Development and Structure of the Nervous System in Phoronids: Evolutionary Significance
The development and structure of the nervous system are traditionally used for phylogenetic analysis and reconstruction of evolutionary events. Phoronids are an unusual group of protostome animals whose organizational and developmental characteristics are similar to those of deuterostomes. This article presents a brief description of the development and structure of the nervous system in phoronids based on published results. Analysis of existing data suggests that the nervous system in the common ancestor of the Bilateria included both a nerve center and a nerve plexus. It is important to note that the nerve center in the ancestors of the Bilateria is homologous to the aboral organ of radially symmetrical blastulas and gastrulas of contemporary members of this group, while the apical organ of bilaterally symmetrical ciliated larvae appears de novo in living organisms in plankton, maintaining a connection with parental individuals and initiating metamorphosis. For this reason, the apical organ disappears during metamorphosis in most protostomes and all deuterostomes.