TYPES OF METACHRONAL PATTERN 177 



Up to several thousand in the diaplectic direction, all tightly 

 compressed together. In other uses of this type of metachronism 

 compounding of ciUa may not be necessary. Coonfield (1936) 

 describes the reversal of the metachronal waves of Mnemiopsts 

 in feeding, and, to judge by the movements of feeding ctenophores 

 described by Hardy (1956), the reversal of beating of ctenophore 

 comb-plates must play an important part in ctenophore behaviour. 

 It appears that when the wave is reversed, the beat is also 

 reversed. 



Diaplectic metachronism is very widespread, especially in 

 metazoa, and Knight -Jones (1954) has found that the distribution 

 of dexioplectic and laeoplectic metachronism is of phylogenetic 

 interest in that both types are seldom found in the same group. 

 In the protochordates, for example, diaplectic waves always show 



M. W. 



Fig. 48. Diagram of metachronal waves of comb-plates of the 

 ctenophore Pleurobrachia. The effective stroke is towards the 

 left and the metachronal waves move towards the right. The 

 sequence of movements of a comb-plate can be followed by 

 moving along the row from right to left. 



the dexioplectic pattern. The basic character of these two 

 patterns is the sme, and only the dexioplectic pattern shown by 

 the membranelles of Stentor will be described. The waves travel 

 along the row of compound cilia around the edge of the peristome 

 of the animal (Fig. 49), and the cilia beat outwards, roughly at 

 right angles to the row. Each compound cilium is built of 60 

 to 75 cilia, being 3 cilia wide in the diaplectic direction (in Stentor 

 polymorphiis) and 20 to 25 cilia wide in the orthoplectic direction, 

 except in the gullet region where fewer component cilia are 

 present. The beating cilia do not touch each other because 

 adjacent cilia beat in parallel planes, although the water move- 



