In protozoans flagella are found always one per cell. Flagella and Cilia can generate substantial force as they try to push or pull a protozoan through water. To be able to do that, plasma membrane needs to be strengthened, and when it is, surface is referred to as the pellicle. In some ciliates even further strengthening takes place with kinetosomes being anchored to each other creating a complex infraciliature. The stronger the pellicle, the less likely it's that shape of the protist can change, and most ciliates have recognizable shapes. A good instance is Paramecium with its oval shape and food groove.