Hydromechanical considerations on the origin of the pentaradial body structure of echinoderms

When echinoderms are conceptualized as hydraulic entities, the early evolution of this group can be presented in a scenario which describes how a bilateral ancestor (an enteropneust-like organism) gradually evolved into a pentaradial echinoderm. According to this scenario, the arms are outgrowths from the anterior/ posterior body axis of the bilateral pterobranchia-like intermediate. These outgrowths developed when the originally U-shaped mesentery of the intestinal tract formed loops, and correspondingly, the tensile chords of the mesentery were attached to the body wall in five loops. The wall faces between these regions of tensile chords could bulge out under the hydraulic pressure of the body coelom. The originally more or less round body cavity was deformed into a pneu with five bulges. The loops of the gut forced a roughly symmetric arrangement, which was enhanced by a physical fact: five pneus as well as one pneu with five internal tethers, naturally adopt a pentaradial pattern of ‘minimum contact surfaces’, as the most economic arrangement. This part of the body became shorter along its longitudinal axis, but gained volume by the five bulges. The bulges came in contact with the tentacular crown, placed directly in front of them, and began to grow under the tentacle bases. The more the bulges extended along the tentacles, forming the so-called arms, the better was the mechanical support for the tentacles, which are completely integrated in the arms. Only the paired end-branches of the tentacles project and the pentaradial ambulacral system with many ambulacral podia was the result. These evolutionary transformations were accompanied by certain histological modifications, such as the development of mutable connective tissues and skeletal elements that fused to ossicles and provided shape stabilization in the form of a calcareous skeleton in the tissues of the body wall. The resultant organism was an ancestral eleutherozoan echinoderm (Ur-Echinoderm) with an enlarged metacoel stabilized by hydraulic pressure working against a capsule of mutable connective tissue, skeletal elements and longitudinal muscles.