A curling structure for use in mechanical swimming creatures and the like includes a series of jointed segments pivotally connected together, with cross members pivotally connected between non-adjacent segments. Each cross member has its first end pivotally connected to a given segment above the pivot point connecting that segment to an adjacent segment, and its second end connected to a non-adjacent segment below the pivot point which connects that non-adjacent segment to the next segment. The result is that as one segment pivots about the joint connecting it to an adjacent segment, the cross members cause all of the segments to curl in a single direction.