ИСТИНА

Development and organization of the nervous system is traditionally used for phylogenetic and comparative analyses that are very important to establish relationships between different taxa. The phylum Phoronida is a monophyletic group, which is characterized by unique body plan. Phoronids are traditionally regarded as relatives of deuterostomia. At the same time, phoronids and other lophophorates are recently united with typical protostomia. This contradiction may be reconciled with the help new data on organization of the nervous system. The nervous system at consecutive stages of embryonic development, larvae, metamorphic animals, and adults is studied by immunocytochemistry, laser confocal microscopy, and transmission electron microscopy. Serotonin-like flask-shaped cells such as the ones described herein for Phoronopsis harmeri are found in the majority of lophotrochozoan larvae and therefore most likely belong to the ground pattern of the last common lophotrochozoan ancestor. The overall organization of the nervous system in phoronid larvae has more in common with the deuterostomian than with the protostomian nervous system. The metamorphic remodeling of the phoronid nervous system occurs in two different ways: with complete or incomplete destruction of organ systems. The nervous system of the phoronid juvenile is organized as an epidermal nerve plexus but exhibits a nerve center in the anterior portion of the body. The simultaneous presence of both the apical organ and anlage of the cerebral ganglion in phoronids at the larval stage, and the reduction of the apical organ during metamorphosis support the Trochea theory and allow to suggest the presence of two nervous centers in the last common ancestor of the Bilateria. In adult phoronids, all nerve elements have a similar cytological organization and stratified structure that is related to the cellular layers. The internal layer contacts the basal lamina and is formed by numerous nerve fibers surrounded by the glial cell processes. In the neuropil of the dorsal ganglion, most neurites have large diameters and electron-light cytoplasm, whereas in the tentacular nerve ring neuropile most neurites have small diameters and dense cytoplasm. The bodies of glial cells constitute the second layer. The third layer consists of perikarya overarched by the bodies of epidermal cells. This stratified structure is penetrated by basal projections of epidermal cells, which contain thick bundles of tonofilaments. According to our preliminary observations of adult P. harmeri, neurites and perikarya that demonstrate serotonin- and FMRFamide-like immunoreactivity occur in the nerve tracts of the tentacle, tentacular nerve ring, dorsal ganglion, trunk nerve plexus, and esophagus. The giant nerve fiber exhibits neither serotonin-like nor FMRFamide-like immunoreactivity. Because phoronid larvae have very complex organization of the nervous system, which consists of numerous elements and exhibits some deuterostome-like features, it is more plausibly to suggest that this pattern of the nervous system was inherited from the last common bilaterian ancestor and then underwent simplification correlated with a sessile life style of adult phoronids.