From Bernard Perier's Zkwyx Meccanaut book.
Grubby is a twisting, metal-eating worm from the planet Hostilia. He is powered by an MR motor. This drives the front segment of the worm from side to side. The wheels ars not powered, but the front ones are on separate axles and each axle is fitted with a ratchet that prevents the wheels from turning backwards - this means that, as the body swings from side to side, each wheel moves forward in turn. Unfortunately, I fitted them with plastic tyres, and they tended to slide on the smooth table surface, stopping Grubby from working properly.