2l6 WILLIAM RAY ALLEN. 



of the mussel, the recovery of only a fraction of them is sufficient 

 in some lakes to equal or surpass the total volume of larger or- 

 ganisms utilized. 



(a) Experiments in Selective Feeding. 



A few experiments were made to separate the nannoplankton 

 from the grosser, in order to feed them separately. 



There is no hard and fast line by which the nannoplankton may 

 be distinguished, except Lohmann's ('n) arbitrary size limit of 

 25 microns. Stomach and intestinal contents were examined for 

 evident of a predominance of either larger or smaller forms. 

 So far as the diatoms are concerned such evidence is not very 

 conclusive, though on the whole the smaller organisms appar- 

 ently have the better of the argument. As for Flagellata, they 

 were commonly seen in the rectal contents, but more often in the 

 stomach. This is as we should expect, for some of them do not 

 possess so resistant a test as the diatoms. 



The fact that even a few of the smaller flagellates and dia- 

 toms are found demonstrates that the gill-meshes are fine enough 

 to accomplish something with the nannoplankton, while the di- 

 minished number of flagellates in the rectum implies the more 

 complete digestion of that group. Certain flagellates are more 

 resistant to digestion than others, e.g., Peridiniwn; some are 

 doubtless, of more frequent occurrence in the alimentary tract on 

 account of their colonial form and greater bulk, e.g., Pandorina. 



We have no very efficient mechanical means for separating 

 plankton according to size. The nets of finest bolting silk allow 

 nearly all flagellates and all but the large or colonial diatoms to 

 pass through. It was found that water, poured through a net 

 suspended in the air, forces more large organisms through the 

 meshes than in the case when the net is suspended in water, and 

 the water containing plankton poured through slowly. 



A concentration of net plankton actually took place in the 

 plankton bucket. A complete separation of coarse from fine 

 plankton is not claimed, nor was it necessary to the experiment. 

 Undoubtedly some of the grosser forms passed through the silk. 

 But these were never in sufficient quantity to be detected in the 



