Dr. C. A. Kofoid on Platydorina. 551 



of origin of the Hagella, being somewhat wiilely separated. 

 At diastole the vacuoles of an adult colony have a diameter of 

 l'0-2 fi. The contraction is rhythnnciil and the two vacuoles 

 usually alternate at regular and equal intervals. At a tempe- 

 rature in the laboratory of 20° C. each vacuole contracted at 

 intervals of forty-tive to Hfty seconds. In rare instances the 

 contractions of the two vacuoles wore 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 lelt over to right or from right over to left. The rounded 

 end of the colony is unitbruily directed forward in locomotion ; 

 at least, no instance in which the caudal cud 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 locomotion is blocked by 

 obstructions the rotation continues, as in P/eodorina, with 

 frequent reversals in direction. In fact, obstruction to pro- 

 gress seems frequently, though not uniformly, 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 rive from left over to 

 right. In another twenty-rive 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 rifihs 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 periods vary greatly in length, that from 

 left over to right lasting at times but a lew 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 Ibrward 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- 

 tion of rotation is doubtless the coordinated action of the 

 flagella, since reversal of direction does not seem to be 

 aceom])anied by any change in the direction of the torsion of 

 the colony. I'he evidence upon this point is not conclusive, 

 but repeated etlbrts have failed to detect any change in the 



