Dr. C. A. Kofoid on Pleodorina illinoisensis. 



147 



and what seem to be the enlarged bases of the flagella are the 

 only areas discernible in the anterior end of the cell which at 

 all resemble a contractile vacuole. Shaw (1894) finds in 

 picronigrosin material a single vacuole in the anterior end of 

 the young cells of P. californica. 



The flagella (/.) are two in number for each cell, and unite 

 with the cell at the anterior end adjacent to the stigma. The 

 two flagella have the same proportions, and in adult colonies 

 they measure 40 fi in length. In the young colonies they 

 are relatively longer. They are visible on the young colonies 

 shortly after the cup closes, and persist upon the maternal 

 colony during the early divisions of the gonidia. 



The locomotion of the colonies of Pleodorina illinoisensis 

 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 ceaseless rotation with its frequent rever- 

 sals begins. Colonies in the life cell, while favourable con- 

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

 another and their neighbours 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 ; — 



10* 



