Measurements of test, lantern, and protractor muscle dimensions in different representative sea urchin species.

These values are based on 3D MRI scans of a single specimen per species and size group. The muscle parameters were measured at the mid-level of each of the ten protractor muscles and then averaged. In case of the frilled protractor muscles, the calculations of muscle area, volume, and surface area are based on the presence of five lobes on average. The surface area of the protractor muscle is that oriented towards the interradial cavity. The values for protractor muscle volume and surface area (last two lines, in bold) served as the basis for the conclusions presented in Fig. 7.