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"Is it golden?" I said. "I don't know. It is Odin's disk and it has only one side." — J. L. Borges, The Disk.		Monday, August 16th, 2004 I've been planning to cover animals that use the wheel to get around, but the wheel is an extensive concept. Many science fiction writers have thought about this subject and most of the idea space has been explored. Since I wish to avoid reinvention, particularly of the wheel, I'm having to do some extended research, and my wheel entry will likely be more of a review of what's been done on the subject. By way of preparation, I'll mention some of the difficulties with designing a land animal that moves by wheel. The primary problem is one I discussed last time in connection with the helix animal — if the wheel is an independently moving piece, how do you keep it attached and supply it with nutrients? Blood vessels would quickly get squeezed or torn. And the wheel and axle can't be covered with the same contiguous skin as the rest of the animal, leaving it vulnerable to fluid loss and infection. There are few animals that use the wheel in nature. Animals that roll into a ball, such as the pillbug and three-banded armadillo, do so for protection, not locomotion. Prokaryotic bacteria use wheels to rotate their flagellae, but what works at their level would not well suit a macroscopic land animal. The concept of 'skin' is much looser for microscopic organisms. Amoebae get about by pushing their endoplasm ('meat') in one direction, and as it's about to break through the ectoplasm ('skin'), they convert it to more ectoplasm. How could a wheel work on a macroscopic level? Possible solutions: The animal's entire body forms a wheel. This is the solution to which nature has come the closest. The animal would need some way to propel itself in wheel form (likely internal shifting of body mass) as slopes would rarely be arranged for its convenience. The animal grows, builds or accretes a non-living wheel or sphere upon which it can ride. The dung beetle sometimes travels this way, but only to transport balls of dung; it normally gets about on legs. The only animal on earth that constructs wheels primarily for locomotion is Homo Sapiens. The animal constructs a hollow ball or cylinder and rides inside it. The Jumping Bean does something analogous, but itself doesn't count as a wheel. The wheel is a living appendage of the animal. This option is perhaps the most interesting (read challenging) from an engineering standpoint. I can't think of any other options offhand; if you can, drop me a line.
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