CILIARY REVERSAL 193 



In metazoa, anthozoan coelenterates (e.g. Metridium, Parker, 

 1928), ctenophores (e.g. MnemiopsiSy Coonfield, 1934) and 

 nemertines (Friedrich, 1933) have been found to show temporary- 

 reversal of beating activity, but information from other groups 

 is very limited. There have been reports that the beat of some 

 cilia may be permanently reversed for the life of the animal, 

 e.g. some frontal ciHa of Mytilus gills (Atkins, 1930), but this 

 may result from a secondary development of special ciha, and 

 comes in a different category. 



It appears that reversal of beat in Metridium is a necessary 

 part of the normal behaviour of the animal, and is required in 

 feeding. Parker (1928) found that in Metridium the ciha on the 

 ** lips " around the mouth normally beat outw^ard, and would 

 carry away a scrap of filter paper soaked in seawater, but would 

 reverse and carry into the mouth similar scraps of filter paper 

 soaked in glycogen. Similarly, mussel extract causes reversal of 

 the beat of these cilia (Parker and Marks, 1928). Reversal of 

 beat in Mnemiopsis and some other ctenophores seems to be 

 associated with the body movements necessary to bring the animal 

 into such a position that it can swallow the food caught by the 

 tentacles; these movements must require a high degree of control 

 and co-ordination of the cilia. 



Reversed beating of the cilia of Paramecium and Opalina is also 

 a normal part of everyday life for these organisms. In fact, in 

 both types the cilia may beat in almost any plane (Okajima, 1953 ; 

 Parducz, 1954), and the animal is not restricted to movement in 

 the forward direction only, but can reverse and turn according 

 to the direction of beat. It has already been mentioned that 

 electric current (p. 90), some ionic changes in the medium 

 (p. 98) and membrane deformations (p. 95) will cause ciliary 

 reversal, and in life a wide variety of stimuli such as contact with 

 obstacles in the environment will Qause a reversal of beat for a 

 time before forward beating is resumed. 



From many of these studies it appears that reversal of beating 

 is accompanied by reversal of the metachronal wave. Some 

 different patterns of metachronal waves in Opalina and their 

 movements over the " dorsal " and " ventral " surfaces of the 

 body at different stages in the change of beat direction are shown 

 in Fig. 53. These patterns are commonly observed in the normal 



