MORPHOLOGY AND TAXONOMY OF MASTIGOPHORA 293 



sets of what are ol^viously unit aggregates of nuclei and kinetic 

 elements (karyoniastigonts and akanomastigonts of Janicki). 



The great majority of forms included here are parasitic and most 

 of them are simple with one cytostome if any (Tribe Monozoa). 

 The kinetic elements are highly developed much more so than in 

 free-living t;y^es. In a small number of genera which are usually 

 grouped as the Order Distomatida (Klebs), the cytostome is double 

 and the organisms are bilaterally symmetrical. In the majority 

 of these the mouths are separated and occupy symmetrical positions. 

 In Giardia, however, the mouths have come together to form a 

 single suctorial cytostome (Fig. 140). There is some justification 

 for Kofoid's (1920) hypothesis that Giardia has resulted from the 

 division of a type something like Chilomasrix with subsequent re- 



FiG. 140. — Giardia muris; ax, axostyle; 6, blepharoplast ; hg, basal body; c, centriole; 

 fc, endosome ; n, nuclei ; p, parabasal body. (After Kof old and Christianson.) 



fusion as in the formation of a double Uroleptus mobilis (see p. 466) 

 or of a double Glancoma scintillans described by Chatton (1921). 

 Like Giardia the majority of these forms are binucleated and each 

 side has its equi\'alent complex of kinetic elements. Kofoid points 

 out that Chilomastix is almost an exact duplicate of the right side 

 of Giardia, and the various types included here may be interpreted 

 as having arisen in a similar manner from flagellates with originally 

 two, three, or four, flagella. For this reason w^e have grouped them 

 here under the Tribe Diplozoa. 



Somatella formation is a frequent phenomenon in the life history 

 of these parasitic flagellates. Within the cell membrane the nucleus 

 and kinetic elements divide, usually from 2 to 4 times, and 4, 8, 

 or 16, future individuals are thus contained within the mem- 



