INTRODUCTION 



Other or both of these facts is not estabUshed, ahhough the 

 structures are usually regarded as cilia and are reputed to have a 

 sensory function. Cilia or flagella are known to be present in all 

 the major animal groups except the Nematoda, and it is possible 

 that some sensory structures there may reveal their ciliary origin 

 in studies of their fine structure. 



The normal function of cilia and flagella is the movement of 

 fluids relative to the attachment of the organelle. If the body 

 bearing the organelle is free to move and is small enough, the 

 movement of cilia or flagella will move the body, otherwise the 

 fluid will be moved over the surface of a stationary body. It would 

 be an easy matter to fill a book of this size with accounts of the 

 ways in which cilia and flagella are used in organisms of all the 

 groups mentioned in Table 1, so that we shall have to be satisfied 

 here with a few general comments of wide application and f 

 reference to useful reviews and other works of value in this coniv^t, 

 which are listed in an Appendix on p. 201. 



Perhaps it is first necessary to emphasize that the difference 



a. 



b. 



^r-rrrrrnrrryy 



Fig. 1. Diagrammatic representation of the characteristic 

 movement of a, a cilium, and b, a flagellum. The solid arrow 

 indicates the movement of water and the dotted lines the 



movement of the organelles. 



between cilia and flagella is a purely functional one. Both 

 structures show an identical structural plan in electron microscope 

 studies, but their mode of use is slightly different. It is character- 

 istic of cilia sensu stricto that the resultant movement of fluid 

 caused by their beating is at right angles to the long axis of the 

 cilium when it is at the middle of the active part of the beat (Fig. 

 la), and therefore parallel to the surface bearing the cilia. 

 Typical " flagella, on the other hand, produce a resultant 



( ( 



B 



