A new system of skeletal homologies for echinoderms is presented. The EAT (extraxial-axial theory) recognizes two major components of the body wall: axial elements are arranged in accordance with specific ontogenetic rules operating in association with the water vascular system: and extraxial elements. Embryological evidence demonstrates that axial and extraxial elements are of different origins. The explanatory power of the EAT is tested by deciphering the architecture of extreme morphologies and teratological phenomena. Several implications of the EAT for our interpretation of the phylum and its evolution are analyzed. (1) The interambulaera of echinoids are not homologous with those of other echinoderms, and echinoids are without doubt among the most atypical of echinoderms. (2) The marginal rings of asteroids and edrioasteroids are not homologous. (3) There are different types of "arms": those of crinoids and asteroids are homologous, and consequently the pelmatozoan grouping is undetermined. (4) The stylophorans are not early echinoderms, but appear to be closely related to the crinoids. (5) Pentameral symmetry is superimposed on a fundamentally linear arrangement which is comparable to that of most other groups of organisms.