Moving through water is much more difficult than moving through air because the viscosity of water is 70 times greater than the viscosity of air. Viscosity is the degree to which a fluid resists flow under an applied force. Water is more viscous than air. Protozoa have evolved ways to deal with the difficulty of moving through water. Protozoa have cilia, flagella, or pseudopods that help them move.
Cilia are thousands of tiny hair-like projections that cover the surface of a cell.
Cilia wave in a coordinated way to move the protozoa in one direction. A flagella is a long tail-like appendage that is waved back and forth to move the protozoa. Pseudopod means false foot . The protozoa extends a pseudopod in the direction it wants to travel. The rest of the body flows into the pseudopod to move the animal.