1922] McDonald: On Balantidium coli and Balantidium suis 255 



an individual measured more than one but less than another whole 

 division of the scale ; e.g., such a preference might result in the tabu- 

 lation of several individuals having a length of 72/t, and of none with 

 a length of 7 1/*,, though in reality all lay within these two limits and 

 as many were as near to one as to the other. 



As previously mentioned, the graduations along the ordinate 

 represent the number of individuals, each small interval representing 

 one individual. In Jennings' (1908) work these intervals represent 

 percentages of the total number of individuals. But it happens that 



I.I 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 



Fig. A. Graphic representation of the variation in the ratio of length to 

 breadth among 200 Balantidium chosen at random from samples of material taken 

 from several different pigs. The number of individuals is measured on the ordi- 

 nate, the ratios on the abscissa. The dotted line is the curve resulting from the 

 combination of the two curves shown in figure B, superimposed here to facilitate 

 comparison. 



in the graphs shown in figures B and C, the number of individuals 

 showing a certain ratio is identical with the percentage of the total, 

 for in these cases the total is 100 individuals. 



Figure A represents graphically the result of the first attempt 

 to determine the existence of different types. Measurements were 

 made of 200 individuals. At least ten slides were used in getting 

 these measurements and they were prepared from samples taken from 

 nearly as many different pigs. It will be noted that the curve pro- 

 duced by plotting the ratios of these individuals is decidedly bimodal. 

 One mode represents those individuals which are approximately 1.2 

 times as long as wide, while the other represents those which are 1.6 

 to 1.8 times as long as wide. 



These findings seemed to fully justify my early suspicions that 

 there were two very different types of balantidia parasitic in pigs. 



