ИСТИНА

Ciliated tentacles are known in different groups of protostomes and deuterostomes. Such a distribution of tentacles within bilaterians and their presence in lower metazoans allow to suggest the existence of tentacle apparatus in the last common bilaterian ancestor (LCBA). Although recent palaeontological and molecular data evidence the simple organization of the LCBA, the ideas about complex LCBA that had tentacles are still discussed. In phoronids, early larva firstly acquires the specialized postoral ciliated band, which in advanced larva extends along the laterofrontal sides of tentacles. Tentacles together with the preoral ciliated band are used for collection of food particles from the water. Both the preoral and postoral ciliated bands beat from anterior to posterior. In metamorphosis, the fate of larval tentacles may be different. In some phoronid species, transformation of larval tentacles is expressed in destruction of the postoral ciliated band; in others, larval tentacles are completely consumed by the juvenile and the definitive tentacles form de novo or from the small basal anlages that arise at larval stage. In metamorphosis, nervous, muscular, and coelomic systems of tentacle apparatus undergo great changes. Experiments with the BrDU show that in competent larva and in juvenile, the most active proliferative zones are located along the postroral ciliated band and at the tentacle basis. The transformation of larval tentacle apparatus in metamorphosis may correlate with the changes of life style from planktonic to benthic. It is likely that the transformation of larval tentacles in metamorphosis reflects some steps of the evolutionary history of phoronids and possibly all bilaterians, whose the last common ancestor initially used the tentacles apparatus to collect food on the substrate, and then this apparatus was transformed into the ciliated bands of bilateral pelagic larvae. This work is supported by Russian Science Foundation (18-14-00082).