FILTER-BED NEMAS AND PUBLIC HEALTH 191 



beds. Beginning with a new period of use, it appears that minute 

 species, e.g., bacteria and protozoa, having a short life cycle and suited 

 to the new conditions, first make their appearance; these multiply 

 and become the food of succeeding species, which in their turn give 

 place to others. The problems presented are of great biological interest, 

 and may not be without some bearing on public health. Beyond 

 doubt they have a more or less important relation to the economical 

 and effective management of the filter-beds. 



DETERMINING FACTORS OF FILTER-BED POPULATION 



Organisms other than Nemas. Many other kinds of organisms are 

 found in filter-beds, some of them in far greater numbers than the nemas. 

 Bacteria, fungi and protozoa occur, of course, in abundance, and are, 

 I believe, the organic basis on which is built up the later animal pop- 

 ulation consisting of organisms of larger size, such as the nemas. 

 Rotifers are not uncommon. Small oligochaetes, earthworms, occur, 

 especially after long use, but never in such myriads as in sewage. Small 

 crustaceans, such as daphnia and cy clops, sometimes occur, but I have 

 never seen them in large numbers. Occasionally aquatic insects are 

 found. The filter-beds of each city present biological peculiarities 

 dependent upon the source of the water supply, for the biological 

 characteristics of filter-beds depend to a considerable extent on the 

 climatic and geological conditions prevalent on the water-shed from 

 which the supply is drawn. 



PHYSIOLOGICAL SIGNIFICANCE OF FILTER-BED NEMAS 



Illness from Change of Water. It is a well-accepted idea among 

 physicians, as well as laymen, that a change of drinking-water may 

 cause intestinal disorders; why they are thus caused is not always clear. 

 If the waters in question are widely different in composition, for instance 

 one soft and the other hard, it is easy to understand how intestinal 

 derangements might follow a change from one to the other; but are 

 the derangements due to change of water always associated with such 

 marked chemical differences? Do not intestinal disturbances follow 

 changes of water in which the usual tests would show but very slight 

 differences? 



Soluble Excreta in Drinking-water. Is it possible that slight quantities 

 of organic substances found in drinking water and of a character as 

 yet unknown, might, under some circumstances, exert a powerful 

 physiological influence? If this question be answered affirmatively, 

 a wide field of investigation is opened up in connection with potable 

 waters, and it is in this connection that the present researches are 



