202 C. CLIFFORD DOBELL. 
INTRODUCTION. 
THE observations recorded in the following pages are the 
results of an attempt to discover the life-histories of the 
protists which inhabit the gut of the common frog and toad. 
My original intention was the investigation of the life-history 
of Trichomonas. But so great is the number of organisms 
which live in company with this form that I very soon 
perceived that it would be almost impossible to confine my 
investigations to a single species. Every organic particle in 
the alimentary canal had to be tested regarding its possible 
relationship to the organism which, in particular, I was 
examining. lor example, I not infrequently found a number 
of cysts occurring side by side with Trichomonas in the 
frog’s gut. To connect the two, without further evidence, 
would be ridiculous. I therefore had to discover to what 
organism the cysts belonged. And thus, time after time, I 
found myself driven to determine, to the best of my ability, 
the life-histories of all the protists which I encountered. 
The number of these is considerable. Therefore there were 
many difficulties in the way of success, and therefore, also, 
my work remains still far from finished. 
My attention from the first has been chiefly directed towards 
the smaller protists, as these were relatively less known. 
There is but one of the larger forms, however—O palina— 
which has really been carefully studied. 
A part of my work has already been published—namely, 
that dealing with the little flagellate which I have called 
Copromonas subtilis (10); that dealing very briefly with 
a very small portion of the bacteria-like organisms (11); a 
preliminary notice of the organisms discussed in the present 
paper (12); a description of a portion of the life-history of 
the yeasts (18). I have also published some observations on 
a peculiar process of degeneration in Opalina (9). 
I will preface my own observations with a brief summary 
of the work which has previously been done by others. 
