In response to amputation, we found that instead of regenerat- ing the lost parts, Aurelia ephyrae reorganize existing parts, and regain radial symmetry (Fig. 1C–E). This process of reorganiza- tion, which we call symmetrization, is fast, often completed in less than 2 days, and occurs at high frequency (∼90%). In amputations that leave a large part of the body (Fig. 1D), symmetrization takes longer. In amputations that leave only part of the arms (Fig. 1E), the remaining arms are often resorbed before symmetrization begins. Symmetrization seems to be advantageous, since ephyrae that symmetrized continued growing into symmetrical medusae, while ephyrae that did not symmetrize were unable to swim, and devel- oped abnormally.