A hexamer origin of the echinoderms’ five rays

de Lussanet M H E

Abstract

Of the major deuterostome groups, the multiple forms of echinoderms are the most mysterious. Although larval echinoderms are bilaterally symmetric, the adult body seems to lack bilateral symmetry. The prevalent pentamer structure, the asymmetry of Lovén's rule, and the variable location of the periproct and madrepore, have placed enormous difficulties to homologizing structures across the groups despite the excellent fossil record. This irregularity apparently projects echinoderms far outside the other deuterostomes. Here I propose that the pentaradial structure might be derived from a hexamer structure. This hypothesis demonstrates that the adult echinoderm body may be derived from a true bilateral symmetry and thus firmly rank the adult echinoderms among the bilaterians. Furthermore, I show that Lovén's rule can be predicted if one assumes that one ray is ``lost'' from the symmetry by the formation of the adhesive pit. The proposal is consistent with the extraxial-axial theory. The theory provides fundamental insight into the Von Ubisch', Lovén's and Carpenter's axes. Finally, in line with the current hypothesis, it can be understood how the varying positions of the madrepore develop.

Projects the publication originates from