Axes of Assymetry

I read Your Inner Fish and got inspired for a new creature:)

So we humans are bilaterally symmetrical, with a functional difference between front and back, and another between back and belly. But there’s no functional difference between left and right. In the same way, jellyfish have a difference between up and down, but no front versus back.

What if we had an animal with three axes of asymmetry? Right vrs. left is as different as back-vrs.-front and top-vrs-bottom. But while it’s easy to see why a worm crawling along the ocean floor would evolve a head and a belly, why would it evolve a specialized structure on its right as opposed to its left side? What force could there be that always acts on an animal orthogonally to gravity and its direction of motion?