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Paleontological and zoogeographic data speak in favour of Mesozoic origin of ixodid ticks. The absence of strict restrictions for the feeding on unusual species of hosts has caused the domination of polyphagy and oligophagy over monophagy among ixodid ticks. The same peculiarities of ixodid ecology are responsible for a restricted part or absence of phylogenetic parallelism with hosts in their evolution. Primary food relations with reptiles are, apparently, preserved only in the genus Aponomma and in many species of Amblyomma while hosts for most species of other genera are mammals and, to a lesser extent, birds. The number of potential hosts in these species can be much greater than that of real ones. Restrictions in the distribution of some species are connected rather with direct effect of unfavourable environmental factors on their non-parasitic stages of the life cycle than with the absence of suitable hosts. During the evolution of natural landscapes and at a shorter stages under the influence of successions or antropogenic factors ixodids easily adapt themselves to feeding on new species of hosts. So the differentiation of primary and secondary hosts of these parasites is rather difficult.

The investigation of a tarsal receptory complex (TRC), the main distal sense organ in gamasid mites, by means of scanning electron microscopy in 50 species of mites, belonging to 32 genera and 17 families of Gamasina, has made it possible to classify all types of sensilla, to realize the perfect topography of each sensillum and on this basis to distinguish the stable features that can be used in taxonomy and phylogeny of the mites. According to these features the dendrogram was obtained where similarity and difference between the genera studied are shown graphically. The number and topography of sensilla within the TRC show systematic closeness between taxons, but not their ecological or biological resemblance.

Changes in the ultrastructure of cells of the intestinal epithelium during the digestion of one blood portion were traced in the fleas L. segnis. It is shown that alongside with the cavity digestion take place elements of intracellular digestion. Hypothetic scheme of the digestive cell functioning is given.

The karyotypes of two distinct populations of Diplodiscus subclavatus obtained from the Lithuanian and Ukrainian SSR were described and their chromosomal morphology was compared. The diploid chromosome number of somatic cells of parthenogenetic generations is 20. The karyotypes are composed mainly of biarmed chromosomes. Karyological analysis revealed interpopulation differences in the chromosome morphology of pair N 7. The occurrence of B-chromosomes in the chromosome set in one of the studied populations was detected. B-chromosomes were the smallest elements in the karyotypes, subtelocentrics. Their number ranged from 0 to 10 in different cells.

Peculiarities of chromosome sets of 4 schistosome species, parasitic in different mollusks from Chauna lowland were investigated. The karyotype of parthenites of Trichobilharzia sp. 1 (the intermediate host Sibirenauta picta) consists of 18 elements, of which the 2nd and 6th pairs are submetacentric-metacentric, the other chromosomes are metacentric. The diploid chromosome set of Trichobilharzia sp. 2 (the intermediate host Anisus acronicus) includes 16 biarmed units. The first pair of metacentric chromosomes amounts to 28.02% of the whole haploid set length. Austrobilharzia variglandis have 14 biarmed autosomes and 2 sex chromosomes. Females are the hetero-gametic sex, Z-chromosomes are of a metacentric-submetacentric type, W have an acrocentric structure. The chromosome complex of Shistosomatium sp. (the intermediate host Lymnaea zazurensis) consists of 14 elements, of which the largest pair is sex chromosomes. The female sex is determined by an exclusively large metacentric Z-chromosomes (relative length 38.36%) and a small acrocentric W-chromosome (relative length only 7.08%). The chromosome sets of Schistosomatium sp. have undergone the most considerable evolutionary transformations among schistosomes, as a result it has the least lineage groups of genes and the most expressed differentiation of sex chromosomes.

Native and glutaraldehyde-treated (for denaturation of glycocalyx) larvae of Aploparaksis birulai were transplanted from naturally infected oligochaetes Lumbriculus variegatus to haemocoel of Gammarus lacustris. The transplantation was accompanied by the loss of glycocalyx external layer of the cyst's tegument and by the secretion of membrane material in a shape of tubes from the surface of microvillous integument of the caudal appendage. The secretion of tubes by the cyst's tegument is well expressed only when glycocalyx is damaged by glutaraldehyde and at passing of inflammatory infiltration on the cyst from the caudal appendage during mechanical damage to its intactness in the process of transplantation. Tegument-secreted tubes caused lysis of haemocytes of the membrane digestion type accompanied by the destruction of internal departments of the forming capsule. Pigment deposits blocked the membrane material secretion from the surface of cyst's tegument and caudal appendage. Membrane material secretion by the larval tegument creates prerequisites for its survival in a case of incapsulation and thus an opportunity for the use of new intermediate hosts in phylogenesis.

The description of different parts of the excretory system of microphallid cercariae studied by means of electron microscopy is given for the first time. It is shown that although fine structure of cyrtocytes, excretory canals, bladder and pore of investigated larvae corresponds to the general plan of trematodes well known from the literature, it has a number of specific features. Obtained data are compared with analogous literature information devoted to trematodes. The question on the nature of excretory bladders in different trematode groups is now posed in connection with the division of the latter according to this sign into the superorders Anepitheliocystidea and Epitheliocystidea.

The life cycle of Proteocephalus thymalli (Cestoda, Proteocephalidae), a parasite of Siberian glame (Thymallus arcticus), was repeated under experimental conditions. The first intermediate hosts, the copepods Epischura baicalensis, Cyclops kolensis and C. vicinus, were determined. The developmental time of P. thymalli in the first intermediate host was determined and the morphology of the larval and adult phases was described.

356 parasitic larvae of the genus Triodontophorus from equids (two Equus hemionus and one E. grevyi) have been investigated. They belong to three fenons, which differ from each other by the shape and dimensions of a stoma, the structure of teeth and other signes. That fenons belong to three different species: T. serratus, T. tenuicollis and T. brevicauda. The differential diagnosis of L4 of that species of Triodontophorus are given.

Mature and larval forms of trematodes of the family Didymozoidae were recorded from pelagic sharks from the equatorial and tropical parts of the Pacific. Marites of Didymocystis orbitalis Yamaguti, 1970 were found in the eyes of Pterolamiops longimanus. Metacercariae of the Paramonilicaecum and Torticaecum type were found in the spiral valve of P. longimanus and Carcharhinus limbatus. Descriptions and figures of the worms are given.

Syncoelicotyloides macruri gen. et sp. nov., a monogenean of the subfamily Syncoelicotylinae Mamaev et Subtschenko, 1978, is described. Members of this subfamily were known only from Notacanthiformes and are found for the first time in Macrouriformes. This fact is considered by the authors as a confirmation of the earlier supposition (Mamaev, Zubtschenko, 1978) about phylogenetic closeness of these fishes.

A description of a new parasitic isopode Lironeca galinae sp. n. (Isopoda, Cymothoidae) from Atlantic is given.

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