The serotonergic components in the nervous system of the planarian Polycelis tenuis were studied by the indirect immunocytochemical method for staining frozen tissue sections and their analysis using a fluorescence microscope. Morphometric measurements were conducted on microphotographs taken from stained serotonin immune-positive sections using a digital photo camera. The P. tenuis central nervous system is represented by a pair of cerebral ganglia connected by a commissure having together a butterfly-like shape and prominent ventral nerve cords that lie along the whole planarian’s body. Each cerebral ganglion had a size of 132–310 µm on serial sections. From 12 to 17 serotonin components were visible in one ganglion on one tissue section. The thickness of the nerve cords was 138 to 60 µm in different areas of the body. Nerve knots located along the way of the ventral nerve cords had 5–6 up to 8 serotonergic neurons of 11 to 23 µm. The obtained results can be useful for comparative analysis of the nervous system in free-living and parasitic flatworms to define their evolutionary development aspects.

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 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.