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flagellum does not rotate ; and the mechanical problem thus 

 involved is one by no means easy to solve. In the case of 

 collared monads the motion observed is the reverse of that usually 

 found in flagellate organisms. The rotation is probably in a 

 left-handed spiral, as against the usual right-handed motion. 

 Flagella of Bacteria had been mentioned ; but it was suggested 

 that the term in this case was not well applied. Passing on 

 to consider locomotion caused by the action of cilia, these may 

 be defined, like flagella, as permanent, whip-like extensions of 

 the living cell-substance, but, unlike flagella, are incapable of 

 moving by individual effort the body to which they are attached 

 (being usually small in comparison with the body). Cilia are 

 always associated in number, and act in harmony. In the 

 case of cilia, we are dealing not with spiral motion, but with 

 whip-like action in one direction, and with motion very much 

 more rapid in one plane than the other. A slight retardation 

 in the moment of lashing of successive cilia or rows of cilia 

 produces the well-known illusion observed, e.g., in the ciliary 

 organs of Rotifers, giving an appearance like rotating cogwheels, 

 and in other cases as of waves travelling over a field of corn. 

 References were made to instances of specialised cilia, as, for 

 instance, when a Vorticella becomes detached it swims away 

 by the action of the mouth cilia. The presence of cilia for 

 locomotive purposes extends to many very different types of 

 organisms. The highest developments of these appendages are 

 found in the ciliated Infusorians ; but cilia also occur either 

 permanently, or in certain stages only of the life-history, in 

 Sponges, Hydrozoa, Echinoderms, Worms, Rotifers, Gastrotricha, 

 and Mollusca, and perhaps Volvox may also be included, as it 

 is extremely doubtful whether the individual threads in this 

 last instance act in the spiral manner characteristic of flagella ; 

 but this particular instance has not yet been fully worked out. 

 Other methods of progression were then referred to. There is 

 what mav be called medusoid motion, which consists of the 

 sudden contraction of the sides of a more or less umbrella-shaped 

 organism. This type of motion is well shown among micro- 

 scopic forms by the curious protozoan Leptodiscus occurring in 



