This is a rotifer (the Latin word meaning “wheel-bearer.”). They are “microscopic aquatic animals… found in many freshwater environments and in moist soil, where they inhabit the thin films of water that are formed around soil particles.” Though they can get bigger, they’re usually around 0.1–0.5 mm long.

These rotifers are feeding… according to the video notes, the first one has a cell caught in a vortex caused by the two sets of cilia near its mouth. “Rotifers play a key role in filtering out the decomposing organic matter contained in water. And those rotifers, in turn, make nice snacks for fish, shrimp and crabs. A single drop of pond water might contain 50 to 100 rotifers.” 

Via Cosmic Log