64 R. GREIG-SMITH. 



toxins. 1 Even the toxins secreted by bacteria may have 

 an effect upon the fauna, for Cropper and Drew 2 believe 

 that the amoebae encyst in order to protect themselves from 

 the toxic products secreted by bacteria in the medium. 



In the soil, one must distinguish an active, a resting and 

 a cultural micro-fauna. Martin and Lewis believe the 

 active fauna to consist chiefly of small flagellates and 

 amoebae. 



It is doubtful whether the active protozoa have much 

 effect in reducing the numbers of soil bacteria. My obser- 

 vations point to the bacteria being ingested accidentally. 

 Colpoda cucullus is very fond of bulky organic matter; I 

 have seen several feeding voraciously upon the slime extrud- 

 ing from encysting Colpodae, like sharks feeding upon the 

 carcase of a horse. The soil amoebae do not appear to find 

 bacteria very attractive, for I have seen bacteria passing 

 over the amoeba surface and being left behind. This 

 occurred in culture media, and it may be that, in such a 

 nutritive fluid, the phagocytic tendency is at a minimum. 

 There does not appear to be any reason for the ingestion of 

 bacteria if enough soluble and suitable nourishment can be 

 obtained by diffusion, a conclusion that Wolff arrived at 

 from experimental work in 1909. 3 The englobing of bac- 

 teria may be quite an accidental phenomenon, for amoebae 

 free from bacteria, have been found byMusgrave and Olegg 4 

 in sterile liver abcesses. They, probably to a slight extent, 

 do diminish the numbers of bacteria in the soil, but the 

 by-products of digestion, secreted by amoebae and other 

 protozoa, are sure to contain auxetics and soluble nitro- 

 genous matter which must enable the surviving bacteria to 

 multiply to some extent which may even counterbalance 

 any diminution through phagocytosis. Indeed it has been 



1 Through Bull. Inst. Past., xin, 515. a Ibid., xin, 521, 



3 Cent. Bakt., 2 te, 24, 465, * Walker, Cent, Bakt. 1 te, Eef. 50, 583. 



