Pentaxonic Body Plan in Adult Living Echinoderms

Adult living echinoderms have a unique pentaxonic (pentaxial: Turner, 2024) body plan and growth pattern. Following metamorphosis of the monaxonic larva (dipleurula type), a single, united pentameric body disk, which includes the ring central nervous system and patterned by the genomic program of anterior development, emerges first. Then the growth and segmentation of the five rays/arms as five body trunks begin, patterned mainly by Hox genes. Each of the rays-trunks exhibits the canonical features of the bilaterian axial body plan: bilateral symmetry, posterior growth, pre-posterior localization of the growth zone, and metamery. It is assumed that the successive increase in the number of body axes (1 → 2 → 3 → 5) in the echinoderm phylogeny was transformed in the chain of ontogenies into the simultaneous emergence of the radial pattern of five axial organizers with primordial growth zones.