CONTROL BY THE ORGANISM 121 



of Stentor appear to stop under some inhibitory control, for these 

 compound cilia cease to beat as soon as the animal contracts, and 

 also stop their normal activity when the animal swims backwards. 



Control by activation also seems to be present in protozoa. The 

 cirri of hypotrichs like Euplotes and the long stout cilia of 

 Cyclidium show sudden activation of the cilia resulting in rapid 

 movements by co-ordinated action, although these cilia are 

 normally quiescent. The co-ordination shown in these movements 

 is not metachronal co-ordination, although a similar mechanism 

 may be involved. Activation of the cirri of Euplotes may be 

 accompanied by inhibition of the membranelles which is abolished 

 by cutting the fibril which connects the " motorium " to the 

 membranelles (Taylor, 1920). 



Even among the simpler flagellates like Astasia and Euglena, 

 the movement of the flagellum may be modified by stimulation 

 of the organism. Borgers and Kitching (1956) found that a change 

 in the carbon dioxide tension caused Astasia to change its direction 

 of movement, while Kitching (1961) reports that a sudden 

 decrease in the carbon dioxide tension caused this flagellate to 

 cease its forward movement and perform repeated turns before 

 settling to forward motion again. Similarly, in the well-known 

 light responses of Etiglena, there must be a change in the flagellar 

 activity in response to a change in the direction or strength of 

 illumination. 



The presence of a well-developed nervous system in metazoan 

 animals allows a means of controlling the activity of cilia, and it 

 appears that both excitatory and inhibitory control are frequently 

 used. It is difficult to be certain whether there are any metazoan 

 cilia which are without any form of control, though there may be 

 some cilia with cleansing or respiratory functions that are 

 continuously active. For example, while the control of lateral 

 cilia of lamellibranch gills has been suggested several times, 

 the frontal cilia of the gill filaments seem to be continuously 

 active. 



Many cases of ciliary control which appear to involve inhibition 

 may in fact be cases of activation, and it is difficult to be certain 

 which method of control is being used without thorough ex- 

 perimentation. The control of the lateral cilia of lamellibranch 

 gills is a case in point. In Mytilus, Lucas (1931 a, b) found that the 



