CHAPTER XVIII 



AFFINITIES AND CLASSIFICATION OF THE MAIN 

 SUBDIVISIONS DOUBTFUL GROUPS 



IN the foregoing chapters the Protozoa have been dealt with 

 systematically, grouped in a somewhat conservative manner under 

 the four old-established and generally-recognized classes. At the 

 same time it has been pointed out that one class at least namely, 

 the exclusively-parasitic Sporozoa comprises two subclasses which 

 are quite distinct from one another, and are descended, in all 

 probability, from ancestors differing t greatty in characters and 

 affinities. And in the case of the three remaining classes, con- 

 sisting mainly of free-living, non-parasitic forms, two whifch exhibit 

 more primitive characters namely, the Sarcodina and Mastigophora 

 are connected with one another by transitional forms which 

 render the distinction between them very arbitrary (p. 213) ; while 

 the third, the highly-specialized Infusoria, are linked closely by 

 structural characters and by transitional forms to the Mastigophora. 

 Many authorities on the Protozoa have put forward schemes of 

 classification which are intended to express the affinities and inter- 

 relationships of the chief groups in a clearer and more satisfactory 

 manner than the fourfold classification generally recognized. The 

 systems proposed have taken the form either of subdividing the 

 Protozoa into more than four classes or of uniting the recognized 

 subdivisions into a smaller number of categories. 



Rolleston and Jackson (15) divide the Protozoa as a whole into three groups : 

 (1) the Rhizopoda (= Sarcodina ); (2) the Endoparasita (=Sporozoa); and 

 (3) the Plegepoda, " referring to their mode of progression by means of a 

 rapidly - repeated stroke (7rX?/y^) of vibratile processes," to comprise the 

 Mastigophora and Infusoria. 



Doflein (891) recognizes two principal stems in the Protozoan phylum: 

 (1) the Plasmodroma, to include the Sarcodina, Mastigophora, and Sporozoa, 

 organisms that make use of locomotor organs which represent true pseudo- 

 podia, or their derivatives or modifications ; and (2) the Ciliophora, comprising 

 the Ciliata and Suctoria, in which the locomotor organellse are cilia. The 

 obvious criticism of this scheme is that, whatever opinion may be held as to 

 the desirability of drawing a line between the Infusoria, so highly specialized in 

 many respects, and other Protozoa, the distinctive character chosen is not a 

 happy one, since whatever may be predicated of flagella as derivatives of pseu- 

 dopodia applies, apparently, with equal force to cilia. 



462 



