10 SPOROZOA. 



Phylogeny and Classification. 



All parasitic forms are assumed to have been evolved from 

 free-living ancestors. The problem natm-aUy presents itself 

 as to what must have been the nature of the ancestral forms of 

 the Sporozoa and what is their relationship to the remaining 

 classes of the Protozoa. The Ciliophora are at once 

 excluded, as they are far too speciahzed, and the Sporozoa 

 do not show any special relationship with them. Two rival 

 theories of Sporozoan ancestry have been put forward in the 

 past, one claiming their descent from the Rhizopoda, the other 

 from the Mastigophora. The Rhizopoda and the Mastigo- 

 PHORA are themselves hnked together. Many Rhizopoda 

 show flagellated stages in then- life -cycle, and there are many 

 Mastigophora which are amoeboid. Most authorities agree 

 with Awerinzew (1910) that an amoeboflagellate type represents 

 the primitive stock of the Protozoa, which gave rise to all 

 existing groups, becoming differentiated into Rhizopoda on 

 the one hand and Mastigophora on the other. Biitschh 

 (1882) was the first to advance the theory of the Euglenoid 

 ancestry of Sporozoa, according to which a typical Flagellate 

 would become adapted first to a saprophytic and then to 

 a parasitic mode of life, and thus lose the special organs of 

 locomotion, nutrition, etc. As remarked by Minchin (1903), 

 " an Euglena or Astasia deprived in this way of flagellum, 

 mouth, chromatophores, stigma, and vacuoles, nutritive 

 or contractile, would be practically indistinguishable from 

 a simple Gregarine." The euglenoid movements of the Gre- 

 garines, and of the motile stages of other Sporozoa, such as 

 the sporozoites and merozoites of Coccidia, the free stages 

 of the HiEMOGREGARiNiDA, and the ookinetes of the IL^mo- 

 SPORiDiA, lend support to this view, and additional support 

 has been furnished by the discovery of flagellated stages in the 

 hfe- cycle in many Sporozoan forms. 



The life-cycles of the Amcebosporidia (=Nbosporidia), 

 however, do not lend support to the above-mentioned view. 

 They have no Euglenoid phases, do not possess flagella at any 

 stage of their life, and are amoeboid throughout their trophic 

 phase. Thus the Amcebosporidia give support to the rival 

 theory of Rhizopod ancestry. It was doubtless this funda- 

 mental difi'erence that led Schaudinn (1900) to divide the 

 Sporozoa into two subclasses, viz., Telosporidia and 

 Neosporidia ; and led Hartmann (1907) to regard the 

 Sporozoa {sensu stricto) and the Amcebosporidia as distinct 

 classes. Minchin (1912) expressed the opinion that from such 

 forms as Cercomonas arose on the one hand the Rhizopoda 

 and their derivatives (Neosporidia) by loss of flagella and 

 speciaHzation of the amoeboid form in the adult, and on the 



