INTRODUCTION 5 



movement of fluid along the length of the flagellar axis (Fig. lb), 

 i.e. normal to the surface bearing the flagellum. It is also interest- 

 ing that a beating flagellum moves fluid continuously, while a 

 cilium is only moving the fluid usefully during part of its beat. In 

 practice, there is no hard-and-fast line to be drawn between the 

 two, for although one can usually see when an organelle is produc- 

 ing a typical ciliary beat, the beat of organelles generally described 

 as flagella may vary from the sine wave pattern shown by some 

 sperm tails, which results from equal and alternate waves of 

 bending on the two sides of the flagellum, to a pattern that is 

 virtually the same as a cilium, in most respects, being asymmetrical 

 and sometimes extremely unilateral. This latter form of beating is 

 common in those protozoa that bear few flagella, but the organelle 

 may show several complete bending waves within its length 

 (see p. 136), which one never finds in true cilia. It is perhaps 

 realistic to regard the flagellum as the original type, which has been 

 specialized in the majority of cases to give the ciliary type of beat. 

 On the basis of this functional distinction between cilia and 

 flagella, it is interesting that a single cilium would be useless for 

 moving a sperm head, while a flagellum is ideal. Similarly, a few 

 flagella are much more efficient than a few cilia in moving a small 

 protozoon. Large numbers of organelles with a flagellar beat may 

 occur in those cases where they can be used effectively by virtue of 

 their orientation in the functional position. Where many flagella 

 occur on the bodies of some multiflagellate protozoa, their basal 

 structures may be arranged in such a way that the flagellar shafts 

 leave the surface at a fairly acute angle and the flagellar beat can 

 only act to move the body in one direction, so that all the flagella 

 of the body work together to move the organism forward (Pitelka 

 and Schooley, 1958). The same may be true of the multiflagellate 

 sperm of some plants. In sponges the flagellate cells are positioned 

 so that the water currents produced by flagellar beating draw 



p, present in at least some members of the group ; -, absent; /, system or stage 

 missing in the group; ?, uncertain (for sensory structures, cilia are presumed 

 to be sensory or the sensory structures are presumed to be cilia); ^, in 

 Onychophora; ^, in Insecta; •, not known; y, in cerebrospinal cavity and 

 therefore not strictly in the coelom. 



Data mostly from Hyman (1939-59) and Grasse (1948-61), with additional 

 information from other references quoted in this book. 



