By contraction of various parts of this muscle the Short-beaked Echidna can change shape . The most common shape change is achieved by rolling itself into a ball when threatened , protecting its belly and presenting a defensive array of sharp spines . By contraction of various parts of the panniculus carnosus , the Short-beaked Echidna can change shape , the most characteristic shape change being achieved by rolling itself into a ball when threatened , so protecting its belly and presenting a defensive array of sharp spines .