PREFACE 



THE Protozoa not only claim the interest of the professional 

 naturalist, but also that of a wider circle of nature students who, 

 with the aid of the microscope, have always found here a fascinating 

 field for observation and research. In writing the present volume, 

 embodying a summary of the more recent discoveries concerning 

 these minute animals, I have aimed to keep in mind the needs of 

 the latter class of naturalists, as well as those who search more 

 deeply in the unicellular organisms for the solution of many morpho- 

 logical problems which remain unsolved in the higher animals, or for 

 vital processes which afford a transition from the manifestations of 

 life in its simplest expression to life as seen in the lower' forms 

 of invertebrates. 



The subject-matter of the volume is treated from three points 

 of view: (i) The historical, to which the first chapter is devoted. 



(2) The comparative, to which five chapters are given : one to the 

 group of Protozoa as a whole, the other four to the main classes. 



(3) The general, to which three chapters are devoted. One of these 

 is given to the phenomena of old age or senile degeneration in 

 Protozoa and renewal of youth through the union of two individuals, 

 and to the bearing of these phenomena upon sexual reproduction in 

 general. Another is given to the special structures of nuclei and 

 centrosomes of the Protozoa ; this, the most technical chapter in the 

 book, is introduced because of the growing importance which the 

 Protozoa have in the problems of cellular biology, especially with 

 those dealing with the origin of the division-centre and its accom- 

 panying structures in the cells of the Metazoa. The last chapter 

 is devoted to a consideration of the physiology of the Protozoa, with 

 especial reference to the Protozoa as organisms endowed with the 

 powers of coordination and of adaptation, which up to the present 

 time have eluded physical and chemical analysis. 



Every one who works with the Protozoa is mindful of the debt 

 we owe to Professor Otto Biitschli, whose indefatigable labors of 



