404 University of Calif or nia Publications in Zoology f Vol. 19 



INTRODUCTION 



Protozoa are commonly regarded as representatives of the most 

 primitive and simplest forms of life. The most salient feature of the 

 phylum is conceded to be their unicellularity, each individual being 

 the morphological equivalent of a single cell. That these characteristics 

 indiscriminately applied to this very large and diversified group of 

 organisms are not only inadequate but somewhat misleading is par- 

 ticularly evident from several recent investigations on various flagel- 

 lates and filiates. The results of these researches point toward com- 

 plexity rather than simplicity and stimulate inquiry into the nature 

 and function of certain intracytoplasmic structures that these animals 

 possess, which may indicate an organization more highly evolved than 

 is usually assumed. 



These structures in both flagellates and eiliates are intimately asso- 

 ciated with ectoplasmic organelles (flagella, cilia, cirri, etc.), a fact 

 which strongly suggests that they share some role in the animal's 

 motor mechanism. Accordingly, investigators are generally agreed in 

 designating the structures with their attached organelles "the motor 

 apparatus." 



Of the organisms possessing such a motor apparatus a larger 

 number of flagellates than of filiates has been studied and compara- 

 tively described. In the former class a series has been worked out that 

 indicates a progressive evolution of this mechanism. A simple type 

 of motor apparatus appears in the biflagellate stage of the soil amoeba, 

 Naegleria gruberi (Schardinger). It consists of two flagella attached 

 to a basal corpuscle, the blepharoplast, which in turn is connected by 

 a fine fibril to the nucleus. This organism spends most of its excysted 

 life as an amoeboid trophozoite, but it may become transformed for a 

 brief period of twenty-four hours or less into a very active flagellate. 

 This interesting change has been described by Whitmore (1911), 

 Alexeieff (1912)., and mure completely by Wilson (1918), who has 

 shown that variations in temperature, media, and other factors may 

 induce tlie change. The actual transformation may be followed in 

 living forms and its stages analyzed in fixed material. 



It was thus observed that the motor apparatus arises by an out- 

 growth from the karyosome, "presumably from the centriole. " Wilson 



