176 



CO-ORDINATION OF BEAT 



naturally work together in this type of metachronal pattern, 

 compounding of the ciUa is unnecessary, and co-ordination by 

 mechanical interaction is possible (see p. 183). The direction of 

 transmission of the metachronal waves in Opalina is not fixed, 

 but changes rapidly so that the waves move transversely across 

 the animal or even pass forwards (Fig. 53, p. 194, and PI. 

 XVIIIa) ; the direction of beat always changes at the same time, 

 and the metachronism is always symplectic. The problem of the 

 change in beat direction is discussed separately (p. 192). 



A well-known example of antiplectic metachronism is provided 

 by the rows of comb -plates of ctenophores. In Pleurohrachia^ 

 for example, the metachronal waves normally travel from the 



Movement of the metachronal wave 



Fig. 47. Diagram of a metachronal wave of Opalina showing 

 the component cilia. The plane of beat is approximately in 

 the plane of the paper, and successive movements of a cilium 

 can be followed by moving along the row from right to left 

 (cf. Fig. 39 and Plate XVI lib). The animal is moving towards 



the left (from Sleigh, 1960). 



aboral pole of the animal towards the oral end, while the effective 

 stroke of the beat is towards the aboral pole, so that the animal 

 moves mouth forwards. During the effective stroke the comb- 

 plates are spread out like the spokes of a paddle wheel, moving 

 separately through the water and obtaining the maximum effect 

 from the broad surface of the comb -plate (Fig. 48). It is evident 

 that the compounding of cilia is here of a considerable advantage 

 because each comb -plate works alone, the larger it is and the 

 larger the force it can exert, the more effective it is in the 

 movement of water. Here compounding of long orthoplectic 

 rows would not be practicable, though the comb -plates in 

 fact consist of 80 to 100 cilia in the orthoplectic direction and 



