i6 



THE PROTOZOA 



metrical" Infusoria including all but one or two known forms. The 

 subdivisions were, however, remarkably happy, and were based upon 

 natural lines which have never been displaced. While Ehrenberg's 

 subdivisions were based upon gross external characters, such as the 

 presence of hairs, the position of the mouth, etc., Dujardin's were 

 based upon the means of locomotion, and in this early grouping we 

 see the first use of our modern terms " rhizopod" "flagellate" and 

 "ciliate." Von Siebold ('45) was the first to divide all Protozoa into 



Fig. 2. Actinophrys sol Ehrenberg, a heliozoon. [After GRENACHER from BUTSCHLI.] 

 An individual with a large gastric vacuole {g ), contractile vacuole (c), and axial filaments (a) 

 in the ray-like pseudopodia. 



two classes, Rhizopoda and Infusoria, a system which formed the 

 basis of our modern classification ; the Mastigophora or flagellates, 

 regarded by von Siebold as plants, found no place in his zoology. 



Three of the four great classes recognized to-day were thus out- 

 lined by Dujardin in 1841. The modern Rhizopoda (Sarcodina) were 

 characterized as " animals provided with variable processes " ; the 

 Mastigophora as "animals provided with one or several flagelliform 

 filaments, serving as motile organs"; and the Ciliata as "ciliated 

 animals" (Fig. i). The further subdivisions, which, little by little, 

 have been developed along the lines laid down by Dujardin, have 

 brought order out of this heterogeneous group of organisms, which at 

 the present time includes nearly sixteen hundred genera and many 

 thousands of species. As the name of a class, the term Rhizopoda, 



