CONTROL OF FILTER-BED NEMAS 193 



found effect upon the human organism. Such reflections lead to the sug- 

 gestion that the study of filter-bed organisms is one that should be 

 prosecuted more vigorously. We have developed a few excellent chemical 

 tests, and, so far as it goes, an excellent system for determining the 

 bacterial content of drinking-water. Why not go a step farther and 

 make at least an attempt to determine the nature of the minute quan- 

 tities of soluble organic substances of physiological significance which 

 may be present, and the origin and nature of these substances. 



POSSIBLE CONTROL OF FILTER-BED ORGANISMS 



If it should be found that the presence of a particular micro-organism 

 in filter-beds is deleterious, is it possible so to manage the beds 

 as to exclude the micro-organism, or counteract its effect? Already I 

 feel sufficiently conversant with some of the facts to predict that such 

 control will prove feasible, at least in some instances. In the case of 

 Mononchus longicaudatus for instance, as soon as we know the natural 

 distribution of the Mononchus; its relationship to the seasons of the 

 year, if it has any significant relationship of that kind; the period of 

 its life cycle; its rate and method of reproduction; its food; its enemies; 

 then, almost beyond doubt we shall be able to suggest means for its 

 control. So with other organisms. 



To secure a reliable filter-bed census it is necessary to examine the 

 sand as soon as collected. After a short period under laboratory con- 

 ditions the population begins to change: e.g., sand which at the time 

 it was removed from the bed contained many specimens of Mononchus 

 and a few of Ironus ignavus, after ten days yielded no Mononchus, though 

 it continued to yield Ironus, and in addition a few adult specimens of 

 Tripyla and Monhystera, differences no doubt due in part to the fact 

 that in stagnant collections Mononchus longicaudatus is subject to the 

 attacks of a variety of fatal diseases caused by fungi and microbes. 



NEMAS A CLUE TO THE FLOW 



Nemas Unequally Distributed in Beds. The distribution of organisms 

 in filter-beds is not uniform. For example, if a series of samples be col- 

 lected, one each from near the main drain-pipe, near a lateral, between 

 the laterals, and at the margin of the bed, the numbers and kinds of 

 nemas will be found to differ in the various samples. This unequal 

 distribution is doubtless a function of the flow of the water, for where 

 the flow is rapid the biological environment differs from that where it 

 is slower; there is a greater supply of oxygen, a greater supply of water- 

 soluble food, and a greater supply of such free micro-organisms as may 

 pass between the grains of sand. It follows that to some extent the 



