FILTER-BED NEMAS: 



NEMATODES OF THE SLOW SAND FILTER-BEDS 

 OF AMERICAN CITIES 



(Including new genera and species) 



WITH NOTES ON HERMAPHRODITISM AND 

 PARTHENOGENESIS* 



CONTRIBUTIONS TO A SCIENCE OF NEMATOLOGY, VII 

 BY N. A. COBB 



United States Department of Agriculture 



The nemas here described were collected from the filter-beds of 

 several American cities, incident to a rather extensive investigation 

 of nemas of economic interest. A study of them has afforded me such 

 an interesting and suggestive glimpse of the biological conditions in 

 slow sand filter-beds that I venture to think an account of it may be 

 of some slight use to sanitarians, and to engineers connected with city 

 and town water works. The biological observations, more particu- 

 larly those on the vanishing series of spermatozoa in syngones, may 

 prove of interest to zoologists and geneticists. 



NATURE AND NUMBER OF THE ORGANISMS FOUND 



Few Green Organisms. Comparatively few green organisms occur 

 in covered slow sand filter-beds, especially if the water passes through 

 a subsidence reservoir before entering the beds. Practically all the 

 living forms found are colorless or nearly so, and most of the larger ones 

 are animal. To me the most striking organisms in every sample of 

 sand examined were the nemas. 



Period of Use. Renewal. From time to time a few inches of the 

 topmost sand of slow filter-beds is renewed; the old sand is removed and 

 fresh sand spread in its place. The period of use, the time between 

 any two successive renewals, varies from a few weeks to a few months, 

 according to the practice of the engineer in charge. Toward the end 

 of a period of use the number of nemas in the topmost three inches of a 

 bed often mounts to hundreds of millions per acre, and sometimes 

 exceeds a thousand millions per acre. At this latter figure each glass 

 of drinking water must percolate through sand containing at least 

 about a thousand nemas. 



Dozens of Species Found. Often the nema population is of a mixed 

 character, but sometimes it is comparatively homogeneous. On 



*Waverly Press, Baltimore, Jan. 11, 1918. 



