Rotifers are multi-cellular animals with two circlets of cilia at the mouth end that wave to give the illusion of a pair of wheels (hence the name -- rotifer is Latin for wheel).

I found this fellow in the algae growing on a dead leaf in a rain puddle outside my house.

You can see the details of the spikes at the tail, and watch as he cements the tail to the glass slide to inch his way around. They can also use their cilia to swim quickly from place to place. As they are transparent, much of the internal structure is visible.