1922J McDonald: On Balantidhim coll and' Balantidium sxiis 273 



maceration remains attached to the basal end of the cilium. So the 

 basal apparatus in Balantidium coli even to the relative location of 

 the basal granules is almost identical with that in Isotricha prostoma. 

 In Balaintidium, however, as mentioned above, the "Grenzschicht" 

 seems to be lacking. The comparison becomes more significant in view 

 of the fact that both ciliates are parasitic in the digestive tract of 

 maimuals, and both are in much the same state with reference to 

 the degree of specialization and degeneration correlated with habits 

 of the parasitic mode of life. So that in general morphology they 

 seem to be more alike, though they are in separate orders, than do 

 Diplodinmm ecaudatum and Balantidium coli, which are in separate 

 suborders only. 



Ciliary Movements. 



Locomotion is the chief function of the cilia except for those of 

 the adoral zone. The balantidia swim in approximately a straight 

 line and not in a spiral course as do paramoecia. They do, however, 

 rotate on their axis as they progress. This rotation is generally from 

 left to right, that is, in a counter-clockwise direction when viewed from 

 a point in front of the animal. A few instances of i-eversal of the 

 direction have been seen, but it is not at all common. The direction 

 of rotation, i.e., from left to right, seemed at first inexplicable, since 

 this was not compatible with the direction of the rows of cilia. The 

 rows of cilia, as described above, are comparable to the threads of a 

 left-hand screw. In order to penetrate, such a screw must be turned 

 in a clockwise direction (when viewed from the point, not from the 

 head). Such, also, is the direction of rotation of balantidia which one 

 would expect to find if the arrangement of the cilia were the con- 

 trolling factor, but the rotation is in the reverse direction. In the 

 further study of the problem, I fortunately obtained some very thin 

 tangential sections of animals. on which the fixative had acted so 

 quickly that the cilia were stopped instantly and left in the relative 

 positions assumed in normal ciliary action. 



Figure N is a camera lucida drawing of such a section. By analysis 

 of the position of the cilia on this and other like sections, it was 

 possible to determine the complete cj^cle of a single cilium. This cycle 

 is diagrammatieally represented in figures M and 0. Figure N 

 includes about two and one-half cj'cles of action as represented by 

 the waves. The dark portions are produced hy the prostrate position 

 of the cilia at the termination of the effective stroke. The lighter 



