Comparative characterization of the nervous system of the Turbellaria

Initial stages of the centralization of the nervous apparatus in the Turbellaria can be traced through a comparison of the structure of the nervous system in various representatives of the class. The most primitive state, found in the Acoela, is predominantly plexiform with a varying number of longitudinal trunks. Three, and in some cases four, longitudinal trunks are found in the Proseriata and Temnocephalida. Commissures appear in the Macrostomida and all higher orders and form an orthogon. Brain shape varies from ring-shaped in the Acoela to bilobed in the Neorhabdocoela. While the nervous system of the Polycladida is peculiar, having numerous lateral trunks and separation of dorsal and ventral parts of the nervous system, the development of the nervous system in Müller's larvae of polyclads shows it is of an orthogonal type comparable to other platyhelminths. Transition to parasitism is accompanied by some progressive transformations in the structure of the nervous system.