0)1 Pleodorbia lllinoiscnsis. 288 



rotations. This is iiulicated in a general way in the second 

 table by the plus and minus signs, which show the directiim 

 in which the rotation was of longer and shorter duration. 

 In conclusion it may be said that both directions of rotation 

 occur, though that from right over to left is more frequently 

 met with, or, in other words, is of longer duration. 



With regard to h)comotion in P. (■(tliforuica, Shaw ('94) 

 says that "the movement of the i)lant in the water was fol- 

 lowed in the case of a few individuals l)earing well-developed 

 gonidia. In swimming through the water the vegetative pole 

 is directed forward and the plant revolves to the right (in 

 observed cases) on the axis connecting the vegetative and 

 reproductive poles. The path is parallel to this axis in up- 

 ward vertical as well as in horizontal movement." The 

 polarity of this genus thus expressed physiologically in the 

 movements of the colony is accompanied by a corresponding" 

 structural differentiation of the cells composing the organism. 



In Goiiitiin, according to Fresenius ('56), the motion of the 

 colony resembles that of a wheel, progression taking place in 

 the line of the axis of rotation. According to Biitschli 

 ('83-'87, p. 858), locomotion is accomplished by the rotation 

 of the plate-like colony around its shorter axis, the direc- 

 tion of rotation being to the right in some individuals and to 

 the left in others. Pfeffer ('84), on the other hand, describes 

 the rotation during the forward movement as alternately 

 from the right and the left, Migula ('90) calls attention to 

 the wavering, often backward, and irregular movements of 

 this genus, and also notes its rotation about an axis through 

 the middle of the colony. This rotation is either to the right 

 or to the left, no predominance being mentioned. Polarity 

 is thus marked in the activity of the Gon'uim colony, though 

 not expressly marked in its structure except as it appertains 

 to the individual cells. 



In StephduosplKcvd the polarity in structure is l)ut slightly 

 marked in the colony, being indicated in some colonies by 

 the asymmetrical position of the cells, but there is a physio- 

 logical differentiation in that one pole of the colony leads in 

 locomotion. In this genus also, according to Colm ('5 2), the 



