CENTRIFUGE IN POND- LIFE WORK. 249 



special captures, and in fact microscopic aquatic animals generally, 

 it is better to place them in water taken from their own pond 

 rather than in ordinary " tap " water. Hitherto there has been 

 no certain basis for this empirical rule, but now it is seen to 

 find its justification in the fact that the special food of the 

 animals is no doubt present in the pond water in the form of 

 extremely minute algae, flagellates, diatoms, etc. 



By the help of the centrifuge it is even possible to in- 

 crease the amount of food material normally present in pond 

 water, and in this way to rear such creatures as Rotifers, 

 Entomostraca, etc., much more certainly and rapidly than before. 

 This is especially true of the plankton forms which have hitherto 

 resisted nearly all attempts at cultivation in captivity. When 

 supplied, however, with their proper food obtained by centrifuging 

 the water of the lake or pond in which they lived, it is found 

 that they can be kept in confinement almost as easily as the 

 hardier littoral species, a fact Avhich will evidently be of great 

 value in various researches on these very interesting forms. 



In conclusion it may be repeated that the principal use of 

 the centrifuge in pond-life work, as in marine plankton work, 

 is not to take the place of other methods of collection such as 

 nets and filters, but to be accessory to them. As such an accessory 

 piece of apparatus the centrifuge has evidently come to stay, 

 and no method of collection depending entirely upon straining 

 or filtering processes can be considered sufficient in the future. 



Since the reading of the above, Lohmann has issued another 

 very useful paper on the subject (9). He uses the word " Nan- 

 noplankton " {vdvvo<s = dwarf) as a convenient term for the 

 organisms which are so small as to pass easily through the 

 meshes of the finest nets, and he gives a great deal of very in- 

 teresting information both as to the method of centrifuging 

 water and as to the results obtained. He particularly insists 

 upon the fact that owing to their great rapidity of reproduction 

 the " Nannoplankton " organisms are of much greater import- 

 ance than might be imagined from their relative abundance at 

 any one moment, a point of view which is sometimes ignored 

 in considering the available food supply of the larger aquatic 

 animals. A number of " Nannoplankton " organisms (all marine, 

 however) are figured, and the excessive minuteness of the majority 



