The major skeletal structures (test, spines, Aristotle's lantern) of a common regular sea urchin Paracentrotus lividus were analyzed for Mg-calcite composition to determine their relative vulnerability to ocean acidification. Percentage of MgCO3 in the test and several sub-components of the Aristotle's lantern were generally similar to one another (mean range = 10.1-11.0 mol % MgCO3), and would be the most vulnerable to partial dissolution. Also vulnerable would be the teeth (mean = 8.8 mol % MgCO3) of the lantern. Primary spines (mean = 3.7 mol % MgCO3) are potentially the most resistant to dissolution, with significantly lower MgCO3 percentages than all other skeletal structures. Our results suggest that in near-future predicted levels of ocean acidification adult regular echinoids will retain their ability to defend themselves with their spines. However, feeding efficiency may be compromised should dissolution weaken the lantern and its associated teeth, decreasing grazing efficiency and/or increasing the energy investment of tooth replacement.