For the convenience of those who wish to study more of the 

 details and particular features of certain forms, there is ap- 

 pended to each chapter a list of important references which 

 will serve as guides to a more comprehensive literature on the 

 subject. 



Since comprehensive monographs on different groups of 

 Protozoa are widely scattered and often unavailable to the 

 average worker, the author has attempted to gather here as 

 much material from them as could be put together within 

 limited space. This handbook is, therefore, believed to be suit- 

 able for practical use by biology teachers in colleges and uni- 

 versities, field workers in pure and applied biological sciences, 

 veterinarians, physicians, public health workers, and others. 



To facilitate the use of this handbook, a comparatively 

 large number of illustrations are inserted; for students can 

 comprehend the form, appearance and structure of a micro- 

 scopic organism more thoroughly and satisfactorily by looking 

 at its picture than by reading a lengthy description. All the 

 illustrations have been especially prepared for this work. The 

 majority have been redrawn from illustrations found in earlier 

 works, and in all such cases the indebtedness of the author is 

 indicated by references to the sources. The original illustra- 

 tions are accompanied by scales of magnification only. In order 

 to make the illustrations as useful as possible for practical pur- 

 poses, the magnification in most cases is not more than five 

 hundred times natural size, this being the maximum afforded 

 by microscopes ordinarily used by students. Higher magnifica- 

 tions are used to show very small forms and to bring out some 

 minute protoplasmic structures. 



The author is indebted to numerous colleagues and investi- 

 gators for their observations which have been included in the 

 present work. Special thanks are due to Mr. H. C. Oesterling, 

 Editor of the Illinois State Natural History Survey, for assis- 

 tance in the revision of the manuscript and the correction of 

 the proof sheets. 



January, 1931. R. R. K. 



