196 CO-ORDINATION OF BEAT 



movements which interfere with the normal excitation of the 

 ciHa in such a way that a different fibril of the cilium is caused 

 to contract first, and the direction of beat is changed. Different 

 fibrils of the cilium may contract first under different conditions 

 of excitation, and it is interesting that the change in wave pattern 

 with depolarization always takes place in the same direction. 



Little evidence is available about the reversal of ciliary beating 

 in the metazoa, but in the well-known cases it is evident that the 

 reversed ciliary beat is started by a contraction in the fibrils on 

 the opposite side of the cilium from those which start the normal 

 beat, and a mechanism similar to that suggested for protozoa 

 could be involved. 



Reversed beating has been reported from cilia whose meta- 

 chronism is symplectic (Opaliiia) or antiplectic (Mnemiopsis), and 

 in both cases it is known that the direction of metachronal 

 transmission is also reversed.* Taylor (1920) reported that the 

 membranelles of Euplotes showed reversal; these compound cilia 

 show diaplectic metachronism, but it is not known quite how the 

 beat changes in reversal or whether the transmission of the 

 metachronal wave is also reversed. 



It is interesting that the cilia forming the cirri of hypotrichs 

 like Euplotes are in isolated groups and cannot therefore be 

 metachronally co-ordinated, but yet these cirri are capable of 

 beating in at least two directions in co-ordination with other cirri 

 (Taylor, 1920). The cirri are very often quiescent, but may be 

 excited to beat forwards or backwards, and perhaps also in other 

 planes. Excitation involves the passing of two items of informa- 

 tion, the stimulus to beat and some indication of the beat 

 direction. Taylor showed that the " neuromotor " fibres of 

 Euplotes were important in co-ordination in that the beat of the 

 various cirri was synchronized when the fibres were intact, but 

 not when the fibres were cut. As a pure speculation, it is possible 

 that the fibres may be used to excite the beat of the cirri, while 

 the electrical depolarization of the surface membrane used in the 

 reversal of beat in other ciliates may be used to determine the 

 direction in which the cirri beat. It is interesting that reversed 

 beating of cirri may be accompanied by the inhibition of movement 

 of the membranelles, and that the membranelles of Stentor cease 



* See footnote on p. 178 



