Answer the following question given this paragraph: 

Sponges are characterized by a feeding system unique among animals. As sponges don't have mouths, they must feed by some other method. Sponges have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella, which line the surface of chambers connected by a series of canals.


Q: The flow of water through the sponge is unidirectional, driven by the beating of what?


A:
flagella