1922] McDonald: On Ealantidium coli and Balantidium suis 281 



very constant, at least throughout the limit of possible observation. 

 These vacuoles seem clearly to originate within the ectoplasm. When 

 fully distended they encroach far upon the region of the endoplasm 

 and it becomes impossible to tell whether or not they are entirely sur- 

 rounded by ectoplasm ; but from their origin such might be suspected 

 to be the case. Also such is the case in many related forms, for 

 example, Diplodinium eodudatum (Sharp, 1914) and Euplotes patella 

 (Yocom, 1918). 



The pulsation of the vacuoles was observed in several instances 

 for a long period of time. The rate of pulsation varies considerably, 

 occurring as rapidly as once in every thirty seconds under some 

 conditions, while under others a complete cycle from discharge to 

 discharge occupies a period of five minutes. In degeneration the 

 pulsation is likely to be very much retarded or may cease entirely, 

 the vacuoles becoming enormously distended and breaking together 

 thus forming one large vacuole occupying fully one-half of the interior 

 of the organism. Following this the animal ruptures and disintegrates. 

 The observation of a considerable number of normal individuals has 

 shown the usual cycle to be as follows. At two points in the ectoplasm 

 small droplets of clear liquid appear. These increase in size and 

 become the vacuoles usually seen. Contributing vacuoles or channels 

 such as occur in Paramaecium have never been noted. When they have 

 reached sufficient size (10/x, to 15/* in diameter in the ordinary indi- 

 vidual), they change from their spherical shape and begin to bulge, 

 each on the side toward the other. These bulges elongate until they 

 meet at a midpoint. At this midpoint a new vacuole arises and into 

 it, through the channels thus formed, the two vacuoles discharge their 

 contents. This large middle vacuole almost immediately discharges 

 to the exterior through the pellicle, and at the same time the other 

 two vacuoles re-form. However, the discharge of the middle vacuole 

 may be delayed until the other two are well formed and then the 

 individual has three vacuoles present. This and other variations are 

 not uncommon and should be taken into account in the use of the 

 number of vacuoles as a basis for classification. Leuckart (1861) 

 described a third contractile vacuole though he did not give its rela- 

 tive position to the other two and he did not describe the process of 

 contraction. He reported that he had observed the vacuoles "drop- 

 like" through the cytoplasm and wandering from place to place. 

 Solojew (1901) described the two vacuoles and observed a canal con- 

 necting the two, but he did not explain its function in discharge of 

 the contents. 



