On Platydorina. 431 



tion of rotation is doubtless the coordinated action of the 

 flagella, since reversal of direction does not seem to be 

 accompanied by any change in the direction of the torsion of 

 the colony. The evidence upon this point is not conclusive, 

 but repeated efforts have failed to detect any change in the 

 form of the plate when the direction of rotation is reversed 

 in the living and moving colony ; and, again, colonies when 

 killed suddenly have always the usual form of spiral, though 

 some of them were moving in the reverse direction. When 

 the usual direction of rotation is reversed, the forward motion 

 still continues in spite of the fact that then the form of the 

 plate favors a backward movement ; the form of the colony, 

 therefore, does not control the direction of rotation though it 

 is correlated with the direction which predominates. The 

 fact that the rotation from right over to left predominates 

 also in Pleodorina illinoisensis and Eudorina elegans, where 

 there are no structural features favoring such a predominance, 

 suggests the possibility that the form of the colony in 

 Platydorina is the result and not the cause of this predom- 

 inance, and that the function of turning from right over to 

 left predominantly preceded the structure which favors it. 

 The organization of Platydorina suggests a descent from a 

 Eudorina-like form, in which event the systematic series and 

 the phylogenetic series alike afford evidence of a function 

 arising in an organism before the structure with which it is 

 correlated appears. 



In another connection ('98) the subject of locomotion and 

 polarity in the different genera of the Volvocince was reviewed 

 and discussed. It will suffice, therefore, for the present to 

 give a brief resume of the facts. In the lower genera of the 

 family, Stephanosphcera and Goniiim, as also in Pandorina, 

 the rotation seems to be indifferently to the right or left, while 

 in Eudorina and especially in Pleodorina illinoisensis it is 

 oftenest to the left, rotations to the right in observed cases of 

 the latter species being to those to the left as 100 to 117-138. 

 With respect to Volvox there are no data at hand. In Pkity- 

 dorina, we find by far the most pronounced predominance of 

 one direction of rotation, the ratio in observed cases being 



