1922] McDonald: On Balantidium coli and Balantidium suis 275 



figure 0, the funnel described by the complete beat of the cilium 

 is somewhat irregular, the rim outlined by the tip of the eilium is 

 elliptical, and the base of the cilium, i.e., the neck of the funnel, is 

 not central but is situated below the anterior focus of the ellipse. 

 The ellipse described by the tip of the cilium lies in a plane which is 

 not parallel to the surface of the animal, but which approaches it 

 much more closely posteriorly. It was possible to corroborate the 

 action of the cilia in observations on the living material. The cilia of 

 the apical cone are somewhat larger than the other cilia of the body, 

 and are closely coordinated in their action. In balantidia which were 

 allowed to cool until action had slackened considerably, these cilia 

 were observed to make their effective stroke at a decided angle to the 

 rows of cilia. For instance, in the boring action in connection with 

 the process of penetration described above, these cilia will beat in an 

 almost exactly transverse direction, always from right to left. In 

 further corroboration of this interpretation of the movements of the 

 cilia is the fact that it explains the rotation of the animal during 

 progression. It will be seen from figure that the effective stroke 

 of any single cilium will be in the direction cd. This line crosses both 

 the arrow, representing the long axis of the animal, and the line ab, 

 representing the rows of cilia, making an angle of approximately 

 20° with the latter. That is, the line cd forms an angle with the arrow 

 on one side about equal to the angle on the opposite side made by the 

 line ab. It clearly follows that if the effective stroke is in the direction 

 cd then rotation will be from left to right and not vice versa as would 

 be the case if the cilia beat in the direction of the rows in which they 

 are arranged. The structure of the basal apparatus is significant in 

 view of the direction of the effective stroke of the cilia, viz., posteriorly 

 and to the left. The cross-section shown in figTire L is viewed anter- 

 iorly and shows that the ciliary rootlet from each basal granule passes 

 to the right and enters the granular band on that side of the hyaline 

 band in which the basal granule lies. Without giving to the ciliary 

 rootlets any motor or skeletal function, it still seems logical that they 

 conform in a general way to the axis of the cilium, for the latter during 

 the greater part of its movement inclines to the posterior and left of 

 the row of which it is a constituent. 



The peristome {per. fig. J) may be said to comprise all that part 

 of the organism which lies within the row of adoral cilia. In the 

 active animal it is variable in shape. At times it is almost round 

 while at other times it may become a mere slit or groove. In what 



