From Bilateral Symmetry to Pentaradiality: The Phylogeny of Hemichordates and Echinoderms

Abstract

Nested within the clade of bilaterally symmetrical animals, variously called the triploblasts or the bilaterians, lies a most unusual group. Although most bilaterians have a bilaterally symmetric body plan, with a clear anterior–posterior axis and in most cases a differentiated head region, the echinoderm adult is constructed on a pentaradiate plan and lacks an obvious anterior–posterior axis (e.g., figs. 22.1, 22.6, and 22.8). Yet echinoderms clearly start out life as bilateral organisms, and their peculiar body plan is a secondary modification that arises during the metamorphosis that transforms them from larva to adult. It is because echinoderms are so very different in appearance from their closest relatives, the hemichordates, that they provide a fascinating and important group for evolutionary and developmental studies. Based on their pattern of development, both echinoderms and their bilateral relatives the hemichordates clearly fall among the deuterostomes. Until comparatively recently, five major groups (Echinodermata, Hemichordata, Chordata, Lophophorata, and Chaetognatha) were considered to be deuterostomes. However, molecular evidence now overwhelmingly suggests that only the echinoderms, hemichordates, and chordates belong together (Adoutte et al. 2000, Cameron et al. 2000, Giribet et al. 2000, Peterson and Eernisse 2001, Winchell et al. 2002).