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On the basis of published and original data some statements on peculiarities of host-parasite interactions of aquatic invertebrates are formulated. Chief inference is that these interactions are expressed mostly on populational level. Some mechanisms are shown how the parasites can influence the number of invertebrates population.

A new flagellate species, Leptomonas mycophilus, has been described from the intestine of Phytocoris sp. larvae. Strain SV—69 of L. mycophilus was isolated and put in the liquid culture medium. The medium chosen for the parasite cultivation appeared to be contaminated with a mould of unknown species. However, in the mould-free culture the flagellates failed to grow to perish eventually. In the presence of the mould decoction and at regular subculturing every 10 days a population of nearly monomorphic flagellates was seen to emerge, composed of choanomastigote-like forms. This is the more surprizing that the parasites isolated from the bug intestine were promastigotes. However, by the 30—40-day cultivation, without subculturing, a great amount of flagellates similar to the bug forms appeared in the original strain SV-69. This transformed strain was designated as strain SV-69cf. The flagellates of this strain formed rosette-like colonies with cyst-like endomastigotes inside, which might have appeared as a result of successive divisions. On the dense nutrient medium containing mould decoction L. mycophilus formed hemispherical colonies. L. micophilus from the bug and both its strains (SV-69 and SV-69cf) were studied comparatively with the light microscope, flagellates of strain SV-69 being investigated, in addition, with the electron microscope.

Six new species of Myxosporidia of the genera Myxidium, Ceratomyxa, Myxobilatus and Kudoa, parasitic in the fishes from the Black Sea, are described.

Morphology, development and behaviour of Leishmania gymnodactyli in the sand fly Sergentomyia arpaklensis at different stages of blood digestion have been studied. It has been shown that leishmaniae reproduce readily and develop normally inside the food ball of sandflies. The dense peritrophic membrane is not destroyed at the end of digestion and is an insuperable obstacle for leishmaniae. Promastigotes of leishmaniae, being involved in the peritrophic membrane, are excreted together with indigested food that excludes their transmission through the bite of S. arpaklensis.

The morphogenesis of M. pirum larvae of hermaphroditic generation, parasitic in the White Sea gastropods, Hydrobia ulvae has been described. The cercariae of this species have a rudimentary tail and are unable of active movement. The transition to metacercarial stage is marked by casting away the tail rudiment. At first young metacercariae swim easily within daughter sporocysts and then encyst. Bilayer cyst is formed both by the secret of cystogenic glands and probably of cells of excretory bladder walls. Two types of marita glands are installed in embryos of metacercariae after degeneration of cystogenic glands. The formation of excretory system is a result of the consecutive division of flame cells and of excretory canals. The excretory bladder lumen is formed by the junction of distal parts of main excretory canals. Its walls consist of large cells, which may be either the cytons of canal cells or be independent of the excretory system cells. The caecum primordium appears as cellular cords. Lumens within them are formed at later morphogenesis stages. The genital primordium is revealed as a friable accumulation of small cells in the hind third of cercaria embryo. Its fore part rounds as a semiring the ventral sucker primordium from the left side. Later the departments of genital primordium, situated before the ventral sucker, on the left of it and behind it are differentiated into a complex of organs of cirrus sac, genital atrium and ducts of female reproductive system, respectively. Testes and ovaries are installed independently of the genital primordium. The presence of cirrus sac in Microphallus metacercariae and maritae has been proved. It represents a thin-walled sac of flattened cells around the seminal vesicle and ejaculatory duct.

he research has demonstrated that 35.8% of Hypomesus pretiosus Girard and 55.9% of Osmerus eperlanus dentex Steindachner in various regions of the Far East of the USSR (Khabarovsk and Primorye territories and Sakhalin) are affected by diphyllobothriid larvae of a hitherto unknown type, indicated as the G type. Parasites are localized on the internal organs of a intermediate host such as esophagus, stomach, liver and genital organs. Diphyllobothriid larvae of the G type do not invade man, thus the indicated fishes are not epidemiologically significant in spreading diphyllobothriasis among the population in the Far East of the USSR.

The life cycle of the trematode Conspicuum popovi (Kassimov, 1952) has been studied. Land molluscs are the first intermediate hosts while in the exrepiment Orthoptera were used as second ones. Maritae were obtained from the chicken Gallus domesticus juv. The comparison of developmental stages of S. popovi to those of the genus Conspicuum has proved their identity. The genus Skrjabinus is regarded by the author as a synonym of the genus Conspicuum and therefore the species S. popovi is assigned to the latter.

Allometry of the exosceleton organs was studied in three species of the genus Dermacentor and Ixodes persulcatus. All the species are characterized by positive allometry between larval and nymphal phases and by negative or weakly positive allometry between nymphal and imaginal phases (excluding one character, width of scutum). It has been shown that the character of growth of sizes of gnathosoma and its appendages as well as the 1st tarsus length with regard to the body length of Dermacentor males and females coincides. As to the scutum length sizes of females' organs increase more sharply than those of males. In I. persulcatus the character of allometry differs. Between the larval and nymphal phases sizes of organs in females increase more markedly with respect to the scutum length than in males. Between the nymphal and imaginal phases allometry in females with respect to the scutum length is positive while in males it is negative. With respect to the body length the difference in the character of allometry of gnathosoma and its appendages remains the same whereas considering other characters it is more negative in females than in males.

The decisive effect of physiological age of ticks on the infection of their saliva was revealed by means of parenteral infection of D. marginatus females with tick-borne encephalitis virus. The virus was not found in the saliva of young individuals of instar II. Maximum number of cases of saliva infection was recorded in mature ticks of instar III (50%). It was established that the infection of saliva did not depend on the amount of virus in the tick's body. Less susceptibility to the virus of young individuals, associated probably with the way of infection, was noted. Under light microscopy there were observed no significant age differences in salivary glands of ticks of instars II and III. The infeciton of saliva of young individuals is, apparently, opposed by the barrier of fat body on the way of virus to salivary glands, depending on the age of ticks. The barrier is supposed to have an influence under natural transphase infection of ticks.

The isolation of viruses of tick-borne encephalitis, West Nile fever, and Omsk hemorrhagic fever from arthropods of nests of colonial birds (rook, sand martin, tree sparrow, Laridae) in different zones of West Siberia (from tundra to steppe) has been analyzed. The role of gamasid mites, hematophags and saprophags, characteristic inhabitants of nests of colonial birds, and of the tick Ixodes lividus in connection with their biology, coadaptation with hosts, microclimatic nest conditions, etc. has been evaluated. It has been concluded that the whole nest community of arthropods in a nest rather than one infected individual has to be regarded as an infective unit in colonies.

Examination of 144 specimens of the dace, caught in the Vychegda and Rbzhjan rivers, has shown that worms are unevenly distributed on the gills, the highest number of parasites being recorded on the 2nd gill. The number of parasites on the gills is correlated with their area. They prefer the middle sector and middle third of the gill leaflet where they usually form aggregations. Under the conditions of hypoxia the number of Dactylogyridae on the gills of fishes decreases and their microlocalization remains unchanged.

Far-eastern shrimps of the families Pandalidae and Hippolytidae were found to be infected by two species of Bopyroides. One of them was identified as B. hippolytes (Kroyer), the second species, found on Heptacarpus spp. appeared to be a new one. The latter was named B. shiinoi after Dr. S. Shiino, who found this species in Japan in 1937 and described it as B. hippolytes. The new species differs from B. hippolytes by the total fusion of the fifth abdominal segment with the pleotelson without any trace between them, by the presence of two pairs of lamellate uniarmous pleopodae (others reduced and being represented by protuberances on ventral surface of abdomen) and by the shape of the head: its length is approximately equal to its width.

The male of the new flea species of Nycteridopsylla collected on Barbastella leucomelas in the Kanigut cave of Kirghizia is described.

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Summary is absent.

Summary is absent.