CLASSIFICATION OF THE MAIN SUBDIVISIONS 465 



tives of the two classes may differ, there are forms of which the 

 systematic position is quite arbitrary. In such a form as Pseudo- 

 spora, it becomes almost purely a matter of opinion or taste which 

 phase of the life-cycle is to be regarded as the " adult ' form 

 determining the class hi which the genus is to be placed. 



Thus, all paths of evolution in Protozoa appear to lead back- 

 wards to one or the other of the two forms that occur so frequently 

 in the actual development as the earliest phases the amcebula 

 and the flagellula. Most of those who have speculated on the 

 phylogeny of the Protozoa have, consequently regarded the an- 

 cestral form of the phylum as one combining amoeboid and flagellate 

 characters. Biitschli (2) considered that the Rhizomastigina re- 

 present more nearly than any other existing group the primitive 

 type of Protozoon. Since then, however, the life-cycle of the 

 mastigamcebae has been studied, and it is seen that the adult 

 amoeboid form is preceded in development by a simpler monad 

 form (p. 266, Fig. 112), which makes it very doubtful if the mastig- 

 amoeba itself can be taken as a primitive type. Awerinzew (890) 

 also regards an " amcebo flagellate ' type as the primitive stock 

 of Protozoa, which gave rise to all existing groups, and became 

 differentiated into the Amcebina on the one hand, the Flagellata 

 on the other. 



If an organism possesses two kinds of locomotor organs pseudo- 

 podia and flagella it is reasonable to suppose that a still more 

 primitive and ancestral form would have possessed only one of 

 these two kinds of organs. It has been seen that there is a gradual 

 transition from pseudopodia to flagella, the intermediate type of 

 organ being a pseudopodium (axopodium) with a firm, rigid, or 

 elastic secreted axis. The question then arises, Which end of the 

 series is to be put first, the flagellum or the pseudopodium ? Inas- 

 much as flagella are found commonly in bacteria, it might be argued 

 that they represent the most primitive type of locomotor organella, 

 and that a simple flagellate monad would represent most nearly the 

 ancestral type of organization in Protozoa. Then it must be sup- 

 posed that the formation of pseudopodia is a secondary character, 

 acquired by the ancestral form, and the pseudopodia themselves 

 would represent either simple outgrowths of the naked body (lobo- 

 podia) or modifications of flagella (axopodia). 



Having regard, however, to the manner in which flagella them- 

 selves arise as simple outgrowths from the body and to the fact 

 that their structure and mode of action are apparently of a much 

 more specialized type than those of pseudopodia, the conclusion 

 seems irresistible that pseudopodia preceded flagella in evolution. 

 We may, then, regard as the most ancestral type in the Protozoa 



a minute amcebula-form, in structure a true cell, with nucleus and 



30 



