430 Illinois State Laboratory of Natural History. 



contracted at intervals of forty-five to fifty seconds. In rare 

 instances the contractions of the two vacuoles were separated 

 by unequal intervals, being almost coincident in one case 

 observed. 



The method of locomotion in Platydorina is similar in 

 many respects to that of other genera of the family. The 

 lashing of the flagella produces a forward movement of the 

 colony and causes its rotation about the major axis, either 

 from left over to right or from right over to left. The 

 rounded end of the colony is uniformly directed forward in 

 locomotion ; at least no instance in which the caudal end 

 led was noticed. The forward movement is, as a rule, 

 accompanied by the rotation of the colony, though the amount 

 of rotation varies somewhat with the individual, the freedom 

 of movement, and the speed of locomotion. When locomo- 

 tion is blocked by obstructions the rotation continues, as in 

 Pleodorina, with frequent reversals in direction. In fact, 

 obstruction to progress seems frequently, though not uni- 

 formly, to act as a stimulus to the reversal of the direction 

 of rotation. 



The two directions of rotation are not equally prevalent, 

 that from right over to left having a marked predominance. 

 Thus, of twenty-five colonies observed in motion twenty were 

 rotating from right over to left and but five from left over to 

 right. In another twenty-five the corresponding numbers 

 were nineteen and six respectively. Keeping a single colony 

 under observation for some time, it is found to rotate from 

 right over to left about four fifths of the period and to turn 

 in the opposite direction the balance of the time, this pro- 

 portion representing the totals of the periods of rotation, 

 while the individual i^eriods vary greatly in length, that from 

 left over to right lasting at times but a few seconds. 



This predominance of one direction in locomotion is doubt- 

 less correlated with the torsion of the colony, whose shape is 

 such that the rotation would necessarily be from right over 

 to left in forward locomotion, as a result of the resistance of 

 the water, unless, of course, there should be some disturbing 

 factor. The immediate and most potent cause of the direc- 



