An illustration combining two sets of diagram shows how the muscles and joints of a vertebrate, endoskeleton and an anthropod, exoskeleton work. The first set of diagram belongs to endoskeleton shown in bend and straight form. The following are marked on the endoskeleton diagram: Flexor (hamstring) contracts Joint flexed Extensor (quadriceps) contracts Joint straight Second set of diagram belongs to Exoskeleton shown in bend and straight form. The following are marked on the exoskeleton diagram: Joint Exoskeleton Flexor muscle contracts Joint flexed Joint Exoskeleton Extensor muscle contracts Joint straight