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The data on natural-nidal diseases typical for cities, urban agglomerations and culture eoenoses are summarized in the paper in the light of academician E. N. Pavlovsky's theory. According to the ecological principle all infections with natural nidality are classified into two large groups. Concrete data from the north-western region show the role of leptospirosis, haemorrhagic fever with renal syndrome, tick-borne encephalitis and pseudotuberculosis in urbanized landscapes. Modern epidemiological tendences of these diseases, associated with functioning of their natural and economic nidi, are stressed.

9 species of fleas belonging to 3 genera were found on bats from the Rostov Region, Krasnodar and Stavropol Territories and North-Ossetian ASSR. Two species, Ischnopsyllus octactenus Kol., 1856 and I. hexactenus Kol., 1957, are first reported from the North Caucasus. New data are given on the distribution of 7 species. The distribution of fleas on hosts is established. Rhinolophopsylla unipectinata Tasch., 1880 and Nycteridopsylla eusarca Dampf., 1908 reproduce at cold time of the year. Fleas are arranged into groupings according to their distribution area types.

Observations were carried out from 1982 to 1983 in the Armenian SSR in the region of Alpine Lake Sevan. The aim of the investigation was to determine physiological age of mature hungry ticks in nature. It was found that D. marginatus imagos have two peaks of activity, the autumn peak being higher than the spring one. Hibernate both the individuals which already were in the active stage and freshly moulted ones. Ticks which became activated before the hibernation spend most of their nutrients. There are noted some differences in the physiological age between males and females after the hibernation. Males appeared to be more exhausted as compared to females. Differences in the physiological age of ticks during one activity season for different years are higher than these during different seasons of the same year.

The response of adult ticks, which were at the distance of 5, 10, 15, 20 and 50 cm from the source of convection thermal currents and infrared radiation, was determined in field experiments. The temperature of the source was from 38 to 42. Ticks were found to respond to convection thermal currents at all the distances. With the increase in the distance the number of responding individuals decreased from 62.5 to 48.1%. Reactions of ticks to infrared radiation with current density of 0.11 cc/cm² and less at the distances of 15 and 20 cm were not observed.

On the basis of data on dynamics of the adundance of mountain ibex in different physical and geographical regions of Tien-Shan and data on the biology of itch mites it has been established that in epizootical nidi of Sarcoptes infection the reduction in the density and crowding takes place. The higher the amount of precipitations, the more intensive the coures of the disease. In natural nidi of Sarcoptes infection the reproductive potential of the ibex population is higher than the mortality of animals due to itch mite. It is good practice to maintain the sex composition in the following ratio: 25 to 30% of adult males and 35 to 40% of females.

In July and September—October two peaks of abundance of Ergasilus sieboldi were recorded on fishes from lake Arakul (Central Ural). The peaks corresponden to the attacks of two generation of the copepod. In 1981 the total abundance of E. sieboldi of the first generation on all fishes amounted to 5204.6 specimens, while that of the second generation to 29616 specimens. There were recorded copepods of the third generation but their number on fishes did not exceed 2000.4 specimens. The main source of infection of fishes with E. sieboldi is Acerina cernua.

The structure of the farynx, oesophagus and gastrodermis of U. murmanica is described. The farynx of udonellids is of massive type. Its structure points to the phylogenetic relations of udonellids with the turbellarians of Neorhabdocoela Typhloplanoida. Udonellids differ from all Platihelminthes studied in the structure of gastrodermis. They lack, apparently, intracellular digestion characteristic of other Platihelminthes. The elements of resemblance with gastrodermis of trematodes have been noted. Analysis of the structure of udonellids, monogeneans and temnocephalids suggests an independent origin of these groups.

In the populations of Littorina saxatilis and L. nigrolineata the infection of males and females with trematodes (resulting in the parasitary castration of the hosts) grows with the increase in the size (or age) of molluscs. With the definite size class the rise in the infection rate stops in females while it continues to increase noticeably in males. In the same size class differences in the sex proportion in littorines are noticed: the per cent of the female species increases and that of the male species decreases. It is shown that if the growth of the infection extensivity with age in females had the same tendency as in males, the existence of the littorines population would be threatened because the females of the senior size classes play an important part in the reproduction of the association. The role of big males in the supporting of the reproduction potential is not great and therefore their high infection rate does not influence much the reproduction of the population.

The problem of dissimilation of larvae of strongylates of ruminants in pasture biocoenosis is discussed. A new form of their migration in horizontal direction on the basis of negative geotaxis is suggested. As a result of alternation of vertical migration of larvae on the tilting leaves of the grass and their subsequent washing off with rain or dew down on the ground proceeds their distribution on the pasture. The description of corresponding experiments is given.

A study of the phenetics of the zoonotic cutaneous leishmaniasis agent in Uzbekistan and subzone of southern and northern deserts has made it possible to separate a northern population (or a group of populations) of the agent. It differs from the southern population (or southern ones) in having smaller sizes of the amastigote stage, greater abundance of parasites in cutaneous affections of gerbils, milder progress of leishmaniasis in white mice and great gerbils. This population can be called priaral, i. e. belonging to the subzone of northern deserts of the Turan lowland.

By gradual (during several months) increase of the concentrations of cycloheximide (C) and chloramphenicol (Ch) in cultural medium there were obtained strains of С. oncopelti resistant to the effect of 20, 50, 100 mkg/ml and 2.5 mg/ml Ch. During 38, 28, 19 and 12 subculturings (that corresponds to 160, 120, 80 and 50 cell generations, respectively) on media without inhibitors parasites preserve their resistence to corresponding concentrations of С and Ch without essential changes. Cross resistence between obtained strains of Crithidia was missing. Therefore, the characters of resistence to С and Ch are rather specific, genetically specified and can be used as genetical markers.

Micromorphology of the myxosporidian Sphaerospora pectinacia found in the lake Baikal for the first time is described and the comparison of the spores morphology of this species from different parts of this region is made. The pathomorphology of perch kidneys at sphaerosporosis is first described. Clearly expressed pathology of kidneys and selective death of infected fishes are explained by the youth of the "parasite—host" system.

By feeding plague infected Xenopsylla cheopis fleas with blood substitutes free of blood formed elements blocked fleas were obtained. These fleas are able to transmit plague agent to white mice and cause their mortality.

Schoutedenichia infrequens sp. n. from Apodemus sylvaticus from Tadzhikistan is described. The species is close to Sch. thracica Kolebinova, 1966 from which it differs by larger sizes, longer PL and less number of idiosomal setae.

A study of the ecology of Cyclocoeloidea has shown that true pathogenicity of this group of trematodes is much higher than the known one. In addition to the mechanical damage done by marites of cyclocoeloids to the host, migrations of cyclocoeloid larvae in the definitive host's body are a great threat to the latter as well as processes connected with the removal of sexual production by adult worms outside that results usually in a serious damage to the host's organs and tissues.

The paper describes a new genus and species of high monogeneans of the family Diclidophoridae from gills of Sуnodus foetens L. from the Gulf of Mexico.

The main differences of marimermithids from freshwater and soil mermithids resolve themselves into preservation of characteristic successive location of oesophagus and midgut (trophosome) as in all free nematodes. In freshwater and soil mermithids oesophagus is transformed into stichosome, loses its connections with trophosome and is located in the groove of tropho-some, in parallel with it over the whole extent of the body. Up to now 2 families of marimermithids are known: Marimermithidae and Benthimermithidae. They include 8 genera and 23 species. Due to great morphological differences between some genera and their groups the author divides the first of the above families into four subfamilies: Marimermithinae comb. n. (I genus); Acronematinae subf. n. (I genus); Thalassonematinae subf. n. (2 genera — Thalassonema and Ananus); Trophomerinae subf. n. (I genus). The family Benthimermithidae is represented by 3 genera: Benthimermis, Adenodelphis and Abos. Key to two families, subfamilies and their genera is given.

Summary is absent.