422 Illinois State Laboratory of N(ttiiral History. 



latter carrying 3 or 5 prolongations or tails formed by the 

 gelatinous substance of the coenobium. The colony is plate- 

 like, and flat except that the plate is slightly twisted in a left 

 spiral. This spiral is scarcely noticeable in a face view 

 (Fig. 1) except by focusing with a high-power objective, but 

 it can be easily detected in a side view (Fig. 8). It varies 

 from one eighth to one thirty-second of a turn of the spiral, 

 and in twenty-five colonies especially examined on this point 

 it was invariably a left spiral, with the location of the twisting 

 always in a definite relation to the colony, the right anterior 

 and the left posterior regions of the colony in face view being 

 high, while the left anterior and the right posterior are low. 

 Eepeated examinations of specimens, both living and pre- 

 served, indicate that this spiral form is a constant feature of 

 structure ; that it is not reversed in direction ; and that it is 

 subject to but slight variation in the degree of the torsion. 

 No movement within the colony which would produce or vary 

 the spiral was noted in living individuals. The form of 

 Platydorina seems to be as constant and as characteristic as 

 that of other genera of the family. 



The size of the colony varies with the age, with the number 

 of cells present, and also, perhaps, with the locality and the 

 season. Colonies of 32 cells in which the first division lead- 

 ing to the formation of daughter colonies is taking place, 

 average about 150 /'in length, 180 /' in width, and 20 /< in 

 thickness. The largest colonies are about 165x145x25 yw, 

 and the smallest free-swimming ones about 25x21x4 /<. 

 Colonies of 16 cells are smaller than those of 32 cells, and 

 are also narrower in proportion to their length, measuring 

 about 70x43x16 /^. 



The colors of Plati/dorina are quite as striking as those of 

 related forms. The cells, which are imbedded in the trans- 

 parent gelatinous matrix of the colony, are a bright chloro- 

 phyll-green, and each has, as a rule, a red stigma, or eyespot, 

 of unusual brilliancy. 



The substance in which the cells of the colony are imbedded 

 is similar in appearance to that in Eudorina. It is a trans- 

 parent colorless substance of considerable consistency, show- 



