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Ultrastructural characteristics of progenetic and monoxenic Archigetes sieboldi Leuckart, 1878 from the oligochaete Limnodrilus hoffmeisteri Claparède are described. Our observations demonstrate that progenetic Archigetes sieboldi shares characteristics of both larval (progenetic) and adult stages. The primary larval characteristics are: the presence of a cercomer; a surface filamentous coat covering the whole worm; the presence of the penetration glands and the absence of tegumental ones; wide sarcoplasmic processes connecting the circular and longitudinal external tegumental muscles; the absence of the dense homogenous zone of the basal lamina beneath the epithelial cytoplasm of all reproductive organs and ducts; non-functional gonopores; and an orthogonal plan of nervous system with three pairs of longitudinal nerve trunks. The principle adult characteristics are: oogenesis, spermiogenesis and vitellogenesis that produce fertilized eggs; the uterine glands; a well-developed longitudinal tegumental muscle layer between tegumental cytons; and the presence of different microtriches. As a result of this progenetic development there has been a secondary reduction in the life cycle of A. sieboldi. It is postulated that a similar process of progenesis may have played a major role in the early evolution of the Caryophyllidea by first appearing in a plerocercoid stage of an ancestral strobilate cestode from fish.

Taxonomic characters for identification of Turgida turgida Rudolphi, 1819, a parasitic nematode of the Virginia opossum Didelphis virginiana, were studied by scanning electron microscopy. The distinguishing feature of the cephalic end is the presence of numerous denticles, structures associated with the internal tooth, and 2 spongelike areas on the inner side of each pseudolabia. The posterior end of male T. turgida differs from that in other species of Physalopteridae by the number of caudal papillae (22), truncated postcloacal papilla, and patterns of ventrocaudal ornamentation.

The formation of the definitive procercoid excretory system of the pseudophyllidean cestode Triaenophorus nodulosus has been investigated. This process can be divided into 3 main stages. In the first stage, active autophagic processes lead to the formation of intracellular excretory canals. In the second stage, the process of the autolysis is enhanced and the system of intercellular lacunae functioning as an excretory system is formed. A definitive excretory system of the procercoid forms at the next stage, by means of the migration and proliferation of undifferentiated cells replacing the lacunar system.

The structural and functional organization of digestive-transport surfaces in some lower cestodes and their fish hosts was studied. It has been shown that the ultrastructure of cestode microtriches and fish enterocyte microvilli being the basis of membrane-linked digestion is quite similar. These organelles increase the digestive-transport surfaces both in helminths and fishes. However, the hydrolytic enzyme activity in helminths is usually 2-4 times lower than that of the fishes. Desorption (adsorption) characteristics of various hydrolases in helminths and fishes are also different. In helminths the easily desorbed fraction of each enzyme is always more abundant than in fishes. In contrast, the intensity of transport processes in helminths is higher when compared with fishes. The adaptation of digestive-transport surfaces and enzyme systems to feeding conditions is discussed.

The content of sodium, potassium, calcium, and magnesium ions was determined in cestodes Ligula intestinalis, Eubothrium rugosum, and Triaenophorus nodulosus. In freshwater fishes the total concentration of Na, K, Ca, Mg ions in cestode tissue water is practically similar to that of fishes muscles and in serum. It was suggested that examined cestodes have isoosmotic regulation of the water-salt balance.

The mass infection of larvae and youngsters of carp fishes with the peritrichoid Ambiphrya ameiuri has been recorded in natural condition in the Rybinsk water reservoir. The uneven localisation of ambiphyrans on the body surface has been observed on the hosts Blicca bjorkna and Abramis brama. Most high density of parasites was on the breast, back and tail fins. The morphological variability of ambiphyrans in dependence upon their localisation on the fish body has been observed. Details being significant for the taxonomy have been studied by means of the scan electron microscope. It has been found that the scopulae (greatly developed attaching organ of A. ameiuri) in cases of heavy infection cover about 50-60% of the skin epithelium of fish larvae. In these cases the significant part of skin epithelium, which performs the respiratory function is excludes of the active oxygen supply that cause the illness and death of young fishes.

The paper presents the results of studies on the effect of wastes of the Cherepovets iron-and-steel works containing phenol, naphthaline and oil products on fish parasites of the Sheksna part of the Rybinsk water reservoir (the Volga river system). It has been shown that the number of highly sensitive ectoparasites of Abramis brama (Protozoa, Dactylogyrus monogeneans, Ergasilus sieboldi crustaceans and Caspiobdella fadejewi leeches) considerably decreases in the zone of pollution. It has been first suggested to use Diplozoon paradoxum and Caryophyllaeus laticeps, parasites of bream highly resistant to toxic effect, the number of which considerably increases in the zone of pollution, as indicators of anthropogenic pollution and ecological state of a water body. Among diplozoons there have been first discovered individuals having structural anomalies (reduction of the attachment organs, violation in the symmetry of arrangement and number of valves) that serves as evidence of mutagenic effect of toxic substances of polluted waters on morphogenesis of the parasite.

Mitotic and meiotic chromosomes of Caryophyllaeus laticeps (Pallas) from Rybinsk reservoir were studied using air-drying techniques. It was established that the diploid set contains 20 metacentric chromosomes. The chromosomes are large--their mean absolute length ranges from 3.00 to 12.90 microns. The last pair of small homologues comprises only 3.4% of the total length of the complement. One specimen of C. laticeps with 2n = 21 (trisomic after the last pair of chromosomes) and one triploid specimen with 3n = 31 is described. Data on the comparative karyology of caryophyllids are discussed.

The diploid chromosome set of Eubothrium rugosum contains 16 elements. The karyotype consists of three pairs of metacentric, three pairs of acrocentric and two pairs of submeta-metacentric chromosomes. Their mean absolute length ranges from 2.20 to 8.80 microns. The first two pairs of large metacentric chromosomes comprise over 45% of the total complement length.

Studies were carried out of the nervous apparatus of Eubothrium salvelini, E. rugosum, E. crassum, E. vittevitellatus, Diphyllobothrium latum and D. dendriticum (Cestoda, Pseudophyllidea) at different stages of their life cycle. Scolexes of adult cestodes are innervated with 5 pairs of longitudinal nerve trunks, the number of which in the strobile gradually increases up to 17 pairs in species of Eubothrium and up to 60 pairs (in the wildest parts of the strobile) in species of Diphyllobothrium. There were noted differences in of longitudinal trunks in different species and in the specimens of the same species but from different hosts, the width of the strobile being the same. The main lateral trunks are situated in the medullar parenchima. In species of Eubothrium they are wide apart while in Diphyllobothrium they are drawn closer to one another. The description of the rough and inner nerve plexuses is given. Judging by the larval developmental phases the well developed orthogon with 3 pairs of longitudinal nerve trunks can be taken for the initial state of the nervous apparatus in Pseudophyllidea. This means that the initial forms belonged to advanced turbellarians rather than to primitive ones.

Experiments conducted have shown that water temperature can affect dactylogyrids in two ways. From one hand, the direct influence stimulates the development and reproduction of the parasites that results in the increase of their abundance. From the other hand, the indirect effect can take place. The rise in the temperature increases the general resistence of the fish organism and favours the formation of immunological reaction to hyperinfection that causes the further fall in the parasite abundance. Thus, the seasonal dynamics of the abundance of dactylogyrids depends on the complex interaction of these two factors.

The anatomy of the nervous apparatus of Triaenophorus nodulosus at all stages of its life cycle was studied by means of Zherebtsov's hystochemical method. Judging by the cholinesterase activity the mass of nerve cells is situated in the oncospheres of the coracidium. At the procercoid stage the ortogonal nervous system arises with three pairs of the longitudinal nervous trunk. In plerocercoids the number of longitudinal trunks increases up to 7 pairs, the rough nervous plexus and inner plexus develop too, In mature, cestodes only secondary changes take place associated with the development of the genital system. Both in larvae and adults all longitudinal trunks are situated at the same level, on the border of the cortical and medullary parenchyma, and only the inner nervous plexus first described by the authors from cestodes passes through the medullary parenchyma. The arrangement of all elements of the nervous apparatus at the same level corresponds from the authors point of view to the most primitive state of the nervous system in the order Pseudophyllidea.

The effect of the anthelmintic preparation fenasal on the fine structure of tegument of the cestode Caryophyllaeus laticeps, a parasite of Cyprinidae, was studied in vitro. 10 mcg/ml concentration of fenasal causes numerous evaginations of the external cytoplasmic layer in the scolex of helminth. Inside these formations there are granules of secretion, mitochondria, vacuoles. At the same time microtrichia disappear from the surface, cytoplasmic layer gets thinner and its mitochondria degenerate. In the middle part of the body the number of microtrichia decreases, the layer of tegumental cells swells and conformational condition of its mitochondria changes. The degree of injury depends on the exposition time which was from 0 to 4 hours.

The effect of procercoids of T. nodulosus on the periods of metamorphosis, fecundity, survival and the capacity to infect C. strenuus at all copepodid stages (I-V) was studied experimentally. It has been first shown that early copepodid stages (I-III) can be easily infected and serve as first intermediate hosts of T. nodulosus. Under the effect of the parasite the metamorphosis periods of the I and 2-nd copepodid stages increase considerably even at the low infection intensity while at the IV and V copepodid stages--only at the high infection intensity. If the infection takes place at the IV copepodid stage the female fecundity decreases even at a high infection intensity. The life span of infected and noninfected cyclops coincides. At early developmental stages the relations in the system "T. nodulosus-C. strenuus" manifest themselves in different ways and result in the breach of the important biological function of the host.