ИСТИНА

Although molecular phylogenetic analysis suggests the position of phoronids among Spiralia, this inference is not supported by any morphological and embryological data. To clarify the phoronids position among Bilateria, we investigated the development of serotonergic (5HT) and FMRF-amidergic (FMRFa) elements in embryos, larvae, metamorphic animals, and juveniles of Phoronopsis harmeri. First neurons of both types (5HT and FMRFa) appear in the epidermis of the apical plate. Young larva has ventral nerve cord, which exhibits both 5HT and FRMFa immunoreactivity and consists of paired parikarya, two longitudinal neurite bundles, and repetitive commissures. In competent larva, 5HT nervous system consists of the apical organ, the hood median neurite bundle with frontal organ, two marginal nerves of the hood, the minor and the main nerve rings, each of which penetrates into each tentacle by two branches, nerurites and perikarya of the trunk and the metasomal sack, and the telotroch nerve ring. FRMFa nervous system includes the same elements plus the oral nerve ring, nerves of the midgut, and the neurites around the anus. The apical organ has complex organization including the presence of 40 flask shaped serotonergic perikarya. Most elements of 5HT and FRMFa nervous system can be traced through metamorphosis, but all nervous elements of the hood are destroyed within first minutes of metamorphosis. Juvenile nervous system is completely inherited from larva. Phoronids exhibit both deuterostome- and trochozoan-like features of the neurogenesis, therefore we suggest their position in the basis of the Lophotrochozoa. The remodeling of the nervous system during metamorphosis corresponds to the appearance of a new body plan. This work was supported by RFBR 11-04-00690, 12-04-33045.