Formation of pentameria and axial symmetry in the evolution of echinoderms

The formation of pentaradial symmetry in the evolution of echinoderms was based on the possibility of the middle–left coelom to terminally forward growth along the anteroposterior axis and the appearance of a second growth vector along the left–right axis during the replication of the formed ambulacra. Both growth vectors were realized into the pentamerism of modern echinoderms due to the development of coelom asymmetry and subsequent torsion associated with the attachment of the larva to the ground by the anterior end of the body. In this process, the molecular genetic mechanisms of anteroposterior growth and left–right regulation, common to bilateria, and associated with the genes of the Wnt, BMP, Nodal signaling cascades, and Hox system genes, were probably used together. In the process of replication of channels extending from the ambulacral ring, the emerging ambulacral system was the organizer of the symmetry of the skeleton and the nervous and muscular systems. Replication in many fossil echinoderms ended on the three channels extending directly from the ambulacral ring. In crinoids, sea urchins, sea stars, brittle stars, and holothurians, the second stage of the formation of a more perfect five-ray symmetry of the ambulacral ring with five radial canals extending from it appeared, associated with a shift in ontogenesis of the branch point to the early stages of hydrocoel development.