asian endemic from the genus Oreoleuciscus. The conclusion is made on the influence of extremely changeable environmental condition in the region on the parasite and host adaptations during their co-evolution.

A total of 383 fleas of 11 species were collected from 428 bank voles (Clethrionomys glareolus) near Babaevo Vill. (59°4' N, 35°8' E). Three species (Amphipsylla rossica, Doratopsylla dasycnema, Palaeopsylla soricis) are not typical for these rodent hosts, moving onto the latter occasionally from other animals, such as the common vole Microtus arvalis and shrews inhabiting adjacent or similar biotopes. Peromyscopsylla bidentata, infesting the bank vole in the most part of its range, was recorded only as a single female. Megabothris turbidus was also very rare, being sporadically recorded from May to October. Probably, study area is the northernmost border of its range. The most abundant species, Ctenophthalmus uncinatus, had two picks of average abundance: in April, when imagoes emerge from overwintering cocoons, and in July. Then the number of fleas decreased and the last solitary individuals were recorded till December. Amalaraeus penicilliger is a perennial parasite, being most abundant in winter. Other species were few in number. Imagoes of Megabothris rectangulatus were recorded from April to August, possessing two peaks of abundance: in April, when they emerged from overwintered cocoons, and in July, when the second generation emerged. Peromyscopsylla silvatica is the monovoltinous species emerging in July-August and parasitizing till September. Rhadinopsylla integella was the most abundant in October—December, but a few specimens were recorded in January. The polyxenous species Hystrichopsylla talpae emerged in late July and occurred till September.

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