ABSTRACT
Because our own bodies are bilateral, with functional distinction between left and right, and dorsal versus ventral, we tend to consider symmetry as a basic character. With regard to echinoderms, this is not the case. True, modern forms are mostly radially symmetric with five radii, but all start with bilateral larvae. As the fossil record shows, post-Paleozoic (including modern) echinoderms present only a minute sector of the possible echinoderm bauplans. This chapter is concerned with alternative echinoderm constructions which all vanished in the end-Paleozoic mass extinction. It also deals with some of the special structures these groups evolved in order to overcome constraints of their unique construction.
