Cambridge University Press, 1979. The speed of a single-celled animal is affected mostly by the cell's physiology and its environment. Unicellular organisms may swim or crawl through their environments, depending on what appendages the organism has for locomotion. Due to a cell's small size and fluid-filled environment, the viscosity of the organism's surroundings produces the greatest effect on its movement, rather than inertial forces that larger organisms would encounter. The speeds of single-celled animals vary greatly along with the special locomotive structures that each cell has.
Water currents can easily carry along unicellular organisms because they are so tiny.
The Reynolds number of an organism determines how easily viscous forces affect the organism's motion. The Reynolds number is equal to the inertial force acting upon the object divided by the viscous force acting on the object. Cells generally have a Reynolds number that is less than one. Swimming unicellular organisms must propel themselves with the use of viscous shear whereby viscous resistance is at its maximum during the cell's power stroke and at a minimum on the recovery stroke. Single-celled animals, most of which are protozoan, have evolved to come equipped with special parts to allow them to move.