<p>The functional activity and morphological characteristics of the musculature of planarians have been studied. A physiological study conducted on isolated muscle cells of the planarian <i>Procerodes littoralis</i> (Turbellaria, Platyhelminthes) showed that caffeine in concentrations of 1–10 mM and depolarization caused by a high content of potassium ions (20–75 mM) had a dose-dependent stimulatory effect on the musculature of planarians. Tapsigargin and cyclopiazonic acid, which are calcium reuptake blockers in the sarcoplasmic reticulum depot, reduced the number of muscle responses induced by both caffeine and depolarization. In addition, contractions of single <i>P. littoralis</i> muscle fibers induced by a medium with a high content of potassium ions were inhibited by ryanodine, which in mM concentrations is a blocker of the ryanodine calcium channels of the sarcoplasmic reticulum. The data we obtained suggest that intracellular calcium depots, as an intracellular source of calcium ions, play a role in the mechanism of contractile muscle response in flatworms. Histochemical examination using frozen sections and staining with fluorescently labeled phalloidin demonstrated the conservative organization of the body wall musculature of <i>Schmidtea mediterranea</i> planarians. The musculature of the body wall contains circular, diagonal, and longitudinal muscle fibers. The thinnest circular fibers are densely packed in the outer layer of the musculature. The longitudinal fibers are the thickest and are mostly assembled in pairs. The diagonal fibers extend in two directions at an angle of 110–115 degrees to each other. Dorso-ventral muscle bundles penetrate the planarian body. The intestinal lumen is surrounded by thin longitudinal muscle fibers. These experiments have shown that planarians can be successfully used as a model object for studying the mechanisms of muscle contraction in flatworms.</p>