282 Illinois State Laboratory of Natural History. 



is of the type prevailing among other spherical or ellip- 

 soidal genera of the Volvocinece ; viz., rotation about the 

 principal or long axis of the colony, either from right 

 over to left or the reverse, frequently with one direction 

 predominating, progression being usually along the line 

 of the axis, the same end of the colony always leading. 

 In P. illinoisensis the vegetative pole always leads in loco- 

 motion in horizontal, oblique, and vertical movements, and 

 is therefore the anterior pole of the colony. Under normal 

 conditions, when under observation, this species is rarely quiet 

 during the period of growth. While still within the matrix of 

 the mother colony the ceasless rotation with its frequent rever- 

 sals begins. Colonies in the life cell, while favorable condi- 

 tions prevail, can be seen in active movement, jostling one 

 another and their neighbors in their seemingly aimless wan- 

 derings. When an object is met which does not yield to their 

 persistent rotation, their movements may slacken for a time 

 to be resumed shortly in some line of less resistance. The 

 rotation of this species is prevailingly from right over to left 

 as the following tables show, which indicate the number and 

 direction of the reversals of rotation in ten individuals in one 

 minute. 



A few days later a second set of observations was made 

 with the following result : 



In the majority of instances where the direction of rotation 

 was observed it was from right over to left, the ratios being 

 22 to 16, and 35 to 30. These tables give some idea of bhe 

 frequency of change in direction and its variation in different 

 individuals, but do not show the duration of the directions of 



