http://en.wikipedia.org/wiki/Flagella So I was reading Purcell's "Life at Low Reynolds Numbers", which I found interesting. So here's a question: The flagellum help propel the cell forward. The question is - the flagella should increase the velocities of the fluid molecules behind the cell, right? But if it increases the velocities of the fluid molecules behind the cell, shouldn't the pressure gradient be decreasing behind the cell, causing the cell to move backwards? (as we know from http://en.wikipedia.org/wiki/Bernoulli's_principle). Am I missing something?