ИСТИНА

Scolex most cestodes has a double-beam radial symmetry and, by default, it is assumed that the cerebral ganglion of cestodes has a symmetry corresponding to the symmetry of the scolex. The study of nervous system in Ligula intestinalis (Diphyllobothriidae) occur an unpaired ventral lobe in the brain in addition to symmetrical pairs of frontal and lateral lobes. The pair of lateral lobes is connected by the massive median commissure; the ventral azygos lobe locates in the center and on the ventral surface of the commissure. Two crossed ventral nerves are originated from the ventral lobe; also lateral lobes send two ventral brain rootlets. Localization of large neurons in the lateral lobes and the commissure has dorsoventral polarity. Thus, the brain of L. intestinalis possesses definite bilateral symmetry. The rate of asymmetry and organization of the brain differs significantly in Cestoda orders. The brain architectonics of Parachristianella sp. (Trypanorhyncha) has traces of bilateral symmetry in organization of the central lobe; the ganglion of Triaenophorus nodulosus (Bothriocephalidea) possesses a double-beam radial symmetry. The radial symmetry is developing in the attached organisms, leading a sedentary lifestyle (Beklemishev, 1964). Cestodes ancestors, free-living flatworms possess a definite bilateral symmetry and dorsal and ventral surface. Contrary to the opinion of the primitivity of the radial symmetry, the structure of the cestodes brain indicates that the radial symmetry of the scolex is a secondary specialized device to the attached lifestyle. The presence of bilateral symmetry in the brain is archaic and the original sign and confirms the origin from bilaterian ancestors.