This example shows how to creep metaballs on a surface. In this case, the surface is a contorted tube and the metaballs look like a "blob" being pushed through the tract.
A tube is created and used as the creep surface. A circle is created by carving a profile out from that same tube. The circle is then animated with a Creep SOP down the length of the tube.
Metaballs are attached to the points on that carved circle to create the "blob".