289 



In making a haul, the net is lowered slowly at first, so as to allow it to fill 

 Avitli water that has filtered through the bolting cloth. This prevents any abnormal 

 amount of plankton which might occur if the surface water, rich in plankton, 

 were allowed to fill the net without first being strained. The net is now lowered 

 to the desired depth and hauled in hand over hand. The number of seconds re- 

 quired to raise it to the surface is noted. This will give the velocity, and from 

 this the coefficient of the net, or its efficiency in straining, is calculated. The net 

 is now raised out of the water, and while the process of filtering is going on some 

 water is dashed against it so as to wash down any organisms that may be lodged 

 on the inside. When the filtering process has progressed far enough, the binding 

 screws are loosened and the bucket is removed. Filtering is now hastened by 

 gently tapping the wire gauze with the hand. As soon as nearly all of the water 

 has filtered. out the bucket is placed over a small, glass-stoppered bottle which 

 will hold about 250 cc, the rubber stopper is removed and the contents transferred 

 to the bottle. The organisms that may be lodged on the sides of the bucket are 

 rinsed into the bottle with filtered water. Enough 95% alcohol is now added to 

 the contents of the bottle to give the whole a strength of 70%. This kills and 

 j)reseives the organisms and facilitates the work by eliminating two or three 

 steps which are necessary when some other killing agent is used. Besides, the 

 organisms were found to be in a very good condition for qualitative work when 

 killed and preserved in this way. 



In measuring the quantity of plankton, a centrifuge manufactured by 

 Richards & Co., of Chicago, was used. This is preferable to letting the material 

 settle twenty-four hours, because it requires less time and puts the mass into a 

 more compact form. This method also has an advantage in that it makes the 

 measurements more uniform. If there is a large amount of light material in a 

 haul, this will not settle very compactly in twenty-four hours, consequently the 

 volume will be large. On the other hand, another haul may have a greater mass 

 of material, but not yield such a large volume, because it is composed of material 

 that will settle more compactly in the twenty-four hours. 



Each bottle is permitted to stand an hour or so after it is taken to the labora- 

 tory. Some of the alcohol may then be removed without danger of losing any of 

 the plankton. The remaining material is shaken up and poured into a graduated 

 cylinder. The bottle is carefully rinsed and the rinsings added. The material is 

 now thoroughly stirred with a small glass rod, so as to distribute the organisms 

 equally throughout the liquid. The two sedimentation tubes, which are gradu- 

 ated to tenths of a cubic centimeter, are filled and placed in the metal cases of 

 the centrifuge. The drive wheel is turned through 50 revolutions and the tubes 

 are permitted to stand a few minutes so that the few small organisms that may 



