Hydromechanical considerations on the evolution and diversification of the echinoderm bauplans

The pentaradial organisation of echinoderms is postulated to have evolved as the result of the reorganisation of the internal U-shaped mesentery of the intestinal tract during inflation of the trunk of a pterobranchlike ancestor. Under this scenario, loops of the mesentery developed between five hydraulic bulges by three different mechanisms: (1) by the early formation of two additional loops resulting directly in five hydraulic bulges, (2) by the subsequent formation of two additional loops resulting in the formation of initially three and then five hydraulic bulges or (3) by the inflation of the body without formation of loops. Accordingly there are at least three main evolutionary pathways within the echinoderms. The anatomical structures, such as the ambulacral system or the skeletal capsule, which characterize the echinoderms, developed independently in both lineages. From all the three pathways various body structures can be derived matching those found in fossil and extant echinoderms. The eleutherozoans and the earliest pentaradial echinoderms most likely evolved from the direct-pentaradial pathway whereas triradiate echinoderms and those which show a pentaradial organisation superimposed on triradial symmetry, likely evolved along the indirect pathway. The asymmetric or bilateral symmetric echinoderms evolved from the third paythway. The most important morphological transformations leading to the directly and indirectly pentaradial echinoderms are discussed