COMPOUNDING OF CILIA 171 



they are coordinated to beat rhythmically by any means other 

 than viscous interaction. 



If many cilia are grouped together, then their beating requires 

 more careful organization because of the fact that the beat is special- 

 ized to move water in a direction at right angles to the longitudinal 

 axis of the cilium. Mere parallel alignment of cilia is not suf- 

 ficient to ensure that they all work together, as it would be for fla- 

 gella, for the ciliary beat must also be lined up so that all the mem- 

 bers of a group beat in the same plane and in the same direction. 



The force of the ciliary beat can be used more efficiently if 

 cilia which beat in the same plane and in the same direction are 

 made to beat synchronously. However, a synchronous beat is 

 apparently unsuitable for use over a large area of ciliated surface, 

 perhaps because it gives a jerky movement rather than a steady 

 progression of water over the ciliated surface, or perhaps because it 

 is not practicable to excite a synchronous beat over a large area. 

 In place of overall synchrony it is found that there are two well- 

 defined lines at right angles to each other in any active ciliated epi- 

 thelium, one in which all the cilia beat together, and the other in 

 which they beat in succession to give the appearance of waves called 

 ''metachronal waves". It is usual to find that the bases of the 

 cilia are arranged approximately in rows in these two directions ; 

 Knight -Jones (1954) has named these ''orthoplectic'' rows — in line 

 with the beat — and ''diaplectic" rows — at right angles to the beat. 



2. The Compounding of Synchronously Beating Cilia 



Cilia of short orthoplectic rows, e.g. the membranelles of 

 Stentor and the laterofrontal cilia of Mytilus gills, of diaplectic 

 rows, e.g. the comb-plates of ctenophores and perhaps the 

 undulating membrane of some ciliates, and occasionally groups 

 of more or less isolated cilia, e.g. cirri of Euplotes or abfrontal 

 cilia of Mytilus gills, may have their bases packed closely together 

 and their shafts united to form compound structures (p. 30). 



The compounding of such synchronously beating cilia has a 

 functional advantage which was pointed out by Harris (1961) 

 (see p. 146). The angular velocity with which a cilium can move 

 is directly proportional to the bending couple that the cilium can 

 exert, but inversely proportional to the cube of the cilium length, 

 and hence the velocity with which the fluid can be moved by the 



