Protozoan Studies 2/9 



continuations of the .1, .2, and .3 mm. ordinates. The height of 

 one curve is not to be compared with that of another as it depends 

 on the number of individuals measured and the number of classes, 

 but the spread and Hmits of the curves are to be compared. The 

 specimens were measured after kiUing in one per cent chromic 

 acid, a mechanical stage was used to prevent any unconscious 

 selection and the measurements for some curves were made with 

 an ocular micrometer, and for other curves with a camera lucida. 

 Fig. 10, a represents the lengths of 218 individuals from a culture 

 of P. caudatum from Hinkson Creek, Columbia, Mo., kept in the 

 laboratory one month. Individuals .2 mm. in length form the 

 class of greatest frequency. The next curve {b) is of 234 indi- 

 viduals from a pond near by. The class of greatest frequency 

 is .19 mm. The third curve {c) is of 219 individuals from Ash- 

 land, Va. They had enough food for health but not enough for 

 reproduction. The class of greatest frequency is .182 mm. After 

 feeding this culture 24 hours on hay infusion, 184 individuals were 

 measured (J), and the class of greatest frequency calculated at 

 .191 mm., showing that the majority had increased in length. 

 It will be noticed that the curve extends farther to the left. This 

 is due to the fact that many individuals had recently divided and 

 were shorter than before feeding. These and all other cultures 

 of P. caudatum gave a nearly symmetrical curve. Such was not 

 the case with P. aurelia (shown in the last two curves). The curve 

 e is plotted from the lengths of 297 individuals from an old cul- 

 ture found at the University of Missouri. The class of greatest 

 frequency is .133 mm. Hence most of them are shorter than the 

 majority of P. caudatum, but it will be seen that the second mode 

 of the curve is composed of individuals longer than the majority 

 of P. caudatum. At first I thought some individuals of caudatum 

 had gotten into the culture, but an examination of the micronuclei 

 of a large number of individuals both large and small proved that 

 not to be the case. The next curve {d) is of 127 individuals from 

 a culture from a drain ditch at Columbia, Mo. It shows two 

 modes similar in position to those in the preceding, but the highest 

 is of the largest individuals (.26 mm. in length). From these two 

 cultures of P. aurelia I isolated individuals of different lengths 



