Symmetry in body form has often been used to infer evolutionary relationships, and is assumed to be stable within taxa. During the Vendian, the earliest jellyfishes were characterized by a wide range of symmetries, including several triradial species. Since at least the Ordovician, both classes of medusae (hydrozoan and scyphozoan) have been characterized by strong tetraradiality, although approximately 2-10% of species and individuals have some other symmetry. In the five scyphozoan species I have studied, this variation arises at the clonal level; in some cases no two clonemates share the same symmetry. In order to test why stasis has been observed for 500 million years despite the high rate of plasticity, I have focused on three hypotheses. H1: development is canalized for tetraradiality. H2: selection is responsible for maintaining tetraradiality. H3: tetraradiality is simply expressed by chance alone. Hypothesis 3 can be rejected because approximately 95% of the extant jellyfish species are characterized by four-partedness, indicating a definite pattern. However, we can use the other side of the same argument in rejecting Hypothesis 1, because 36 species in different clades are not tetraradial. Rejection of Hypothesis 1 is further supported by the high rate of variation that occurs in natural populations and among clonemates. For Hypothesis 2, preliminary analysis indicates possibly higher mortality for non-tetraradial individuals, indicating capacity for selection for tetraradiality. These results provide an interesting comparison with the pentaradial echinoderms, a body plan thought to be the optimum design for moving large numbers of food particles to the central mouth.

75/125 YEARS