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 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.

Melatonin is a derivate of biogenic amine of serotonin identified in all classes of animals including flatworms. Melatonin demonstrates different physiological functions the main of which is circadian rhythm regulation. Via specific G-protein coupled receptors, melatonin affects the target cells changing the levels of other hormones. On early stages of embryonic development, biogenic amines as well as melatonin play a role of specific signal cell molecules that regulate processes of cellular renewal. This work has studied physiological function of melatonin in free-living flatworms, planarian Schmidtea mediterranea. The influence of melatonin on diurnal dynamics of stem cells proliferation was investigated using an immunocytochemical method and confocal laser scanning microscopy. The specific antibodies against H3 phosphohistones were applied for immunocytochemical identification of proliferative cells. It was shown that melatonin (1 µМ) decreased the total number of proliferative cells in planarians. It was also found that the diurnal dynamics of cells proliferation in planarians was changed by melatonin: regular rhythmic oscillations observed in the control group of animals were smoothening. Further researches are required to clarify mechanisms of melatonin actions.

Serotonin has been identified in all classes of parasitic and free-living Platyhelminthes. Its function in the body of Platyhelminthes remains poorly studied. In present work, the serotonin effect on morphogenetic processes in planarian Schmidtea mediterranea was studied for the first time. Pharyngeal regeneration was studied in Schmidtea mediterranea planaria. Pharynx is a relatively autonomous organ with well differentiated morphological structure and specialized function. The dissected planarian body fragments were able to regenerate the new pharynx and restore its function – food uptake and feeding. It was observed that in group of animals, the pharynx regeneration occurred from day 5 to day 10 after amputation. The regeneration time varied in different experimental series and seems to depend on the season of the experiment. The maximum number of animals restored their feeding response for 6–8 days after the intervention. Under the serotonin treatments (10–0.001 µМ), the acceleration of the restoration of a new pharynx function has been observed as compared to the control animals. The preliminary results exhibit the stimulatory action of serotonin on pharyngeal regeneration in tail fragments of S. mediterranea, thus, indicating the morphogenetic properties of serotonin.

The serotonergic components in the nervous system of planarian Girardia tigrina was shown by immunocytochemical method. The whole-mounts are used to extract the maximum information and conduct a quantitative morphometric analysis of serotonin-immunopositive elements in different body regions. The planarian nervous system is represented by cephalic ganglion in the anterior body part and a pair of well-defined ventral nerve cords running along the body. The density of serotonin components is greatest in the head region, which reflects in thickness of cephalic ganglion arch (123–94 µm) and of nerve cords, gradually decreasing from head (111–97 µm) to middle (83–42 µm) and tail (64-28 µm) zones. Nerve nodes contain 4–10 serotonergic neurons in different body regions and connected by a few transverse commissures with a distance from 70 to 145 µm. Results supplement the available data and can be useful for comparative analysis of serotonergic components in free-living and parasitic flatworms.