In the regular echinoid Diadema setosum Leske the central ligament that connects the spine to its tubercle is mainly composed of closely-packed collagen fibres which are arranged in the longitudinal axis of the ligament. The mechanical properties of the ligament are quite different from ligament to ligament: the viscosity determined by creep tests ranges from 0·02 to 6 GPa·s. The viscosity of a ligament changes reversibly in response to stimulation. Adrenaline and noradrenaline (10-6-10-3 M) decrease the viscosity. Acetylcholine (10-8-10-3 M), artificial sea water containing high (100 MM) potassium concentration, and electrical stimulation increase the viscosity. As the ligament contains no muscle cells in it, the viscosity change cannot be attributable to muscle activities. The functional significance of having a central ligament with variable and controllable viscosity is that it binds the spine base onto the articulation surface of the tubercle, so as to reduce the possibility of spine dislocation, while being flexible enough to allow the spine considerable freedom of movement when necessary.