  Cardiac tissue and the Belousov-Zhabotinsky reaction provide two notable
examples of excitable media that support scroll waves, in which a filament core
is the source of spiral waves of excitation. Here we consider a novel
topological configuration in which a closed filament loop, known as a scroll
ring, is threaded by a pair of counter-rotating filaments that are
perpendicular to the plane of the ring and end on the boundary of a thin
medium. We simulate the dynamics of this threaded ring (thring) in the
photosensitive Belousov-Zhabotinsky excitable medium, using the modified
Oregonator reaction-diffusion equations. These computations reveal that the
threading topology induces an exotic motion in which the thring swims in the
plane of the ring. We propose a light templating protocol to create a thring in
the photosensitive Belousov-Zhabotinsky medium and provide experimental
confirmation that this protocol indeed yields a thring.
