ZOOFLAGELLATES 145 



(Kirby, 1949) . Few other arrays of organisms provide such elegant 

 material for the study of evolution of architectural complexity 

 within cells. 



Most metamonads have centrioles that appear in the light 

 microscope to be distinct from the kinetosomes of the nagella; 

 during cell division in many species the old nagella and their basal 

 bodies, as well as other elements of the mastigonts, are dedifferen- 

 tiated or topographically segregated well before new ones appear, 

 and the latter then grow out from the centriole region. The 

 centrioles are remarkable structures, often assuming an elongate 

 form during part or all of the life cycle, with a proximal end 

 duplicating to produce new centrioles and the distal end serving 

 as the mitotic center. The extensive and meticulous studies of 

 Cleveland and his colleagues (see Cleveland, 1957, 1960) on the 

 extraordinary mitotic and sexual cycles of these organisms have 

 provided new insight into the behavior of centrioles. 



Ancestral trichomonad flagellates are considered to be central 

 to the evolution of several other metamonad orders (Kirby, 1949; 

 Grasse, 1952). Electron-microscope studies of species of Tri- 

 chomonas (Ludvik, 1954; Inoki, Nakanishi, and Nakabayashi, 

 1959; Inoki, Ohno, Kondo, and Sakamoto, 1961) and Tritricho- 

 monas (Anderson, 1955; Anderson and Beams, 1959, 1961) have 

 been reported. These organisms (Text-fig. 8) bear three or four 

 anterior free flagella and one recurrent flagellum attached by an 

 undulating membrane to the cell surface. Below this attachment 

 is an intracytoplasmic curved fiber called the costa. The axostyle 

 is a hyaline rod that may protrude from the body at its pointed 

 posterior tip ; anteriorly it curves around one side of the central 

 nucleus as a cup-like capitulum and extends to the apex of the cell. 

 Just below this tip, a deep-staining zone called a centroblepharo- 

 plast by light microscopists is presumed to enclose a centriole 

 and marks the point of convergence of all nagella, costa, and a 

 parabasal filament to which the " elongate, fusiform parabasal 

 body adheres. 



fibrils of recurrent flagellum; AK, kinetosomes of anterior flagella; 

 AX, axostyle; NE, nuclear envelope; NCL, nucleolus; NP, nuclear 

 pore; N, nucleus; ER, granular endoplasmic reticulum; IB, 

 inclusion body; M, mitochondrion; CR, elements of the chromatic 

 ring. From E. Anderson, unpublished. 



