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Summary is absent.

Up to 1700 parasite species have been checked for freshwater and anadromous fishes and sea fishes which enter fresh waters. They are represented by different systematic groups. This is 50% more than listed in the first key (Bychowsky, 1962). Most numerous groups are the Myxosporidia and Monogenea. Mostly all freshwater fishes of the USSR have been investigated parasitologically using the methods worked out by V.A. Dogiel (1933) and his pupils.

The ultrastructure of the so-called "cysts" of Myxosporidia has been studied. It was found that species investigated (Kudoa quadratum, Henneguya oviperda, Myxobolus disparoides) possess large plasmodia which have nothing in common with cysts of Protozoa. Large and fusiform plasmodium of Kudoa quadratum localizes inside a muscle fiber. The development of trophozoite up to spore maturity takes place inside the same muscle fiber. A lysis of myofibrills takes place only inside of the infected fiber and does not spread throughout the muscle tissue. The plasmodium surface of Kudoa quadratum and Henneguya oviperda is covered with a single unit-membrane. Plasmodium of Myxobolus disparoides has a complex coat outside of plasma membrane. The relations between the surface structure and biology of plasmodia and taxonomic value of the former are discussed.

Three generations of schisonts in the life cycle of Eimeria akeriana, the intestinal parasite of Meriones blackleri, were determined. Gametogony begins in 94 hours, the first oocysts discharge in 5.5—6 days and lasts 14.5 to 15 days after the oocyst administration. A cytochemical study of the distribution of the nucleic acids, proteins and amylopectin at the stages of endogenous development of E. akeriana has revealed a considerable similarity among the parasites of Eimeria though each type is characterized by some cytochemical peculiarities.

In the smears taken from 98 specimens of 11 fish species from the Kurish gulf of the Baltic Sea the presence of 22 trichodinid species was ascertained: Trichodina domerguei domerguei (Wallengren, 1897), T. acuta Lom, 1961, T. tenuidens Faure-Fremiet, 1943, T. reticulata Hirschmann et Partsch, 1955, T. jadranica Raabe, 1958, T. esocis Lom, 1960, T. pediculus Ehrenberg 1838, T. nigra Lom, 1960, T. modesta Lom, 1970, T. rectangli rectangli Chen et Hsieh, 1964, T. urinaria Dogiel, 1940, T. nobilis Chen, 1963, (?) T. tisae Lom, 1970, T. convictor Chardez, 1979, Trichodina sp. from nasal pits of Acerina cernua, Paratrichodina corlissi Lom et Haldar, 1977, Tripartiella copiosa (Lom, 1959), Trichodinella epizootica (Raabe, 1950), T. subtilis Lom, 1959, (?) T. percarum (Dogiel, 1940), Trichodinella sp. I from gills and fins of Gobio gobio, Trichodinella sp. II from skin of Scardinius erythrophthalmus. Three species (Trichodina convictor, T. modesta, Paratrichodina corlissi) are mentioned for the first time in the fauna of the USSR. Three new hosts (Gobio gobio, Scardinius erythrophthalmus, Perca fluviatilis) are determined for Trichodina tenuidens. It was concluded that Trichodina meridionalis sensu Kandilov, 1964 is a synonym of Trichodina rectangli rectangli Chen et Hsieh, 1964.

A study of the trophic envelope (trophamnion) ultrastructure around the stolon of Polypodium in autumn and summer oocytes of the sterlet and sturgeon has shown that the trophamnion is a large hollow cell lining the parasitophore vacuole. The maximum trophic activity is displayed by the trophamnion in oocytes with intensive vitellogenesis, i. e. in August—September. In May the trophamnion degenerates. The ultrastructure of the trophamnion functioning to full extent (the abundance of trophic inclusions, anastomosing microvilli in a shape of sieve, etc.) and that of the degenerating one (pycnotisation of the nucleus, membrane degeneration of the cytoplasm) is described.

Investigations conducted on the lakes Tavatui of the Sverdlovsk district and Arakul of the Cheljabinsk district have shown that D. amphibothrium occurs on the ruffe within the whole year. The abundance of the helminth and the ratio between its age groups depends on water temperature, immunity of fishes, biological peculiarities of D. amphibothrium and other factors. The helminth has two generations a year: an early summer generation which appears in the end of June and dies off in August-September and late summer one which appears in August-September and dies off in May-June of the next year. The worms lay eggs in all seasons. Their reproduction rate increases with water temperature rise. Within its life one helminth of the early summer generation produces about 411 eggs and that of the late summer one — about 853 eggs. Most laid eggs perish.

There is no common opinion among specialists on the synonymy of species of the genus Bothriocephalus parasitic in Cyprinidae. It is noted that in the identification of the species of this genus a special attention should be given to the scolex structure. According to different scolex structure B. opsariichthydis Yamaguti, 1934 and B. acheilognathi Yamaguti, 1934 are distinct species. Synonyms of the former are B. gowkongensis Yeh, 1955 and B. phoxini Molnar, 1968, a synonym of the latter is Schyzocotyle fluviatilis Achmerov, 1960.

Postembryonal development of cestodes of the family Dilepididae is described. Larvocysts of the cestodes Dichoanotaenia gallinagilis and Platyscolex ciliata were obtained experimentally and belong to the same cysticercoid — monocercus. The developmental periods of larvae (at the stable temperature of 20° C) are established: 26 days in D. gallinagilis and 11 days in P. ciliata. A morphological description of individual developmental stages is given.

A comparative study of the skin-muscle sack structure of nine species of trematodes was made. It has shown that the thickness of the whole sack and its muscle layer, the presence and deepness of the fibrillar one may undergo some adaptive changes depending on its defensive and motor functions and on localisation of the parasites in the host's body.

Not infrequently lice can reach great abundance on rodents. The distribution of lice on animals is uneven. The infection with lice depends not only on the physiological and ecological properties of the host but also on the environmental conditions. The distribution of lice on rodents can be affected by the development of transport communications. If a population of rodents is parasitized by two species of specific lice, they belong to different genera. On single animals only one species of lice occurs as a rule; if two species occur, then one of them is dominant. In this way decreases the interspecific competition between lice. Seasonal and age changes in lice infection are associated with those in host's biology and behaviour under certain climatic conditions. It is clearly displayed in rodents breeding once a year. The infection with lice increases with the rise in rodents abundance. Along with the confirmation of the general rule of poor infection with specific parasites at the borders of the species distribution a case of high infection with lice at the border of the host's distribution was noted. In addition to specific lice, alien species are recorded on small mammals which appear due to exchange of parasites between hosts. Examples are given of conquering new hosts by lice; as a result close species of lice can parasitize phylogenetically distant hosts.

The marine fauna of isopods of the family Cymothoidae s. str. is arranged into two ecological and geographic groupings: neritic (most species) and oceanic (the genus Glossobius, in general). The distribution area of cymothoids represents four coastal regions which are characterized by a high level of endemism and isolation. The nature of distribution of endemic species and genera points to the cymothoids distribution centre in each region: Indo-Malayan, Mediterranean, Caribbean and Tropical Eastern-Pacific. The distribution of most cymothoid species in the Tropical area connects the origin of these isopods with the Thetis Sea. The most ancient place of the cymothoid fauna formation is the Indo-Malayan Archipelago. Paleontological data on teleost fishes, with which cymothoids are closely associated, suggest that the first ancestors of these isopods with stationary parasitism appeared in the Chalk. The main differentiation occurred in Paleogene.

Asymphylodora progenetica known earlier as a progenetic form of trematodes from Bithynia tentaculata (L.) was found in the fish Leuciscus idus (L.). Its belonging to the genus Parasymphylodora Szidat, 1940 was established and the transference to this genus validated. A description of adult specimens of P. progenetica with larger sizes and a description of the designated lectotype of the progenetic form of this species are given.

Visual acuity of Phoxinus phoxinus L. and young of Salmo salar L. infected to different extent with metacercariae Diplostomum was studied by means of the optomotor reaction. Most specimens of Ph. phoxinus with normal vision have no more than 30 parasites in the lens. The rise in the number of parasites leads to a noticeable aggravation of vision and at the number of over 70 — to its complete loss. Due to a different localisation (vitreous body) the aggravation of vision in young of S. salar proceeds at a greater intensity of infection.

A male and female of H. (O.) phasiana were found on Emberiza fucata Pall. and Tetrastes bonasia L. in the Vjazemsky region of the Khabarovsk territory and in Spassky region of the Primorje territory in July of 1979 and 1980.

Summary is absent.