456 Journal of Comparative Neurology and Psychology. 



It must be added that the extent of body surface on which 

 the cilia show any of the characteristic strokes mentioned is ex- 

 ceedingly variable. Often, for example, the body cilia of only 

 the anterior tip. or the anterior half, show the transverse stroke, 

 while posterior to this they do not. Farther, the cilia of the 

 posterior half of the body frequently cease beating effectively, 

 showing only a slight quivering, while the anterior cilia are still 

 very active. As a result of a long study of the ciliary move- 

 ments, one retains the impression that almost any combination 

 of forward, reversed, oblique or transverse strokes is possible 

 among the different areas of the body, and that those mentioned 

 above are only typical combinations, produced under more or 

 less definite conditions. As a rule a combination is produced 

 such as brings about a well ordered movement of some sort, 

 but under certain conditions the movements of the cilia are 

 such as to produce only a disordered quivering or jerking, with- 

 out movement in any definite direction. This is sometimes the 

 case for example when the animal is immersed in a strong chem- 

 ical. Under some conditions a similar result is produced also, 

 as we shall see later, by the electric current. 



What are the conditions on which depends the direction of 

 the effective stroke of the cilia in any given region of the body ? 

 The question is a very difficult one. According to the tropism 

 theory, the direction of the effective stroke of the cilia — that 

 is, whether the "contraction phase" or "expansion phase" was 

 the effective one in producing movement — depended on the di- 

 rect action of stimuli on the part of the body bearing the cilia 

 in question. Certain agents impinging on any given region of 

 the body caused the "contraction" or backward stroke to be 

 more effective ; others had the opposite effect. But we now 

 know that this conception was far too schematic. As a result 

 of a stimulus applied to a single definite region of the body, 

 certain cilia beat effectively in one way, others in a different 

 manner, and the first effect is soon followed by a second one, 

 equally complicated. Thus, a touch at the anterior end with a 

 glass rod, or a chemical acting on the surface, (i) produces re- 

 versal of the stroke of the cilia over the entire body ; (2) then 



