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University of California Puhlications in Zoology [Vol.20 



areas between are due to the fact that the cilia are recovering their 

 vertical position and hence are viewed very nearly endwise. The 

 position of consecutive cilia in any single row, from one point in a 

 wave to a similar point in a following wave will fairly represent the 

 successive positions taken by a single cilium in making one complete 

 cycle. Figure M was made in this way. The arrow represents the 

 long axis of the animal. From this diagram it is seen that the cilium 

 at the end of the effective stroke lies rather close to the surface of 

 the body and not along the row but decidedl.y to the left from it. In 



ciL 



hy fc. .) 



i 



Fig. M 



I'M^'- 



Fig. N 



Fiff. O 



Fig. M. Diagrammatic representation of the successive steps in one complete 

 beat of a cilium of Balanl idium coli. It also illustrates the positions of the respec- 

 tive cilia of a single row between the points a and b in fig. M, at which points the 

 cilia are in a prostrate position at the end of their effective stroke. The arrow 

 indicates the long axis of the animal. 



Fig. N. Tangential section of Balantidium coli. The cilia still retain respec- 

 tive positions which they had in the normal swimming movements of the organism. 

 X 1500. 



Fig. O. Diagram illustrating the effect of the ciliary action in tlie rotation of 

 the organism. The arrow represents the long axis of the animal; ab., the direction 

 of the rows of cilia; cd., the direction of effective stroke of a cilium attached 

 at the point of intersection of the tliree lines; the ellipse is described by the tip 

 of the ciliimi. 



recovery it straightens up and passes anteriorly, thence to the right, 

 crossing the row, of which it is a part, at right angles. At this point 

 the cilium leans anteriorly only slightly from the vertical. The 

 effective beat is produced by the quick stroke of the cilium posteriorly 

 and to the left, and continues until the cilium has crossed the row again 

 and lies close to the surface and extends to the left as represented 

 by the position of the last cilium shown in figure M. According to 

 the classification given by Piitter (1903), the movement of the cilia 

 of Balantidium would be called infundibular. As will be noted from 



