INTRODUCTION 15 



change was first made out by Balbiani ('82), by Jickeli ('84), by Gruber 

 ('86, '87 1 ' 2 ), and by Hertwig ('89), and in great detail by Maupas 

 ('88, '89), by whom the conditions leading to conjugation were for 

 the first time made known. 



B. MODERN CLASSIFICATION OF THE PROTOZOA 



Although the Protozoa are the simplest forms of animal life and the 

 most generalized of cells, it does not follow that they are simply 

 organized and devoid of complicated structures. On the contrary, in 

 many cases they are highly differentiated, and in the Infusoria, the 

 highest group of the Protozoa, they become so complex, that Stein, 

 who was never an ardent advocate of the simplicity of Protozoa, re- 

 marked : " The adult Infusoria must ever be considered doubtful 

 single-celled organisms, for they are not simply cells which have 

 undergone further development, but the original cell-structure has 

 given place to an essentially different organization entirely foreign to 

 typical cells." 1 On the other hand, there are simpler forms of 

 Protozoa in which the undifferentiated protoplasm falls within the 

 description of sarcode, as given by Dujardin in 1835. Between the 

 two extremes of structure lie the vast majority of Protozoa, showing 

 among them all gradations from extremely simple to extremely com- 

 plex forms. A partial explanation of their frequent complexity of 

 structure lies in the fact that, unlike tissue-cells, they live free and 

 usually motile lives, and, like other independent organisms, are subject 

 to changes of form and to intracellular modifications in response to 

 their mode of life. 



Notwithstanding the innumerable forms and the various intracellular 

 modifications, differentiation, as a rule, has followed comparatively 

 .few general lines (Fig. i). It thus becomes possible to arrange the 

 Protozoa in groups or classes, with numerous divisions and sub- 

 divisions. The four classes which are now generally recognized are 

 the Sarcodina, the Mastigophora, the Sporozoa, and the Infusoria. 



The first attempt to classify the Protozoa was made by O. F. 

 Miiller in 1786. Rude and simple, and based upon the^ presence of 

 visible motile organs (Bullaria), or upon their absence (Infusoria 

 s. str.\ this nevertheless was retained as the chief system of classifica- 

 tion until the time of Ehrenberg, who made use of it in his own 

 system, .based upon the presence or absence of "stomachs." 



Despite the progress made by Dujardin, his classification, in its 

 main divisions, based upon unnatural differences of symmetry was 

 equally imperfect. His two divisions were very unequal, the " asym- 



1 Stein, Organismus, etc., II. ('67), p. 22. 



