THE NAUTILUS. 19 



On August 14, twenty-five liters of water was dipped from, 

 the surface out in the river in fairly deep water and in the 

 channel. In the part examined one very minute glochidium 

 was taken. 



On August 19, in taking a surface towing by dragging the 

 townet from a boat going down stream from a bar above the 

 station, and in fairly deep water, three glochidia were cap- 

 tured. 



To sum the matter up, there was not a single collection of 

 surface plankton taken from the river in which there was not 

 one or more glochidia, and indeed, until the river became low 

 and calm, permitting the development of plankton organisms, 

 the glochidia usually outnumbered any other organism; the 

 river, except during the conditions above mentioned, being 

 remarkedly plankton-poor. In every instance, too, where ex- 

 amined repeatedly and at long enough intervals, the glochidia 

 proved themselves alive by a feeble snapping of their valves. 

 The flapping of the valves was always too feeble and too 

 widely separated in time intervals to be effective as a means 

 of locomotion. It may, of course, have been much more vigor- 

 ous and frequent for a time after first discharged, but there 

 is no probability that it could ever have resulted in swim- 

 ming." 



On the assumption that the glochidia lie on the bottom 

 where discharged, and there die unless they become attached 

 to a fish, one of the most important advantages served by 

 parasitism is that of dissemination. In the light of the obser- 

 vations recorded above, it becomes evident that distribution 

 down stream is common and that perhaps many, if not most, 

 natural infections take place some distance from and below 

 the place of discharge. The importance of parasitism as re- 

 gards dispersal is therefore confined chiefly to up-stream 

 migration, although of course dispersal in other directions is 

 greatly assisted and accelerated by means of the fish. 



The surface-floating habit of glochidia explains also the 

 occurrence of Anodonta imbecillis, a species which is capable 

 of developing without parasitism, in floating crates, the bot- 

 toms of which are considerably above the level of the bottom 



