ABSTRACT
This chapter starts by presenting information that is visible in the light microscope, surveying the types of cells that swim and their means of propulsion. It discusses some of the special problems encountered by cells traveling through water because of their minute size. Even without cilia and flagella, however, a cell suspended in water moves passively by diffusion. Like any other particle of similar size, a cell in water is pushed first one way and then the other by the thermal motion of surrounding molecules. A powerful advantage is gained by any single-cell organism that can move more rapidly than its competitors toward a source of food or away from a potentially harmful environment. Many eucaryotic cells also swim by means of flagella: this is true of many species of protozoa and algae as well as most types of animal sperm. Thus cilia and eucaryotic flagella form the basis of modified structures that perform functions other than swimming.
