200 CO-ORDINATION OF BEAT 



the flagella of a small group is usually synchronous, for the waves 

 of bending of the flagella are apparently aligned by viscous 

 interaction. CiHa or flagella may clothe large areas of surface, and 

 here synchrony of all of the organelles is not found, for, while cilia 

 in one line on the surface tend to beat synchronously, those in the 

 line at right angles tend to beat one after the other, metachronally. 



Metachronal patterns have been classified according to the 

 relationship between the direction of movement of the metachronal 

 waves and the direction of the effective stroke of the ciliary beat. 

 The following four groups have been named : symplectic 

 antiplectic, dexioplectic and laeoplectic. The form of metachronal 

 waves on the ciliated surfaces showing several of these patterns is 

 well known, but they do not all appear to use the same mechanism 

 of metachronal transmission. Thus, in Opalina, the transmission 

 appears to be mediated through the viscous interaction between 

 the shafts of the moving cilia, while in the membranelles of 

 Stentor a more refined internal transmission mechanism seems to 

 be involved. Evidence is available to suggest that this transmission 

 moves step -by-step along the row of cilia, and involves two stages 

 in each step : an interciliary conduction process and an intraciliary 

 excitation process. In both types of metachronal co-ordination 

 a ciliary pacemaker could control the rate of beat of all the cilia 

 that beat after it. Under certain conditions the membranelles of 

 Stentor may be co-ordinated to form metachronal waves under 

 the influence of viscous interaction. 



The beating of cilia and flagella usually seems to be under some 

 form of control by the organism. In flagella, the eflFective 

 movement of water relative to the organelle is restricted to one 

 direction — from flagellar base to tip — and the only eflFective form 

 of control is by activation or inhibition. The beat direction of 

 many cilia is also fixed and they are controlled in the same way. 

 In other cilia, however, a mechanism is present for changing the 

 effective direction of the beat, so that a further means of control 

 of ciliary action is available. The reversal of ciliary beating and 

 metachronal waves seems to be closely connected with electrical 

 changes; these may in nature result from ionic movements. 

 There is no sound evidence to show that metachronism, or any 

 other form of ciliary co-ordination for that matter, involves any 

 form of fibrillar conduction. 



