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In the parasitology it is accepted to subdivide the parasites into two categories based on their spatial relations with the organism of a host. To first category — ectoparasites — includes organisms, which live "on external covers, on a skin, on gills". To the second category — endoparasites — includes organisms living "in internal cavities, tissues and cells of the host" (Dogiel, 1941). Feizullaev (1971) offered the third category — mesoparasites — where he placed "the parasites inhabiting places, which are connected to cavities being open to external environment'. According to him the places are oral, nasal, eye cavity, cloaca etc. Since some parasitic copepods living in certain kind of interrelation with a host can be referred neither to ectoparasites nor to endoparasites, the author offers these following criteria for the mesoparasitism definition. 1. Morphological and anatomic subdivision of the body of the parasite into two main parts: ectosoma located in the external environment (environment of the second order) and endosoma located inside the organism of the host (environment of the first order). 2. Functional subdivision of parasite body into two parts: endosoma which performs the trophic function and ectosoma with genital organs performsthe reproduction function. 3. Existence of the parasite on the border of two environments (of the first and second order) which have an influence on parasite directly and simultaneously. The mesoparasites are known among the parasitic Copepoda and Rhizocephala only. As for the copepods all members of the family Herpyllobiidae may be placed in this category; the members of the families Phyllodicolidae and Chitonophilidae may be also included in it, albeit tentatively (because their anatomy has not yet been adequately studied), as well as five genera of the family Nicothoidae (Aspidoecia, Cephalorhiza, Diexanthema, Nicorhiza and Rhizorhina), and Bradophyla pigmaea.

Data on a hook morphology (in plerocercoid and mature helminths) and gene systematics of Triaenophorus nodulosus from the Baikal Lake are presented for the first time. Sequence analysis of rDNA fragments (5'-end portion of 18S rDNA) yelded 562 nucleotid positions.

The results of ultrastructure study of the giant muscular cells located at the base of proboscis receptacle are given. Peculiarities of cell ultrastructure suggest the presence of the "nuclear secretion" in these cells. By their location and morphological features they are identical to the retinacular cells formerly described by some authors.

The effect of three ferric chloride concentrations (100, 200, 300 mg/l) on the acid-alkaline balance of haemolymph and haemoglobin content in Planorbarius corneus under normal conditions and in the case of infection with parthenites and larvae of Echinoparichium aconiatum was examined. In the medium with an effective concentration of ferric chloride, all snail specimens proved to have acidified haemolymph, by 40—50% in infected samples and 80—85% in uninfected ones. Statistically reliable differences in the haemoglobin content in the haemolymph of infected and uninfected specimens between the control and test media were absent, that proves a weak toxic effect of ferric ions on P. corneus.

Four species of the genus Echinatrium Spassky et Jurpalova, 1965 are redescribed. The material was collected from ducks of the Chaunskaya lowland (Chukotka). Morphological study of genital apparatus in cestode species found was carried out. According to the priority rule, it is suggested that the genus Monosaccanthes Czaplinski, 1967 is a junior synonym of the genus Echinatrium.

In the Archangelskaya Bay (North Island of Novaya Zemlya), 25 specimens of sea birds have been collected: 5 kittiwakes, 5 murres, 5 little auks, 4 common eiders, 4 purple sandpipers, and 2 glaucous gulls. Following numbers of helminth parasite species have been recovered in this material: trematodes — 1, cestodes — 5, proboscic worms — 2, and nematodes —- 2 species. The description of morphological characters of Anomotaenia micracantha micracantha (Cestoda: Dilepididae) from the kittiwake is given. The dependence of helminth fauna composition on the character of bird's feeding is traced. Differences in the infection with parasites of gulls and auks habitating the Archangelskaya Bay are revealed. Data on a life cycle of parasites found and ways of their circulation in the ecosystems of the Barents Sea are given. As for Triaenophorus erostris, which was indicated for gulls of the Barents Sea by Belopolskaya (1952), Galkin e. a. (1994) and Galaktionov e. a. (1997) as a common parasite, it was absent both in the material collected by Markov (1941) and in our material.

Thirty nine specimens of passerine birds belonging to 19 species and eight families were investigated by blood smear technique in four localities of Southern Turkmenistan in 3—18 August 1991. The overall prevalence of infection was 59%. Protists from the orders Haemosporida (genera Haemoproteus, Plasmodium, Leucocytozoon), Kinetoplastida (Trypanosoma), and Adeleida (Hepatozoon), as well as Microfilaria were found. Haemoproteids (the prevalence of infection is 44%), leucocytozoids (23%), malarial parasites (13%) and trypanosomes (13%) were most frequently recorded. Only low chronic infections (< 1% of infected cells for the great majority of intracellular parasites, and a few trypanosomes and Microfilaria in each blood smear) were seen. Haemoproteus belopolskyi, H. balmorali, H. dolniki, H. magnus, H. minutus, H. fringillae, H. majoris, Leucocytozoon dubreuili, and Trypanosoma avium were recorded for the first time in Turkmenistan. The former five above-mentioned species of haemoproteids are new records for the fauna of Middle Asia. Gametocytes of leucocytozoids in fusiform host cells were found for the first time in passerine birds in the Holarctic. The host is Parus bokharensis. Due to the wide distribution and the opportunity to collect a large parasitological material using harmless for hosts methods, bird haemosporidian parasites can be used as convenient models for ecological and evolutionary biology studies in South Turkmenistan. The heavily infected Orphean Warbler Sylvia hortensis is an especially convenient host for such purposes.

Ichthyophthirius schlotfeldti Yunchis, 1997 (Ophryoglenidae) was described by the junior author from aquarium fishes from South-East Asia. In the present paper this species is separated into a new genus Neoichthyophthirius gen. n. The reproduction of this species takes place within the epithelium of fishes and not in water as in Ichthyophthirius multifilius. Neoichthyophthirius schlotfeldti has a ring-shaped macronucleus with ends overlapping. Maximum temperature, when the reproduction is possible, is about +34°. The parasite is pathogenous and causes a mortality of aquarium fishes.

Results of long-time (1980—1998) investigations of wild animals, which are potential hosts of the pathogen of alveolar echinococcosis in Belorussian Polesie, have been already generalized. One species of carnivores, two species of insectivores and eight species of rodents take part in circulation of infection.

A new species Syringophiloidus delichonum sp. n. (Acari: Syringophilidae) is described from quills of Delichon urbica (Hirundinidae) and S. glandarii (Fritsch, 1958) from Garrulus glandarius (Corvidae) is redescribed. Syringophiloidus glandarii, female: body length 650—680, width 160—180. Peritremes: longitudinal branch with 11—14 chambers, lateral — with 2 chambers. All propodosomal setae sligthly serrate. Length of setae: vi 33—36, ve 87—96, sci 150—180, sce 180—240, h 200—230, d1 210—230, d2 140—170, d4 25—32, d5 33—40, l1 190—210, l2 150—170, l4 250—280, l5 310—320, pg1 140—150, pg2 120—160, pg3 190—200, g1, g2 40—45, a1, a2 25—33. Male. Body length 500—520, width 150—170. Peritremes: longitudinal branch with 13—14 chambers, lateral with 4 chambers. Length of setae: vi 32—36, ve 50—56, sci 74—80, sce 120—135, h 90—100, d1 135—140, d2 13—22, d5 20—22, l1 24—45, l2 13—22, l5 180—200, pg1 90—110, pg2 70—90. S. glandarii is closely related to S. presentalis Chirov et Kravsova, 1995. It is distinguished from the latter species by following characters. In female S. glandarii, setae vi, ve are thick and serrate, ve 87—96 in length. In female S. presentalis, setae vi, ve are thin and nude, ve 40—70 in length. Syringophiloidus delichonum sp. n., female (holotype): body length 900, width 135. Peritremes: longitudinal branch with 8 chambers, lateral — with 1—2 chambers. All setae nude. Length of setae: vi 45, ve 90, sci 200, sce 200, h 225, d2 90, d4 35, d5 33, l1 140, l2 ?, 155—160 in paratypes, l4 230, l5 320, pg1 135, pg2 120, pg3 170, g1, g2 40, a1, a2 17. Male. Body length 506, width 135. Peritremes: longitudinal branch with 9 chambers, lateral with 2 chambers. Length of setae: vi 33, ve 78, sci 90, sce 123, h 130, d1 135, d2 15, d5 17, l1 22, l2 15, l5 180, pg1 105, pg2 45. S. delichonum sp. n. is closely related to S. presentalis, but differs by following characters. In female S. delichonum sp. n., body length is 850—920; longitudinal branch of peritreme includes 6—8 chambers; setae ve 85—90 in length; in male, longitudinal branch of peritreme include 9 chambers. In female S. glandarii, body length 630—700; longitudinal branch of peritreme include 10—11 chambers; setae ve 40—70 in length; in male, longitudinal branch of peritreme include 11—13 chambers.

The Lernaea cyprinacea population distributed in the water basin of the medial part of the Vychegda river has one generation a year. It is probably the result of lower temperature of water during the year, than in south parts of L. cyprinacea range.

A new cestode species, Proteocephalus pronini sp. n., is described from the Lake Hovsgol Grayling Thymallus arcticus nigrescens (Mongolia, Hovsgol Lake). The new species differs from P. thymalli from the Baikal Lake by the length of strobile, number of proglottids, form of mature proglottids, and number of diverticles of uterus.

In the course of helminthological studies of sheep in south Uzbekistan, 44 helminth species have been revealed. Biocenotical relations between helminth and sheep in desert ecosystems of the south Uzbekistan have been examined.

An ultrastructure of a microsporidian belonging to the aberrant and poorly studied genus Issia is discribed in this paper. The first description of Issia globulifera (Issi, Pankova, 1983) was made on the basis of light microscopic analysis of midgut smears of the infected insects. Both late merogonic and sporogonic stages can be met on the sections from midguts of Anopheles messae. Meronts are ameba-like cells (3.0—3.5 x 2.0—2.5 µm) forming conglomerates of cells without any regular organization. Meronts and sporonts directly contact with the host cell cytoplasm; they are surrounded by numerous mitochondria. Sporonts divide into 2 sporoblasts, but remain connected with each other by posterial ends. In the vicinity of the contact the paramural bodies (structures, presumably participating in the formation of the exospore) can be seen. Sporoblast morphogenesis is accompanied by the rapid grow of electron dense layer in the region of the contact of two sister sporoblasts. Due to such intensive growth the sporoblasts are finally located at sharp angle to each other. Envelopes of sporonts and sporoblasts possess typical 2-membrane structure. Their nuclei lay in pairs like in Nosema species. Sporoblasts (unlike meronts and sporonts) are surrounded by the electron lucid zone of the cytoplasm with numerous tubule-like and spherical inclusions. Host mitochondria are closely adjacent to the periphery of this zone. The formation of characteristic spherical structures (globules), in which the posterior ends of two (in rare cases of 1 or 4) sister spores are downsinked, starts immediately after the sporont division. Globules rapidly increase in size during the sporogenesis. The homogenous globule (6—8 µm), which is essentially larger than a spore, seems to represent the modified exospore; it is not limited by any visible membrane. Fine structure of spores (2.5—3.0 x 2.0—2.3 µm) is typical for the nosems: spherical polar disk; lamellas polaroplast; nuclei located along the long spore axis; basal part of a polar tube forming a fork. A thin (25—40 nm) heterophilar polar filament forms 12 rows of various structure (8 + 4) packed in two rows. The posterior vacuole was no revealed. Endospore is very thick up to 250—400 nm in mature spores. The exospore covering the 2/3—4/5 of the spore length passes into the globule on the posterior end. The refined diagnosis of Issia globulifera: type host species — Anopheles messae (larvae, pupae); tissue localization: mitgut epithelium; terra typica: Siberia, Tomsk region, Ob river basin; developmental stages: multicellular merogonial Plasmodium, sporont gives rise to 2 (rarely 1 or 4) sporoblasts connected with each other by a "globule", a specific formation homologous to an exospore of other microsporidians; globule (6—8 µm) possesses a homogenous inner structure of average electron density; sporophorous vesicle not formed; spore structure is typical for Nosematidae. Embeddings, negatives (##2301, 2303, 2308, 2311—2313, 2532, 2533) and photos of type material in the collection of All-Russian Institute for Plant Protection.

Summary is absent.