THE FORM OF BEAT OF CILIA 143 



p. 175). The form of beat of Opalina cilia appears to be the same 

 in both normal and reversed swimming, although the direction of 

 beat is changed. 



Abfrontal cilia of the gills of Mytilus have been found to be 

 ideal for the study of the form of ciliary beating. Gray (1930) 

 published an excellent sequence of cine -micrographs showing the 

 cycle of beat of these cilia, and several workers have used them 

 for physiological studies on ciliary movement (e.g. Kinosita and 

 Kamada, 1939; Yoneda, 1960). The cilia at rest lie along the 

 gill surface, and during each cycle of beat the cilium first straightens 

 out, by the passage of a wave of flexure from the base to the tip, 

 and then swings back to the gill surface as an apparently stiff 

 structure (PL XVI la). 



Unfortunately this pattern of beat is unusual if not unique. It 

 is difficult to tell which part of the beat is the effective stroke in 

 the sense that it is responsible for the main movement of water. 

 The effective stroke of most cilia occupies a small fraction of 

 the total beat, say J to | at most, and during this stroke the 

 cilium moves stiffly through the water as a result of a bend at 

 the base. The second part of the beat of Mytilus abfrontal cilia 

 is equivalent to the effective stroke of many cilia in that it is the 

 part of the beat in which the cilium is moved stiffly through the 

 water, but it occupies something like I of the total cycle (Kinosita 

 and Kamada, 1939). For much of this part of the beat the 

 cilium moves very slowly through the water. In the first 

 (preparatory) phase of the beat the cilium straightens out very 

 rapidly, appearing to " flick " the water. Most of the effective 

 water movement must be done during this phase, and perhaps 

 this reflects the function of these cilia, which is probably to prevent 

 the settlement of small organisms (protozoa, etc.) on the abfrontal 

 surface of the gill. These cilia are also unusual in that they occur 

 singly among many short cilia; all abfrontal cilia on any one gill 

 filament do not necessarily beat with the same orientation. 



The appearance of the beating of these cilia suggests that the 

 most satisfactory explanation of the cycle of beat in this case is 

 that only the preparatory phase is active, while the " eftective '* 

 phase is passive; in most cilia both phases are believed to be 

 active (see pp. 153 ff.). Thus, if the natural shape of the cilium 

 were to hold it close to the gill surface, and a wave of flexure 



