The work discussed in this paper is a continuation of that described in a previous communication to the Centralblatt für Bakteriologie. Unfortunately it has been found impossible to bring all the problems taken up to a satisfactory conclusion in the time available. And as, owing to unforeseen circumstances it is necessary, for the present at least, to leave this subject, the results so far as they have been arrived at are brought together in this publication and the lines on which it had been intended to work are indicated. It is hoped, however, that an opportunity of continuing this work on the soil Protozoa may present itself at some future date.