Dr. C. A. Kofoid on Platydorina. 547 



colony the amount of the matrix substance is much less in 

 P/afi/donna than in related forms such as Eudorina. 



The cells of the colony arc ail of one type, alike, in structure, 

 and apjdoxiinately similar in size. Each is bitl.ii^ellate and 

 has a central body of protoplasm with a nucleus, two contrac- 

 tile vacuoles, one stigma, and one chroniatophore with a single 

 pyrenoid. 



The number of cells in the colony is either 16 or 32 ; at 

 least no normal colony with cells of any other number has 

 been detected among the hundreds, if not thousands, of 

 colonies examined. Colonies are frequently seen which, by 

 reason of parasites or from other causes, have lost one or more 

 cells — indeed, in some cases all but one or two ; but the form 

 of these colonies is usually preserved, and the empty secondary 

 sheaths frequently remain as evidence of the original comple- 

 ment of cells. The 16-cell colonies are not jnere stages in 

 the development of the 32-cell form, for division of the cells 

 of this type in observed cases leads to the development of new 

 colonies and not to the formation of the 32-cell stage. As in 

 other nearly related genera of the family — for example, Eado- 

 rina^ Pandorina, and Phodorina — the number of cells in the 

 colony varies, within narrow limits, in the ratio of geometrical 

 progression. In Platydorina, however, this pleomorphism is 

 manifested not only by this difference in the number of cells 

 in the colony, but also by a structural distinction — the presence 

 of three tails in the IG-cell and five tails in the 32-cell colony. 

 Inasmuch as the two types always occur together, and since 

 this pleomorphism is in some respects similar to that of 

 related genera, it does not seem justifiable to regard the two 

 as distinct species of the genus. They are, 1 believe, two 

 forms of one species. 



The arrangement of the cells is characteristic and is 

 strikingly different from that of any other genus of the family. 

 The gelatinous matrix and sheath contbrra to the horseshoe- 

 shaped plate of cells, and even the caudal appendages bear a 

 fixed relation to the plan of cell-arrangement. The 32-cell 

 colony is composed of a marginal U-sliaped row of 12 cells 

 about three sides of a 20-celled somewhat rectangular plate, 

 which, in turn, consists of an outer row of 12 cells on three 

 sides of a row of four pairs of cells. The colony might also 

 be regarded as made up of three U-shaped rows of 12, 12, and 

 8 cells lesptctively, nested in such a fashion that the inner 

 two project one ceil beyond the outermost. The cells also 

 fall into six quite irregular transverse rows of 4, G, 6, G, G, 

 and 4 cells respectively, and into the saine number of corre- 



