The muscles of this echidna have a number of unusual features. There is an enormous muscle that is just beneath the skin and covers the entire body. 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. It has one of the shortest spinal cords of any mammal, extending only as far as the thorax.