Reading your comment made me wonder why we have even numbers of limbs for different body plans. I realized it's due to bilateral symmetry for most organisms must some how be advantageous to asymmetric body plans, but I'm unsure how symmetry is advantageous. What are the evolutionary pressures selecting against asymmetric body plans?
I'm not really a biologist, but I think this has something to do with the balance: when you move in some direction, it would be extremely disadvantageous to have one side significantly heavier than another.
Take two runners, give one a 15 kg weight in one hand, and give another 15 kg weights in both hands (30 kg total). I think the runner with weights in both hands would run faster than the one with one weight.
Thus, moving species evolve to be bilaterally symmetric.
If the organism has no need to move in certain direction, it doesn't really have bilateral symmetry, like trees or bushes (though big trees generally tend to have their center of mass close to their vertical axis).
