V7 
tiver with a view to determining the diurnal fluctuation in the 
amount of the dissolved gases. In all these operations parallel 
collections of the plankton have been made at the same time 
and from water collected in the same manner, the plankton 
pump being used for the collection both of the plankton and 
the water for gas analysis. The twenty-four-hour series col- 
lected in 1898 can be brought into correlation with the move- 
ments of the water-bloom, which is a marked feature of the 
plankton of the river during the warmer months of the year. 
About one hundred bottles are comprised in the collections 
belonging in this series. 
The serial plankton work rests upon the supposition that a 
single catch made in a typical locality of a lake or river will 
‘give a fair sample of the microscopic life of the water, both as 
to its quantity and constituent organisms. With a view to 
testing the validity of this supposition, Thompson’s, Quiver, and 
Mantanzas lakes and the river have in previous years been sub- 
ject to extended examinations, collections being made on the 
same date at a considerable number of localities at regular 
intervals throughout the body of water examined. In 1897 
Thompson’s Lake was reéxamined on this plan and a biological 
cross-section of the river was made at Havana. This series of 
collections has been increased by twenty-five oes during the 
period covered by this report. 
The tests looking toward the detection and correction of 
sources of error in the plankton method and the justification of 
changes in it which we have made, have been continued during 
the past two years. Of the collections made in these tests about 
one hundred and fifty have been preserved. Tests have been 
made of the errors resulting from leakage, from the progressive 
clogeing of the drawn net and the consequent variable coefficient, 
and from the active escape of the larger organisms of the plank- 
ton. Several types of funnels for the in-take of the plankton 
pump have also been devised and tested. ‘Tests of the leakage, 
through the silk and efforts to correct it by some form of micro- 
filter or precipitation method have also been continued. A 
variety of filters have been examined, including the Sedgwick- 
tafter sand filter, using sharp Berkshire sand according to the 
