ИСТИНА

Because ctenostome bryozoans retain some ancestral features of organization, the investigation of their nervous systems may contribute to the reconstruction of the bryozoan nervous system bauplan. A detailed investigation of the polypid nervous system of the ctenostome bryozoan Amathia gracilis is reported here. The cerebral ganglion displays prominent zonality and has at least three zones: proximal, central, and distal. The neuroepithelial organization of the cerebral ganglion is revealed. The cerebral ganglion gives rise to five groups of main neurite bundles. Based on the characteristics of their innervations, the tentacles can be subdivided into two groups: four that are near the anus and six that are near the mouth. Two longitudinal neurite bundles—medio-frontal and abfrontal—innervate each tentacle. The zonality of the cerebral ganglion, the presence of three commissures, and location of the main nerves emanating from each zone might have resulted from the fusion of the three ganglia of the three body parts in the bryozoan ancestor: epistomal, lophophoral, and truncal. The presence of a remnant lumen in the cerebral ganglion of A. gracilis supports the idea of its ancestral state among all bryozoans. The presence of lophophoral dorso-lateral nerves, described here for the first time, suggests the reduction of the complex ancestral horseshoe-like lophophore within bryozoans. The six nerves within each tentacle is the ancestral state for all bryozoans. The presence of the outer nerve ring makes the lophophore innervation within the group (clade) of lophophorates similar and provides morphological evidence of the lophophorates monophyly.