In common with a number of other echinoderm collagenous structures, the intervertebral ligament ofOphicomina nigrabehaves like a viscous fluid when stressed by a constant load. Behaviour under constant load is affected by potassium ions which increase the rate of extension and cause rupture. Since this response is specific to potassium and is blocked by anaesthetics, it is concluded that it occurs by nervous mediation. This supports the view that brittlestar arm autotomy involves a nervously mediated change in the mechanical properties of the intersegmental connective tissues. The mechanical behaviour of the ligament is sensitive to ambient pH and divalent cations. It is suggested that changes in mechanical properties may be produced by the availability of calcium ions within the interfibrillar matrix of the connective tissue. The ophiuroid intervertebral ligament is another example of an echinoderm collagenous structure whose mechanical propertiesare subject to nervous control, a phenomenon which on present knowledge is unique to the Echinodermata.