360 D. J. SCOURFIELD ON THE LOCOMOTION OF 



one extremity of the body only, the most constant arrangement 

 being a single flagelliim attached to the anterior end and directed 

 straight forward in locomotion. But they may occur at both 

 ends (e.g. some Bacteria), or in the middle (Dinoflagellata), or in 

 various other ways. 



The question now arises, How can a single, simple whip- like 

 outgrowth produce a movement of the whole organism to which 

 it is attached ? It is evident that mere lashing or vibrating 

 cannot effect this. Even if such lashing were imagined to be 

 stronger in one direction than another, it would still be impossible 

 to account for the steady progress in the line of the long axis of 

 the body which actually occurs. No ; the only way in which such 

 locomotion can be brought about under the conditions named seems 

 to be by a succession of waves travelling backwards along the 

 flagellum, and there is good reason for believing that this really 

 takes place. The waves, however, are probably never confined to 

 one plane, but rotate as they progress, thus throwing the 

 flagellum into a spiral form. The effect, in fact, is exactly the 

 same as would be produced by a rotating corkscrew, but there is 

 this essential difference, that the flagellum does not rotate, and 

 the mechanical problem which is thus involved is one by no 

 means easy to solve. For my own part I can see no way of 

 making a piece of apparatus which could do the work of a 

 flagellum, and do it in the same way, although it is very easy to 

 imitate the action by means of a piece of cord, one end of which 

 hangs freely in the air, while to the other a circular motion is 

 imparted by the hand. 



An important by-product, so to speak, of the spiral action of a 

 flagellum is the rotation of the whole body of the organism on its 

 long axis. This can be easily shown to result from the oblique 

 backward thrust of the spirals of the flagellum, which, by the 

 application of the parallelogram of forces, can be resolved into 

 two factors, one pulling the organism forward, the other acting 

 circularly at right angles to this, i.e. tending to rotate the whole 

 body in the opposite sense to the spiral movement of the flagellum. 

 The usefulness of this rotation, from the point of view of the 

 organism keeping a straight course when swimming, has already 

 been referred to. 



Flagella, unlike cilia, are practically confined to the lowest 

 forms of animal and vegetable life, and are by no means of 



