The skeletal mechanics of Astenas forbesii are examined in order to erect a theoretical framework for the evaluation of starfish skeletal systems. Guided by engineering theory, the skeleton is dissected into its functional parts and an approximation made of the stresses sustained by each. All relevant mathematical operations are demonstrated. It is shown that the whole body acts as a unit sustaining compressive forces across its oral surface and tensile stresses across its aboral surface. Two points of maximum stress are identified: the aboral ridge of the ray close to the disk and the inferior transverse amhulacral muscle (ITAM). Structural stability is conferred upon the former by virtue of its position in the body and upon the latter by the inclusion of strong tensile fibres in parallel with the muscle.