The article analyzes own and published data concerning muscle elements present in various parts of the trematode reproductive system. The data on the presence of serotonergic and peptidergic (FMRFamidergic) nerve elements evidence the innervation of the trematode reproductive system departments by these neurotransmitters. The available literature information was supplemented by results of the study carried out on Prodistomum alaskense, a representative of the family Lepocreadiidae, an intestinal parasite of deep-sea fish (Zaprora silenus and Aptocyclus ventricosus). The localization of serotonergic and FMRFamidergic nerve structures was determined by immunocytochemistry. The musculature was stained using TRITC (tetramethylrhodamine-isothiocyanate) conjugated phalloidin. The slides were examined using a fluorescent microscope and a confocal laser scanning microscope. An analysis of the data and information in the literature suggest that the muscular system of the reproductive system is well developed in trematodes of various taxonomic groups and represented by circular, longitudinal, and, in some cases, by diagonal muscle fibers. The musculature of the trematode reproductive system is innervated by serotonergic and peptidergic (FMRFamidergic) neurons and their fibers which may be involved in regulating the contractile activity in various parts of the trematode reproductive system.

Trematodes of the Bucephalidae family differ from all other trematodes by the number of morphological characteristics including the structure of their attachment organ as well as the digestive and reproductive systems. In this study, the muscular system and nerve components containing serotonin and neuropeptide FMRFamid were investigated in Rhipidocotyle campanula (Bucephalidae family) from the intestine of the pike. Using immunocytochemical methods and confocal scanning laser microscopy (CSLM), data on the presence and distribution of neurotransmitters serotonin as well as the neuropeptide FMRFamide in the central and peripheral nervous system of Rh. campanula were obtained. When studying the relationship of nerve structures containing the studied neurotransmitters, muscle fibers of the muscle elements of the parasite were stained using the phalloidin associated with the fluorophore. Information on the organization of the muscular system, attachment organ, digestive and reproductive systems is given. The data obtained suggest that serotonergic and FMRF-amidergic nerve structures are involved in the regulation of the function of the attachment organ, as well as the digestive and terminal departments of the eproductive system of Rh. campanula. A comparative analysis of the obtained results and related data available in the literature for other trematodes was carried out.

The distribution of muscle elements in adult Fasciola hepatica was studied on frozen sections using histochemical staining and fluorescent microscopy. Measurements were taken from the stained sections on photomicrograms. The histochemical staining detected actin in muscle filaments of the muscle system. The body wall consisted of circular, diagonal and longitudinal muscle fibers. The thickness of the dorsal muscle layer of the body was 25–50 µm, and the ventral muscle layer was 18– 25 µm. Numerous dorsal and ventral muscle bundles of 10–28 µm thick connected the dorsal and abdominal walls of the body. The musculature thickness of the oral sucker was 196 to 238 µm. The thickness of the pharyngeal wall was 64–98 µm; it was represented by circular, longitudinal and radial muscle fibers. The round and oval lumens of the branched blind intestine had a diameter of 63–119 µm. The intestine was surrounded by thin circular and diagonal muscle fibers of 2–3 µm thick, with a distance of 5-7 µm between them. The ventral sucker consisted of three types of muscle fibers, namely, circular, longitudinal and radial fibers. The maximum thickness of its musculature was about 340 µm.

The work provides data on the presence and localization in the nervous system of the causative agent of dicroceliosis, Dicrocoelium lanceatum serotonergic and FMRFaminergic structures. Using the immunocytochemical method and confocal scanning laser microscopy, the test substances were found in the central and peripheral parts of the parasite's nervous system — in the head ganglia, the central nervous commissure, the longitudinal nerve trunks and the transverse commissures connecting them. The innervation of the nerve fibers of the attachment organs of trematodes by immunoreactive to serotonin and neuropeptide is shown.Trematode Dicrocoelium lanceatum (Stiles and Hassal, 1896) (Dicrocoeliidae) is the causative agent of Dicroceliosis, a widespread helminthiasis of domestic and wild animals. Dwelling in the bile ducts of the liver and in the gallbladder, dicroceliums cause irreversible pathological changes in the host organism, chronic inflammation of the bile ducts, cirrhosis of the liver, and metabolic processes in animals occur. As a result of the disease, animals are depleted, their productivity decreases, growth and development lag, and the nutritional and caloric value of meat decrease.In the cycle of development of the causative agent of dicroceliasis, the intermediate hosts are land mollusks, while the additional hosts are ants. The eggs of the parasite along with the bile enter the host intestine, and then with the feces into the external environment. Mollusks swallow eggs, miracidia come out from the eggs in the intestines of the mollusk, and penetrate the liver, where daughter sporocysts develop, in which cercariae form. Of the daughter sporocysts, cercariae are introduced into the respiratory cavity of the mollusk, where they are covered with sticky mucous substance. The formed mucous lumps are pushed by the respiratory movements of the mollusk into the external environment. Infection of ants occurs when eating these mucous balls. Once in the intestinal tract, cercariae enter the ant's abdominal cavity, where they turn into metacercariae. The definitive owners infect on pastures, swallowing invasive ants along with grass.

Original long-term studies of the authors on the sheep infestation by D. filaria in conditions of pronounced vertical zonal character of climate and landscape belts (300–2000 m above sea level) have shown a presence of wide infestation among all age groups of the animals: in lambs up to 60%, in young sheep up to 57% and in adult sheep up to 45%. Dictyocaulus is also present in moufflons and bezoar goats in Armenia. The main reasons for such a wide infection are the following: • a monoxenous character of D. filaria life cycle, i.e. the parasite being a geohelminth; • its survivability in wide range of biotic conditions; • an insufficient volume of planned prophylaxis measures against dictyocaulosis. A study of seasonal and age dynamics on author's own and literature data was also performed. Dynamics of infestation of lambs with Dictyocaulus is in both lowlands and mountain zones characterized with 2 peaks: summer and autumn ones (prevalence reaching 29.0% for lowlands, 42.0% for highlands in contrast to 5.7% and 2.8% respectively in the spring) with no invasion at start of the year. For young and adult sheep, dynamics of invasion with Dictyocaulus is characterized with high prevalence in spring and autumn periods (start of the year prevalence up to 45.0%, decrease to 15–25.0% in summer, rising to 40.0% and higher in October-December).