190 FILTER-BED NEMAS 



one occasion, at the end of a period of use, I found ninety-six per cent 

 of the nemas to consist of but a single species. About thirty species 

 were found inhabiting the various beds examined, twenty-five species 

 being found to inhabit the beds of one city. My observations make 

 no pretence of being exhaustive, and I presume further research may 

 easily double these numbers. Most of the species are of only occasional 

 . occurrence ; those figuring prominently in the activities of the beds are only 

 about half a dozen in number, and of these not all are equally important. 



THE MORE COMMON SPECIES 



Commonest Species Carnivorous. Of the four more important species, 

 (1) Mononchus longicaudatus, (2) Ironus ignavus, (3) Tripyla monohystera 

 and in a lesser degree (4) Ironus longicaudatus, I have shown that all 

 are carnivorous, that they feed upon a variety of living organisms, 

 and that no one of them confines itself to a single kind of food. Mon- 

 onchus longicaudatus, for instance, feeds upon several species of nemas, 

 upon rotifers, and upon a variety of protozoa. The same is true of 

 Tripyla monhystera. In the struggle among these filter-bed organisms 

 it appears that sometimes one species may almost annihilate others. 

 This accounts for such cases as that in which Mononchus longicaudatus 

 constituted ninety-six per cent of the nema population of a bed. 



ROTATION OF THE FLORA AND FAUNA 



Seasonal Fluctuations. I know little about the seasonal fluctuations ; 

 merely that they exist and that sometimes they are very marked. 

 For instance, on January 6 samples of sand were gathered from various 

 locations on a Washington filter-bed. This bed had been in operation 

 about six months, a period longer than in the case of any previous 

 examination (summer collections). About the same range of species 

 was found in this sand as had been found in all previous experience 

 taken together, but the smaller nemas, including Monhystera, were 

 much more abundant in this than in previous collections, and seemed 

 to be thriving. A large Dorylaimus that had been extremely rare 

 in previous collections was fairly common in this January collec- 

 tion. Achromadora minima was also more common. Finally there 

 were one or two small species not hitherto found, Cylindrolaimus ob- 

 tusus and a Rhabdolaimus. Whether these faunal differences were 

 due to the winter season or to the long time the bed had been in use 

 remains in certain instances undetermined, very likely some of them 

 were due to both causes. 



Economic Bearings. Apart from seasonal fluctuations, there is a 

 rotation in the fauna and flora incidental to the management of the 



