The 2000 novel "The Amber Spyglass", by English author Philip Pullman, includes a type of alien known as the Mulefa. The Mulefa have diamond-shaped (◊-shaped) bodies with one leg at the front and back and one leg on each side. The Mulefa use large, disc-shaped seed pods as wheels. They hold the pods on axles made of bone on their front and back legs, while pushing themselves with their side legs. The Mulefa and the seed pod trees have a symbiotic relationship, or a relationship that helps both sides: the Mulefa use the seed pods as wheels, and the seed pod trees depend on the rolling to crack open the pods and allow the seeds to spread.