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Two moult processes of the tegument at earlier development stages of procercoids in pseudo-phyllid and caryphyllid cestodes have been studied. It is recovered, that coracidium of Triaenophorus nodulosus during the penetration into the first intermedial host looses both ciliated layer and external cytoplasmic layer, which covers the six hooked germ. Mace-like processes of tegumental cells stretch along the basal plate and form a primary tegument. At earlier stages of parasitism of pseudophyllid and caryphyllid cestodes the primary cover layes are specialized in secretion tissues to answer an immune attack of the host. It is shown for the first time, that an external syncitial layer and its primary cover cells degenerate at certain period of development, and non-differentiated cells form the secondary covers.

Based on a morphological re-investigation of Baylisiella tecta (Linstow, 1892) this species is removed from the family Diphyllobothriidae Luhe, 1910. A new family Baylisiellidae Jurachno et Maltsev fam. n. and a new superfamily Baylisiellioidea Jurachno et Maltsev superfam. n. are based on this species.

In the northern part of Franz Joseph Land eider ducks Somateria mollissima (2 juveniles and adult) were investigated. Both were found to be infected with hymenolepidid cestode Microsomacanthus ductilis (Linton, 1927). The rate of infection was very high and the worms attained gravidity in their unusual host. This tapeworm is known as a specific parasite of gulls and has never been registered in eider ducks. It is supposed that Microsomacanthus spp. being common for the eider duck in all other parts of the host's area do not attain such high altitudes and in this case M. ductilis realises the opportunity to include in its life cycle the representative of extraneous order — Anseriformes.

The formation of oncospheral hooks in the embryos of Passerilepis crenata preceeds embryop-hore formation and full separation of embryonic epithelium. The cytoplasm of the early oncoblast contains numerous free ribosomes, large mitochondria, Golgi complex and abundant microtubules. The hook anlage consists of three layers: the outer granular zone, a middle fibrous layer of more high electron density and crystalloid central core. Lately the outer granular zone of the blade becomes electron dense too. Numerous microtubules are situated along longitudinal axis of the hook anlage. When the hook anlage becomes elongate the microtubules seem to be joined to the surface of the handle. The extensive development of microtubules associated with hook anlage in P. crenata seems to be similar to the electron-dense "streaks" in the zone of active keratinization of hooks in Inermicapsifer madagaskariensis (Swiderski, 1976). Numerous large mitochondria were observed in oncoblasts during all stages of hook development but their direct role in hook formation (Nieland, 1968; Swiderski, 1976) was not ascertained. At the process of the hook morphogenesis a part of the inner envelope lately undergoing the separation to form the embryonic epithelium gives a deep invaginations into the embryo surrounding the oncoblasts. The formed blade does not penetrate the cellular membrane of the oncoblast. This membrane is adjacent to the blade encircling it. The routine point of view exists about the penetration of the oncoblast membrane by the blade after its full formation (Collin, 1968; Swiderski, 1973, 1976, 1983; Chew, 1983; Gabrion, 1981; Fairweather, Threadgold, 1981). In P. crenata the formed blade does not project the cellular membrane of the oncoblast but is surrounded by it during the hook development. The penetration of cell membrane by any intracellular structure must cause the destruction of the cell in consequence of breaking of the ion balance and membrane potential. Invagination of the inner envelope forms a thick cytoplasmic sheath around the blade. Oncoblast elongates and forms guard and handle of the hook. At that time the cellular structure, usually named collar, surrounding the hook above the base of the guard appears. In P. crenata this structure is formed by the process of adjacent myocyton. This collar is joined to the embryonic epithelium by the circular septate desmosome. Desmosome-like structure exists between collar and oncoblast formed by the concentration of electron-dense material. At the last stage of hook morphogenesis the oncoblast does not degenerate. It is thin cytoplasmic layer with elongated nucleus surrounding the handle and encircling the blade and guard as adjasent membrane. The penetration gland consists of two large lobes situated laterally within the oncosphere and joined together by a narrow isthmus. The ducts of penetration gland begin from the isthmus. Oncospheres have one ore two short ducts, ore up to eight long ducts. The short ducts open at the anterior half of the oncosphere while the long ducts usually are withdrawn to the middle line of the oncosphere. The bilobed form of the penetration gland of the oncospheres of P. crenata resembles that of some other cyclophyllidean cestodes (Lethbridge, Gijsberg, 1974; Collin, 1969; Pence, 1970). Howewer, it differs in that the ducts from the gland do not open in the hooks region of the oncosphere but open in the anterior end or in the middle area of the oncosphere. Previouly the anterior position of the penetration gland ducts were found in H. nana only (Fairweather, Threadgold, 1981). Moreovere the absence of the connection of penetration gland ducts with the embryonic epithelium and their anchoring to the protrusions of the inner envelope is unique feature among all cestode species studied thus far. The cytoplasm of penetration gland is filled by the abundant disciod membrane-limited secretory granules of one type only with the same density as in the preoncosphere as in the oncosphere.

The concentration of the haemocyanin (respiratory pigment) in haemolymph, hepatopancreace and visceral mass (internal tissues, generative and alimentary system) in the pond snail Limnaea stagnalis infected with trematode parthenites Echinoparyphium aconiatum was examined. A normal concentration of heamocyanin in the snails is about 163—302 mg %. In a case of infection it rises up 1.2 times. In a case of a low rate infection it is the result of protective and restoring processes in the host, in a case of high rate infection it is the result of pathological processus.

The water factor in transfer of the activators toxocarosis was not taken into account by parasitologests earlier. Yet in the urbanizeid ecosystems (first of all — in the megalopolis) it can have rather appreciable importance in distribution of toxocarosis in animals and man. Our researches which have been carried out in reservoirs of Moscow with the "wild" and the "organized" beaches, have revealed various, but as a whole their significant, of contaminations by invasion eggs Toxocara (Toxocara canis, T. mistax, and Toxascaris leonina). It specifies an opportunity of their hit in organism of the man and, as a consequence — toxocarosis. The examination of the patients toxocarosis of the people (first of all of children at the age of 3—10 years) have confirmed the fact of their bathing and involuntary swallow of water in urban unflowing reservoirs. In ecosystems of the megalopolis at a high level of parasite of pollution increased superinvasion of dogs and cats and free access of animals to internal reservoirs — the risk of infection of the people toxocarosis gradually grows.

The parasite fauna of grayling (Thymalius thymalius) has been studied. Its genesis is discussed in connection with the occurrence of the host in individual river systems.

First data on a parasite fauna of Limnocottus griseus are presented. 7 parasite species were observed in our materials: Trichodina baicalensis, Myxidium perniciosum, Myxobolus talievi, Dactylogyrus colonus, Crepidostomum farionis, Metechinorhynchus salmonis, Codonobdella truncata. Data about another hosts are presented. The questions about the infection ways of L. griseus are discussed.

The first description of Isospora oocysts from Certhia familiaris. Isospora certhiae sp.n. has round oocysts 21.6—29.7 µm in diameter (27.7 µm in average). The cyst wall lacks a micropile. A triangular polar granule appears, but no oocyst residium. The sporocysts 12.2—14.9 x 14.9— 17.6 µm size (average 13.5 x 16.2 µm), bear a knob-like Stieda body and contain pyramid-like sporozoites 8 µm length and 3 µm width. The species was found in Tree creepers on the Courish spit of the Baltic sea (55° 12' N, 20° 46' E) during the autumn 1996.

The description of the first trypanosomatid from insects of the family Dixidae (Diptera) in North-West part of Russia is given.

Data on a parasite fauna of Thymallus arcticus grubei from different rivers of the Primor'ye region are given.

Parasitological researches of 5 fishes species from Courish Lagoon of the Baltic Sea revealed 7 species of Myxosporidia, such as Myxidium pfeifferi, Zschokkella nova, Myxobolus bramae, M. dispar, M. muelleri, M. muelleriformis, M. pseudodispar. Among them M. muelleriformis was found for the first time in the mentioned area. Two species were found in new hosts: Zsch. nova in bream (Abramis brama L.) and M. dispar in roach (Rutilus rutilus L.).

The nonspecific species of the saprotrophic fungi of the genus Acremonium — A. kiliense W. Gams has been found for the first time in Dreissena polymorpha (Pallas). It appears, that A. kiliense is a potential inducer of mycosis. At the same time it is an indicator of the waterbodies pollution.

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