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The paper deals with peculiarities of Cestodes which distinguish them from other classes of Plathelminthes.A study of these peculiarities is of great importance for understanding phylogeny and evolution of these specialized parasites of the intestine of vertebrate animals. Basing on morphological and biological characters of different groups of cestodes and reviewing critically the systems of existing orders the author accepts the autonomy of 9 orders (Trypanorhyncha, Tetraphyllidea, Lecanicephalidea, Diphyllidea, Pseudophyllidea, Caryophyllidea, Nippotaeniidea, Proteocephalidea, Cyclophyllidea) and one order (Aporidea) — conditionally. The main diagnostic feature of the above orders is the character of their attaching organs. The structure of genital system, as a conservative feature, is of secondary importance. The analysis of distribution of cestodes of various orders among vertebrate animals shows that every class of definitive hosts has its own specific fauna of cestodes which consists of members of definite orders or of members of their families. A study of phylogenetic relations and trends in the evolution of different groups of cestodes parasitic in various systematic and ecological groups of vertebrates as well as the elaboration of systems of taxons of these worms must be based on a careful investigation of morphology, ontogenesis and interrelationships between the parasites and their hosts at every stage of the life cycle. Closely connected processes of the evolution of these highly specialized parasites and their definitive hosts also must be taken into account.

17 species of parasites were found in pink salmon acclimatized to the White Sea. The species composition of parasite fauna remained constant over a period of 1964 to 1965; only slight quantitative changes were observed. With the transition to rivers a qualitative enrichment of the parasite fauna was noted as well as the increase in its infection intensity. The comparison of the pink salmon parasite faunas of the White, Barentz and Far East Seas has shown that they possess a number of common species. The infection with nematodes of the genus Anisakis pathogenic for man was noted.

Experiments conducted on mice and rats have revealed the formation of cross immunity between Trichinella spiralis (Owen, 1835) and T. pseudospiralis Garkavi, 1972. As a result of superinvasion in mice developed 39.8 and 66.8% of muscular larvae as compared to control animals. Rats happened to be more resistant to repeated infections with Trichinellidae. In one group of rats the repeated infection did not take place at all, in the other there was found only 1.8% of muscular larvae as compared to control ones.

Studies were undertaken of the fecundity of Paraphelenchus pseudoparietinus, the mycohelminth frequently occurring in the root soil and tissues of plants. It was established that during the life cycle the female lays 40 eggs (M ± m=40 ± 2). The egg laying lasts about 21 days. On the average, the female lays 2.5 to 3 eggs within a day. The curve of egg production rises till the 13th day, then decreases and ceases completely by the 21st— 22nd day. The female perishes in I to 2 days after the end of egg laying. The duration of one generation (from egg to egg) is 13 days. The reproduction of P. pseudoparietinus proceeds parthenogenetically.

The analysis of copepods of the genus Salmincola collected from the gills of Paracottus kessleri and P. kneri has shown that there is only one species, S. cottidarum, rather than two ones as some investigators previously have been revealed. It was established that S. cottidarum is most close morphologically to S. edwardsii, parasite of Salvelinus alpinus. S. cottidarum seems to originate from the latter since the subspecies of the Alpine char, Salvelinus alpinus erythrinus, lives in the Baikal Lake and its water system.

The topical application technique and set of tests suggested by the World Health Organisation were used to determine the susceptibility of Ixodes persulcatus to acaricides. Acaricides were found to cause a long (within several days) development of ticks injury; in this connection periods of registration of results were changed as compared to recommended ones. The injury development rate depends on the acaricide, concentration of its solution and season.

The fomerly unknown larvae of a mosquito known in the USSR as Aedes riparius ater Guts. were found in the Amur region (Soviet Far East); they are quite different from those of A. riparius, therefore the former should be regarded not as a subspecies of the latter but as a distinct species of A. cantans group. It is found that this species is identical to A. stimulans from Alaska and Yukon Territory of Canada differing from "typical" A. stimulans from southern areas of North America by some characters (branching of upper and lower head hairs in the larva, structure of basal lobe in male genitalia and ciloration of the female) and originally described as A. mercurator Dyar, this name being later synonymized with A. stimulans. It should be restored as a specific name for a mosquito, not abundant but widely distributed throughout the taiga zone of the USSR and occuring also in the northwestern part of North America; A. riparius ater should be considered as a synonym of A. mercurator. Some data on its breeding places and bionomics are given, too.

A key to Aedes species is given as well as additional notes on species which are identified with difficulty by the preparations of females heads. The key contains the main quantitative indices for each of 46 species.

Comparative analysis of interrelations between Leishmania and their invertebrate and vertebrate hosts points to a great significance of the transmissible factor in the complex of characters typical of leishmanioses nidi. The proofs are as follows: obligateness of the transmissible way of transmission of leishmanioses agents; phylogenetic priority of Leishmania relations with invertebrate host — sand fly (subfam. Phlebotominae); specificity of biological interrelations between Leishmania and sand flies; relative specificity of food connections of sand flies with vertebrate hosts of Leishmania; unequal level of anthropophily of sand fly species determining differences in the epidemic danger of nidi; relative stenotopy of sand flies, — restriction of some species to areas characterised by special landscape conditions. In this connection the transmissible factor (species composition of sand flies-vectors) should be regarded as the main character for typification of leishmanioses nidi. Other correlating characters (composition of vertebrate hosts of Leishmania, epizootic and epidemic tension of nidi, peculiarities of their landscape adaptation) are considered to be additional ones. Their analysis will enable to distinguish taxons subordinated to definite types of leishmaniosis nidi (species and variants of nidi).

On the basis of studies of sections of fleas (Nosopsyllus consimilis and Callopsylla caspia) infected with the plague causative agent there were described changes occurring in fleas with partially or completely destorted intestinal permeability. Mass stopping up the intestine fills up the proventriculus and stretches (in width and length) anterior rows of spines. In completely "blocked" fleas the destruction of bacterial clot usually takes place on the border of the stomach and cardiac valve.

Eluation of DEAE-cellulose of lyophilized midgut extracts of 1st stage larvae of Hypoderma bovis, Hypoderma lineatum and Oedemagena tarandi has revealed three active fractions: toxic, proteolytic and collagenase. Toxic effect is most prominent in H. bovis larvae and less prominent in O. tarandi whereas proteolytic effect of the extract of O. tarandi larvae is greater than that of H. bovis and H. lineatum. Maximum of proteolytic activity in all three species is in a range of pH from 4.5 to 5.0. Collagenase activity is very poor in the above species. The differences found reflect apparently the nature of adaptive reactions of hypoderma species for the life in the hosts's organism.

The paper describes three species of microsporidians from part he no genetic generations and cercariae of trematodes, two of which are new ones. The systematic position of these species is discussed in the light of recent changes in the diagnosis of the genus Nosema.

Amino acid content of Eimeria tenella was established by means ot the automatic amino acid analyser Hd 1200 E which enabled the authors to identify 17 amino acids in the investigated specimens. In the protein of oocysts and membranes of oocysts of E. tenella there were found 16 amino acids: lysin, histidine, arginine, asparaginic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine, methionine, isoleucine, leucine, tyrosine and phenylalanine. Protein of oocysts membranes differs in the quantity of some amino acids from that of oocysts of E. tenella. Differences in the quantitative content of amino acids of oocysts and membranes of oocysts of E. tenella are caused, apparently, by specific functions of some structures of coccidian oocysts.

Parasitological investigations were carried out in 15 lakes and 7 ponds of the Sverdlovsk and Chelyabinsk regions from 1966 to 1972. 5008 specimens of fishes (17 species) of different age were examined. In them 23 species and forms of urceolariids were found. Microphotos, biometrical indices of Infusoria and the infestation degrees of fishes with them in waters of different types are given.

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