519 



spirally round the cylinder (Fig. 2) the resulting tube will be spiral. 

 Such tubes may be used to afford an illustration of the mode of 

 action of cilia and flagella. For this purpose the tapered tube is tied 

 at the base on to a short, straight piece of glass tubing and this is 

 connected with an india-rubber ball. By gently squeezing and relaxing 

 the ball, variations of fluid pressure are caused within the artificial 

 ciliura, and these produce movements of a character which exactly 

 imitate the movements of natural cilia. Thus, when the pressure is 

 increased, in the simply curved form, the artificial cilium straightens 

 out, when diminished, it bends over again; and by rhythmically 

 repeating these movements a current is produced in any fluid in which 

 the model is immersed, the direction of the current depending upon 

 the relative rate at which the increase or diminution of pressure 

 within the cilium is brought about. If the increase of pressure be 



Fig. 1. 



Fig. 2. 



the more rapid, so that the artificial cilium straightens itself more 

 quickly than, on relaxing the pressure, it bends over, a current is 

 produced in the direction of the straightening ; but if, on the other 

 hand, the increase of pressure be slow and the relaxation rapid, the 

 current is then in the direction towards which the cilium bends. 



The movements of a flagellated organism can be imitated by the 

 spiral model, as follows: The short glass tube to which the artificial 

 flagellum is attached is connected by a long piece of fine india-rubber 

 tubing with the pressure ball, so that movements of locomotion of 

 the flagellum and its attached tube are permitted to take place. By 

 means of this arrangement it can be shown that the model moves 

 either forwards, i. e. away from the end to which the flagellum is 

 attached, or backwards, i. e. in the direction of the flagellum, according 

 as the increase or the decrease of pressure within the flagellum is 

 produced more rapidly. 



The demonstration is a striking one, although I would not be 

 understood to argue that the mere action of these models is to be 



