552 Dr. C. A. Kofoid on Platydorina. 



form of the })lalc when tlie 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 favours 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 lUinoisensis and Eudorina elegans, where 

 there are no structural features favouring such a predomi- 

 nance^ suggests tlie possibility that the form of the colony in 

 riatydorina is the result and not the cause of this predomi- 

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

 left predominantly ])reccded the structure which favours it. 

 The organization of Platydorina suggests a descent from a 

 Eudorina-Y\kQ 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 connexion (1898) the subject of locomotion and 

 polarity in the different genera of the Volvocina? 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, Stephanospha^ra and Gonium, 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 Platy- 

 dorina we find by far the most pronounced predominance of 

 one direction of rotation, the ratio in observed cases being 

 100 rotations from left over to right to 355 from right over 

 to left. In this respect, then, so far as there is evidence, 

 Platydorina is the most highly differentiated genus of the 

 family. 



The polarity of the lower genera, Stephanosphcera and 

 Gonium^ is likewise of the simplest form, being merely 

 physiological, the same pole or face of the colony always 

 leading in locomotion. In Pandorina, Eudorina, and Volvox, 

 however, there is the added feature of tlie greater brightness 

 of the anterior stigmata, and in Pleodorina the two poles are 

 differentiated by the two types of cells, as well as by the 

 characters found in the genera just mentioned ; but Platy- 

 dorina is the only genus of the family in which polarity is 



