Introduction

This is the second basic swimming mechanism in the Stokesian realm. Although the organelles are apparently identical in ultrastructure and physiology, we use the term “flagellum” when there is only one, or a small number of these hairlike appendages on a cell, as in spermatozoans and the flagellates, and “cilia” to denote large numbers of them on the same cell, as in the ciliates. Cilia, then, are just a large number of flagella on the same cell. The ciliates tend to be larger than the cell bodies of spermatozoans by an order of magnitude. Cilia tend to be shorter than ftagella, however, and this fact puts their mechanism firmly in the Stokesian realm.

Confusion about terminology can arise from two sources: (1) in bacteria there are organelles called flagella with very different ultrastructure and physiology (Chapter 4); and (2) the term “cilia” is usually used in connection with various ciliated tissues in metazoans (manycelled animals) as in the lining of our respiratory tracts, but there is usually only one or a small number of cilia per cell in Lhese tissues.

Because of their proximity to the cell wall, we can expect to find that optimal propulsion using cilia will involve movements quite different from those of a single large flagellum. In this regard it would be valuable to understand the possible adaptive advantages that could have led to the evolution of ciliary motion. As both flagellary and ciliary modes are widespread and both are evidently successful strategies, we might suppose that there are trade-offs between them.