DISTINCTIVE CHARACTERS OF THE PROTOZOA 



method of subdividing them is purely arbitrary and artificial ; it 

 leads to the result that many forms are claimed by both sides, and 

 are always to be found described in both botanical and zoological 

 treatises. It is nevertheless convenient for 

 many reasons to retain the group Protozoa, even 

 though we are obliged to include in it some 

 forms which are plants in every sense of the 

 word. The systematist who desires to give a 

 rigidly logical definition of the Protozoa is, 

 then, confronted with a dilemma : either to 

 exclude from it forms with plant-like metab- 

 olism which naturally belong to it, or, by 

 admitting such forms, to impair the universal 

 applicability of the definition given. Such 

 difficulties arise in every attempt to apply I/^^BH^H'' 

 rigid verbal definitions to natural groups of 

 living things ; they are the direct outcome of 

 the fact that all organisms have undergone and 

 are undergoing a process of evolution, whereby 

 they adapt themselves to new conditions of 

 life and acquire new characters, as a result of 

 which any two forms now distinct are or 

 have been, connected by intermediate forms. 



-N 



FIG. 6. Trypanosoma rcmaki of the pike. A, Slender 

 form (" van parva "). B, Stout form (" var. 

 magna "). After Mirichin, x 2,000. 



FIG. 7. Qregarina 

 polymorpha, parasite 

 of the digestive tract 

 of the mealworm ; 

 " syzygy " of two 

 individuals attached 

 to one another. In 

 each individual, N., 

 nucleus; pr., proto- 

 merite, or anterior 

 segment of the body; 

 d., deutomerite, or 

 posterior segment. 

 After Schneider. 



