180 CO-ORDINATION OF BEAT 



e.g. the membranelles of Stentor, the velar cilia of nudibranch 

 mollusc larvae, or the laterofrontal cilia of Mytilus gills. Narrow 

 diaplectic bands are common on gill structures, feeding tentacles 

 and swimming appendages or zones of the larvae and adults of 

 many invertebrates including protochordates, as well as in some 

 protozoans. They may form long bands such as those which 

 run the length of the gill filaments of lamellibranchs, or they may 

 form encircling rings around cylindrical gills, as in some polychaete 

 worms (Knight-Jones, 1954). The greater specialization of 

 antiplectic and diaplectic metachronism may be correlated with 

 a more complex mechanism of m.etachronal co-ordination in these 

 types, for the viscous interaction between beating cilia is normally 

 much less here than in symplectic metachronal waves. 



4. The Mechanism of Metachronism 



The autonomy of beat of isolated cilia was stressed by Verworn 

 (1889), for he had observed many times that cilia on cell fragments 

 beat until they died, provided sufficient basal cytoplasm was 

 present. He was concerned to discover how this autonomy of 

 the cilia could be suppressed or controlled in order that the 

 observed metachronal co-ordination should take place. 



For many years there have been tw^o widely supported theories 

 concerning the mechanism of metachronal co-ordination. Thus, 

 Engelmann (1868) believed that nerve-like impulses were respon- 

 sible for co-ordination, and while some of Verworn's studies led 

 him to believe that co-ordination was by mechanical action 

 between ciliary shafts, he could not explain all of his observations 

 on this basis (Verworn, 1891). The investigations carried out 

 by Gray (1930) led him to consider these two possible co-ordina- 

 tion mechanisms, i.e. (1) a neuroid transmission mechanism in 

 which cilia are excited by stimuli transmitted through the basal 

 protoplasm, and (2) a mechanical interaction mechanism in which 

 the movement of one cilium may influence the next by the viscous 

 drag communicated through the surrounding water. He believed 

 that the second was the more likely because the first could not 

 be reconciled with the facts that (a) since cilia are autonomously 

 active, the only stimuli that could control their beating were 

 inhibitory ones, (b) the rate of wave transmission was so slow in 

 comparison with the transmission of nerve impulses, and (c) meta- 



