Extracellular electrodes attached to the radial nerve cord of the ophiuroid Ophiura ophiura (L.) recorded unitary potentials in response to a range of mechanical stimuli. Detection within the mechanical mode extended from the near field region of a vibrating source, through the far field propagated pressure wave, to the movement of the aquatic medium as a whole in the form of water flow. Experiments on the behaviour of the ophiuroid in response to the same mechanical stimuli are described. Unitary potentials were also recorded from the radial nerve cord to movement of individual lateral arm spines. The removal of the epithelium and cuticular structures from the arm spines and the subsequent inability to diminish the recorded responses to spine movement, implied the presence of an interoreceptor within the arm spine. It is proposed that movements of the arm spines which are detected by the interoreceptors are responsible for the detection of mechanical stimuli. The possibility that the arm spines in this ophiuroid function in proprioception and orientation is also discussed.