256 
The factors determining the decrease are the proportions 
of silt and plankton and the character of each. When floccu- 
lent debris is abundant, or when filamentous diatoms or alge, 
Copepoda, or the Cladocera with long antenne are present in 
numbers, the decrease upon centrifuging is greater. When the 
silt is earthy or contains considerable quartz, and when the 
plankton consists of Protozoa such as Synura, or Rotifera such 
as Syncheta, or Cladocera such as Chydorus or Bosmina the de- 
crease 1s less. The amount of plankton placed in the tubes of 
the centrifuge also slightly affects the ratio of decrease in vol- 
ume. For example, one of our largest planktons, measuring 
11.15 cm.’ by the gravity method, fell but 8 per cent. when cen- 
trifuged in a single tube. When divided among three tubes 
the decrease became 14 per cent. This was a plankton largely 
composed of Chydorus and Bosmina. Another large plankton, 
measuring 11.85 em.? by the gravity method, fell to 7.6 cm. upon 
centrifuging in a single tube—a loss of 36 per cent. This con- 
sisted very largely of Synura. As other large catches decreased 
as much as 50, or even 60, per cent., it is clear that large vol- 
umes do not necessarily yield only shght decreases. 
The instances in which the decrease exceeded 70 per cent. 
are 5 in number. Of these, 3 contained Melosira or Fragillaria, 
1 was rich in Oscillaria, and 2 contained considerable floccu- 
lent debris from aquatic vegetation. All of the 12 whose de- 
crease was less than 30 per cent. occur in April and May, when 
Chydorus and Bosmina are at their maximum and constitute 
a large, if not the greater, part of the plankton. In a few in- 
stances these catches which showed slight reduction in volume 
contained Hydra, insect larve, and other adventitious forms 
from surrounding vegetation. 
Both extremes contain numerous instances in which the 
plankton catch is made up of typical plankton organisms, and 
consequently the range in the decreases here recorded is normal 
Jor the range in pianktons occurring in our waters throughout the 
year. Itistherefore reasonable to assume that the centrifuged vol- 
umes here reported for plankton in the Illinois must, onthe average, 
