Based on the literature and own data, consecutive stages of development of the central nervous system (CNS) in the lower Bilateria are considered-separation of brain from parenchyma, formation of its own envelopes, and development of the trunk and orthogonal nervous system. Results of histochemical (cholinergic and catecholaminergic) and immunocytochemical (5-HT- and FMRF-amid immunoreactive) studies of the CNS in representatives of Acoela, free living and parasitizing Platyhelminthes and Rotifera are considered. The comparative analysis makes it possible to describe development and complication of the initially primitive Bilateria plexus nervous system. A special attention will be paid to the Acoela phylogenesis, based on molecular-biology data and results of study of their nervous system.