The Ambulacral System and Body Plan in Extant Adult Echinoderms

This review examines data on the emergence and transformation of the ambulacral system associated with the transition from ancestral bilaterality to pentameral symmetry in adult extant echinoderms. It is assumed that this macroevolutionary transformation of the body plan included the loss of the ancestral pharyngeal gill slits, heterotopy of the mutual position of the oral opening and the pharynx resulting in the emergence of an ambulacral primordium as part of the preoral ectodermal epithelium while maintaining the pharyngeal ability to transport nutrients. Duplication of the primordium with the transition through trimery to pentamery increased the number of body axes in the sequence 1 → 2→ 3 → 5. The expansion of ambulacral system and intensification of its functional activity including food-gathering ensured a significant increase in adaptability and evolvability resulting in evolutionary success of extant echinoderms.