Juday—Limnological Apparatus. 
583 
quently are as follows: 1. Filtering a certain quantity of 
water through hard surface filter paper and then washing 
the organisms off the paper. 2. Filtering a definite quantity 
of water through sand as in the Sedgwick-Rafter method. 
3. The direct counting of the organisms. 4. The sedimenta¬ 
tion of the organisms with a centrifuge. 
When filter paper is used for concentration, some of the 
nannoplanktonts adhere very closely to the paper so that 
they can be removed with difficulty or not at all. Others 
become embedded in the meshes of the paper and are lost. 
In the sand filter many of the organisms adhere to the grains 
of sand and are lost. The direct count is the best procedure 
for the more abundant forms; but it is impractical for the 
rarer forms because it would be necessary to count a con¬ 
siderable number of samples in order to obtain a fair enu¬ 
meration. 
A centrifuge is very effective in securing the nannoplank- 
ton organisms, but a high speed type of centrifuge is re¬ 
quired, one with a speed of not less than 3,000 revolutions 
per minute. The machine that is now in use in the Survey 
investigations has a speed of 3,600 revolutions per minute 
and carries two 15 cc. sedimentation tubes. At this speed 
most of the organisms are thrown down in six to eight min¬ 
utes. A second run of this duration generally serves to 
recover the remainder of the material, but a third, or even a 
fourth run is sometimes required. It is desirable also to 
centrifuge a larger sample, say 50 cc. or 100 cc. in order to 
study the very rare forms. 
The results of various investigators seem to indicate that 
the best procedure for an enumerative study of the nanno- 
plankton is a combination of the direct count method and 
the centrifuge method, the former being used for the more 
abundant forms and the latter for those that are less abun¬ 
dant. It is best also to use 15 cc. and 50 cc. or 100 cc. sam¬ 
ples of water for the latter. The enumerations both in the 
direct count and the centrifuge methods should be made in 
duplicate and, if convenient, in triplicate. 
The Survey is also making a study of the quantity of 
nannoplankton contained in a body of water by determin¬ 
ing the dry weight of it per unit volume of water. Chemical 
analyses of this material are then made for the purpose of 
