EMBRYOLOGY SUPPORTS A NEW THEORY OF SKELETAL HOMOLOGIES FOR PHYLUM ECHINODERMATA

The new axial/extraxial theory for skeletal homologies in the Echinode rmata recognizes 2 major components in the body wall: axial elements a ssociated with the water vascular system that follows the ocular plate rule, and extraxial elements. Sea urchins comprise an appropriate mod el depicting major events in development: (1) the hydrocoel produces 5 primary lobes that expand into the radial water vessels; (2) larval e ctoderm, the primary lobes and the somatocoel interact to form the rud iment; (3) the rudiment everts as the larval structures are resorbed. We show that axial and extraxial elements display radically different origins: axial elements are formed in association with the rudiment wh ereas extraxial elements are inherited from the rest of the larval bod y. Other echinoderms follow the same generalized development trajector y leading to 2-part somatic construction. Application of the theory im plies radical reinterpretation of holothurians whose radii can no long er be considered homologous with those of other echinoderms.