On Platydorina. 433 



the bottom rather than in the superjacent strata of water 

 where plankton collections are usually made. Aquaria about 

 to dry up were also searched in vain for sexual stages of 

 Platydorina . 



The mode of development of Platydorina is significant of 

 its systematic position and its relationships. The number 

 and the original arrangement of the cells, the type of develop- 

 ment, and the character of the envelope, all indicate that 

 Platydorina is a more highly specialized form descended from 

 some Eudorina-\i\.e ancestor, and that it is more closely 

 allied to Eudorina than to any other existing genus. 



Throughout this paper the customary term " colony " has 

 been used to designate the organism herein described and 

 others related to it. The wide use of the term in the literature 

 of the subject is doubtless due to the fact that, as a rule, the 

 organisms are composed of similar cells arranged in symmet- 

 rical form with no pronounced axial differentiation, without 

 contact or protoplasmic connection, separated from each 

 other by a non-living gelatinous matrix, and each capable of 

 performing all the functions necessary for its own life and the 

 continuance of the species. Furthermore, the destruction of 

 individual cells does not impair the life of the other cells of 

 the organism, for so long as a single cell remains it continues 

 its customary activity. The use of the term colony is, however, 

 objectionable. A number of facts militate against this con- 

 ception of the organism, and the discovery of the new genus 

 here described adds to the array. (1) The cells are not 

 always similar, for in all forms with poles physiologically or 

 otherwise differentiated the anterior stigmata are brighter 

 than the posterior, and in Pleodorina there are two kinds of 

 cells, the vegetative and the gonidial, the former distinctly 

 smaller than the latter. (2) There is in all of the higher 

 genera a well-defined physiological polarity accompanied by 

 the difference in the anterior and posterior stigmata, and also, 

 in Platydorina, by a differentiation of the poles by the 

 arrangement of the cells and the structure of the envelope, 

 and by the further differentiation of a transverse axis. 

 (3) In Pandorina the cells are almost in contact, and in 



