248 D. J. SCOUEFIELD ON THE USE OF THE 



long delicate spines or other out-growths, or are themselves of 

 a very attenuated form, which is especially the case with the 

 desmids and diatoms. It is somewhat difficult at present to- 

 name all the different kinds of organisms which occur in 

 collections made with the centrifuge, as comparatively little 

 attention has yet been given to these very small forms. I have 

 seen quite a number of organisms which are new to me, and, 

 although I cannot be sure that they are really new, I believe 

 that almost certainly some of them have never been named. 

 Very probably some may prove to be immature forms or special 

 stages in the life-history of somewhat larger organisms. 



The application of the centrifuge to the investigation of lakes- 

 and other fresh waters has already thrown considerable light 

 on various hitherto obscure phenomena. For example, at the 

 Biological Station at Lunz, in Austiia, it had long been observed 

 that the plankton Daphnias of the upper and lower lakes^ 

 although belonging to the same species, were in much better- 

 condition in the former than in the lattei-, in spite of the fact 

 that the small plankton algae, etc., obtained by the usual net 

 methods, and on which the Daphnias were supposed to feed, were 

 actually more abundant in the lower than in the upper lake. 

 It looked very much like a case which might possibly support 

 the idea of Putter, that aqviatic animals get some at least of their 

 nourishment direct from organic compounds dissolved in the 

 water. When the water was centrifuged, however, it was^ 

 found that the explanation of the diffei'ence was very simple, 

 for it was at once observed that the very minute organisms w^ere 

 far more plentiful in the upper than in the lower lake. Thus 

 there was no necessity for any extraordinary hypothesis as to- 

 how the " Obersee " Daphnias obtained their extra food. 



Arising out of this experience, Woltereck (11) was led to 

 investigate the question of the natural food of Daphnias and 

 similar creatures, and he was able to prove experimentally that it 

 really was the extremely minute forms of algae, etc., which counted 

 most in this connection. He fed his Daphnias on pin-e cultures of 

 small algae of various sizes, and found that the animals thrived 

 better on the excessively minute forms than on the forms which had 

 formerly been considered to be the principal source of their food. 



These results very obviously explain the fact known to all 

 experienced " pond-hunters," that in order to keep alive an3r 



