The Mechanism of Ciliary Movement. 



119 



concentrations of acid stop the heart in diastole, stronger concentrations 

 stop it in systole. 



Finally, if the cilia of Pleurdbrachia are considered, it will be obvious how 

 very closely the known facts agree with the hypothesis of muscular action 

 outlined by Hill and Hartree (15). The position of rest of these cilia is 

 at the end of the recovery stroke, so that (like a striated muscle fibre) they 

 possess a definite amount of potential energy, which can be released at the 

 moment of stimulation. The cilium may be regarded as a series of fibres, B,* 

 which are in communication with a network, A ; the walls of the latter 

 are kept stretched by the presence of water in the interstices of the 

 network. It is this turgidity which provides the cell with the potential 

 energy possessed at the position of rest. At the moment of stimulation, 

 some chemical substance is set free at the surface of the fibres, B, which 

 promptly take up water from the network, A, so that the cilium flies 

 forward, owing to : (i) the liberation of the energy stored in the walls of A ; 

 and (ii) the tension developed in B. At the end of the effective stroke, the 

 chemical substance is removed from the fibres, B, and the water flows away 

 from the fibres {e.g., by osmosis) into the interstices of the network, A, 

 thereby stretching the cilium back to the resting position. 



It should be understood that the above analysis of the ciliary mechanism 

 is nothing more than a working hypothesis. At the same time the remark- 

 able similarity between the conditions necessary for ciliary and muscular 

 activity, coupled with the apparent similarity in the fibrous structure of the 

 two types of cell, leads to the conclusion that the two mechanisms may be 

 essentially similar. It is also clear that the same scheme might be applied 

 to pseudopodial movement. 



Summary of Uzperimental Results. 



1. The cilium is capable of expending potential energy in the form of work 

 as long as it is in organic connection with the cell protoplasm. 



2. Each ciliated cell of Mytilus is capable of independent movement when 

 isolated. The cilia of the Ctenophore Pleurdbrachia require a definite stimulus 

 to induce their beat. Both types of cilia show metachronial rhythm. 



3. The cilium is an elastic fibre or bundle of fibres. In the large majority 

 of cases the cilia are in communication with the cell protoplasm by means of 

 intracellular fibril lee. 



4. The ciliary beat consists of a rapid effective stroke and a slower recovery 

 stroke. The form of the recovery stroke often differs markedly from the 

 effective stroke. 



* The fibrous nature of these cilia is well seen in preserved specimens. 



K 2 



