The mechanical properties of the aboral body wall of the starfish Coscinasterias calamaria were analysed by uniaxial tensile tests. All the load curves were J‐shaped. The load curves were divided into 6 segments: tail, toe 1, toe 2, linear, damage and fracture. Video and photo analysis of body wall tissue undergoing stress‐strain tests revealed that each segment of the load curve could be differentiated by its load‐bearing mechanisms. Stretching of the interfibrillar proteoglycan matrix dominated the tail segment, shear of collagen fibre tracts through the matrix characterised the toe segments and direct loading of the collagen fibres and movements of the skeletal ossicles marked the linear segment. Collagen fibres began to fail in the damage segment. The average stiffness calculated from the linear portion of the curves was 10.6 MPa; the average ultimate tensile stress was 3.3 MPa. The stiffness computed for each segment of the curve was not strain‐rate dependent. The stiffness of the linear segment was the best predictor of breaking stress, while breaking strain was inversely related to the stiffness of the tail segment.