  Biological cells and many living organisms are mostly made of liquids and
therefore, by analogy with liquid drops, they should exhibit a range of
fundamental nonlinear phenomena such as the onset of standing surface waves.
Here, we test four common species of earthworm to demonstrate that vertical
vibration of living worms lying horizontally of a flat solid surface results in
the onset of subharmonic Faraday-like body waves, which is possible because
earthworms have a hydrostatic skeleton with a flexible skin and a liquid-filled
body cavity. Our findings are supported by theoretical analysis based on a
model of parametrically excited vibrations in liquid-filled elastic cylinders
using material parameters of the worm's body reported in the literature. The
ability to excite nonlinear subharmonic body waves in a living organism could
be used to probe, and potentially to control, important biophysical processes
such as the propagation of nerve impulses, thereby opening up avenues for
addressing biological questions of fundamental impact.
