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The paper deals with the morphology and ultrastructure of the ovaries of many times fed nonovipositing females of trombiculid mites Hirsutiella zachvatkini (Schluger, 1948) and Euschoengastia rotundata (Schluger, 1955). Brief account on the development of the ovaries during the course of ontogenesis is also given. The ovaries are two nonfollicular large sacs, connected with one another by transverse junction, and laying in the middle part of mite body. Compact groups of interphase oocytes, of I order oocytes in prophase of the 1 meiotic division (pachytene and diplotene stages) forming quite characteristic clusters, and also somatic cells with long processes separating oocytes groups are represented and scattered freely in ovaries of nonovipositing females. Oocytes in each cluster, which may yet be found in deuto- and thritonymphs, are mitotic progenies of the one gonidial cell, which are divided in early ontogenesis. At the onset of the growth of clusters their dimensions axe increased and in the centres of their rosettes the specific fusomal structure is forming, which functions as a so called trophic core. The fusome is built up of tiny microtubules and also of friable material with moderate electron density, which is possibly originated from small Golgi complexes located in the basal regions of the oocytes near large nucleus with decondensed chromosomes and netted nucleolus. At the final stages of clusters differentiation their oocytes are divided completely and fusomal material is demolished and gradually disappeared that are determined the beginning of the small growth of oocytes. Therefore, in the females of trombiculid mites, contrary to other so far arachnid studied, the typical clustering of germ cells may be seen, that is so characterized to insects with polytrophic ovarioles. The following stages of the oogenesis in trombiculid mites may be obseved only in the ovaries of ovipositing females, where somatic and also trophic (nurse) cells are occurred. The comparative morphology of the ovaries in various groups of Acari and also the functional significance of specific cluster organization are discussed.

It is shown that morphological characters, such as a form of gnathosoma, usually considered as qualitative one, may be properly analyzed on a quantitative basis by means of the geometrical morphometric methods. In particular, an extraction of relative warps (RW) from a shape variation using TPSRW computer program makes it possible to run standard statistical procedures over matrices of RW values. This approach is demonstrated by the analysis of gnathosoma shape differences in 4 Ixodes species and 3 Dermacentor species, with a discriminant analysis being employed as a statistical routine. It is shown that differences among I. persulcatus and I. ricinus are statistically significant (by F-criterium) in all ontogenetic phases, while in the case of I. laguri and I. redikorzevi this is true only for the larval phase. Magnitudes of the gnathosoma shape variation among species (by F criterium) are similar within both Ixodes (s. str.) and Serdjukovia subgenera. The differences among subgenera of the genus Dermacentor are more significant.

The results of feather mite study of terns (Laridae: Sterninae) distributed in north-western coastal region of the Black Sea are briefly displayed. One new species, Alloptes chlidonicus sp. n. from Chlidonias hybrida is described. The examination of 8 tern species commonly occured in the region in question discovered 21 feather mite species of three families. The table of host-parasite associations is provided.

As a result of the revision of the taxon the generic name Astega Enderlein, 1930 is adopted as junior synonym of Cnephia Enderlein, 1921; the neotype of species name C. angarensis is designated; for C. intermedia Rubzov, 1956 (praeocc.) the new replacing name C. orenburgica Yankovsky, 1996 is used; the material of Palaearctic species of Cnephia in the collection of Zoological Institute, Russian Academy of Sciences, is checked (94 preparations, including available type-series and Enderlein's exemplars, and 108 pined flies); a new species Cnephia toptchievi Yankovsky sp. n. is described; as practical addition to this work, key for identification of species of the genus is given.

In our previous paper (Gaikin e. a., 1994) a Cestode collection from fish-eating birds of Barents Sea was presented. In the material came from Franz Joseph Land the youngest developmental stages of Tetrabothrius erostris Loennberg, 1889 in its final host, the kittiwake Rissa tridactyla, were found. The infective larva has a big muscular apical sucker and shows no traces of the adult tetrafossate scolex. In the intestine of a bird stabilization begins and simultaneously the first anlages of bothridia appear. Apical sucker ensures the fixation of a young strobile until the adult scolex is fully developed. Then this sucker atrophies and finally is shedded. The same transfiguration of the scolex region was demonstrated (Hoberg, 1987) in the ontogeny of Tetrabothrius sp., parasite of the petrel Puffinis gravis. Outside Tetrabothriidea just similar picture takes place only in the postlarval development of cyclophyllidean Catenotaenia pusilla. Its larval stage (merocercoid) is unifossate. Adult stage on the contrary has 4 lateral suckers while apical one completely vanishes (Joyeux, Baer, 1945). In Tetraphyllidea and Proteocephalidea organs of tetrafossate scolex (bothridia and suckers respectively) emerge on the larval stage. The fate of the apical sucker is various. Its loss in the end of the adult scolex morphogenesis is experimentally shown (Hamilton, Byram, 1974) for tetraphyllidean Acanthobothrium. In some Proteocephalidea the apical sucker ("fifth sucker" or "apical organ") differentiates first and being well developed in the larva it may be lacking in the adult. The unique group which posesses single apical sucker in the adult phase is Nippotaeniidea. To our opinion the highly discontinious area of the order and its other characteristic biological and morphological peculiarities may be explained admitting a hypothesis of its neotenic origin. Dubinina (1980) traces two lines of evolution in polyzoic cestodes, two-rayed and four-rayed, basing on the symmetry of attachment organs and some other characters. Two-rayed line includes orders with difossate scolex (Pseudophyllidea - Diphyllidea - Trypanorhyncha), the four-rayed line those with tetrafossate scolex as well as unifossate Nippotaeniidea. We suppose that the initial separation of these lines was caused by the inclusion of the second intermediate host in the cestode life cycle. Groups with predominantly intestinal plerocercoids belong to the four-rayed line and the ancestral apical sucker serves here for fixation of larvae. Worms who became tissual or cavitary parasites on the second larval phase had to develop powerful organ of penetration even on plerocercoid phase. Their adult scolex arose anew as difossate. It so, the lines indicated by Dubinina have systematic value and schould be recognised as Difossaria subcl. nov. and Tetrafossaria subcl. nov. The life cycle of Tetrabothriidea remains unknown. But numerous larval forms from the intestine of teleostean fishes known as Scolex bothriosimplex are in fact tetrabothriid plerocercoids rather than the young tetraphyllid larvae designated as Scolex polymorphus. Infected kittiwake mentioned above fed on Arctic cod, Boreogadus saida. This fish is thought to be the second intermediate host of Tetrabothrius erostris.

The analysis of 10 main size characters of the cells was carried out in the lower trypanosomatids that were isolated from the insects and plants in 16 various geografical points. Only two of them - the distance between the anterior end of the body and the kinetoplast and the kinetoplast index may be employed for the discrimination of the species and populations of these flagellates. It allows to apply these characters when it is necessary to compare descriptions and to identify species in the cases when other characters are inaccessible for comparison, p. e. when compare the old descriptions with contemporary ones in order to solve some taxonomic problems. There was pointed the existence of the flagellates populations that differ considerably from the others by their morphological and some other characters.

Ultrastructures of the microsporidium Nosema dikerogammari, the parasite of amphipods Dikerogammaris villosus, Chaetogammarus ischnus, and Pontogammarus crassus in Ukraine, are described. The diplocaryotic meronts, sporonts and spores, the lamellar polaroplast, the isofilar polar filament puck up to 7-8 coils with different angles, are the characteristics of N. dikerogammari.

It has been shown that the three fold immunisation of 30 day old chicken whith 10, 50 and 100 thousand oocysts of E. tenella at intervals 15 day induces increasing amounts of T- and B-limphocytes in thymus, spleen and bursa of Fabricius. The mode of lymphocyte dynamics at immunization is considered in terms of host-parasite relationships, possible ways of anti-eimerian immunity formation being discussed.

The analysis of population systems is carried out on the basis of the classification of spatial and functional,structure of populations developed by V. N. Beklemishev. Two aspects of the structure of population systems are established. Firstly, population systems are composed of the smaller groups characterised by different self-maintenance ability. Secondly, different functional parts are included into these systems in accordance with different stages (phases) of a life cycle. Peculiarities of the population systems are discussed from these points of view. The population system is a functional part of a particular community. Steady interrelationships between population systems in the community ("community links") are the basis on which the complexes of population systems in different species are formed. A prominent example of this is the parasitare systems, that is the population system of a parasite and all connected populations of its hosts. The structure of a parasitare system is examined. In general, it is characterised by a) peculiarities of the life cycle of the parasite, since its population systems are the organising component of the parasitare system; b) subdivision of the environment for parasites. The first trait is discussed from the standpoint of phase structure of populations which is could be clearly seen in parasites, and the second one - from the viewpoint of the availability of distinct microbiotopes connected with different parts of the population system of parasites. It is the subdivision of the parasites' environment and its organisation according to the scale (interspecies, interpopulation or intrapopulation) variability of the hosts, that make it possible to recognise spatial and functional parts in the framework of the parasitare system. The critical review of the terminology used in the population parasitology is presented.

It has been shown, that in the Mezen river bassin the grayling Thymallus thymallus plays main role in supporting the population of Cystidicoloides tenuissima. It carries more individuals of the parasite than youngsters of the Atlantic salmon Salmo salar, ruff Gymnocephalus cernua and burbot Lota lota. It has states that dispersion of frequency of this nematodes in different host species is satisfactory described by negative binomial dispersion, but the value of the coefficient of aggregation "К" of parasites in the grayling is greater, than it is in other species of fishes.

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