  We study the dynamics and interaction of coaxial vortex rings in the
FitzHugh-Nagumo excitable medium. We find that threading vortex rings with a
vortex string results in significant qualitative differences in their evolution
and interaction. In particular, threading prevents the annihilation of rings in
a head-on collision, allows generic ring overtaking, and can even reverse the
direction of motion of a ring. We identify that an important mechanism for
producing this new behaviour is that threaded vortex rings interact indirectly
via induced twisting of the threading vortex string.
