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The Mechanism of Ciliary Movement. 



By J. Gray, M.A., Fellow of King's College, Cambridge, and Balfour Student, 



Cambridge University. 



(Communicated by Prof. J. S. Gardiner, F.R.S. Received June 1, 1921.) 



The mechanism of ciliary movement has been extensively studied from the 

 morphological point of view, and although there is a general consensus of 

 opinion as to the structure of the " ciliary apparatus," there is no adequate 

 account of the functions of the various parts of the mechanism. 



The material used for this work has been the gills of Mytilus edulis, and 

 has already been described (Orton, 27). It is entirely due to the movement 

 of the cilia that an efficient stream of water is kept passing on to the face of 

 the gill, and that the food is moved up to the mouth of the animal. By 

 means of carmine particles the existence of these currents is easily detected 

 by the naked eye. 



The production of a constant current of water in a definite direction implies 

 that the cilia are capable of performing work in a remarkably efficient manner. 

 If we watch the movement of a single cilium, it is obvious that the beat is 

 divisible into two phases : (a) a very rapid forward or effective stroke ; and 

 (&) a slower backward or recovery stroke. It is during the rapid effective 

 stroke that the cilium performs work on the surrounding medium, and in 

 doing so, of course, expends energy. At the conclusion of the effective 

 stroke these cilia possess no energy which can be used for work, but by the 

 time the recovery stroke is completed a new supply of potential energy is 

 available and is in turn converted into kinetic energy during the next effective 

 stroke. 



We are, no doubt, entitled to assume that the energy expended by a cilium 

 has its origin in some chemical compound, either in the cilium itself or in 

 the cell to which it is attached. Our main problem is to throw what light 

 we can on the sequence of events which leads to the conversion of chemical 

 energy into the kinetic energy of movement. 



The first evidence which will be presented is that gained by an observation 

 of the living cells under normal conditions. 



I. The Structure and Behaviour of Normal Cilia. 



On the gill filaments are three main groups of ciliated cells — the lateral, 

 the latero-frontal, and the frontal cilia (see Gray, 12). These cilia, like all 

 other living cilia, appear to be optically homogeneous ; they are strongly 



