224 University of California Publications in Zoology [Vol. 20 



SUMMARY 



Giardia enterica has the typical structures of the genus. It lacks 

 basal granules on the anterolateral and posterior flagella, its blepharo- 

 plasts are not enlarged so as to expand the head of the axostyle. The 

 parabasals are rather slender, curved, comma-shaped, tapering, deeply 

 staining structures lying as a pair parallel to each other in various 

 positions posterior to the cytostome in the dorsal region. It has the 

 structurally integrated neuromotor system typical of the genus. The 

 cytostome is an organ of adhesion. There is no evidence of the 

 ingestion of any formed food particles. 



There are four chromosomes ; a paradesmose forms between parting 

 centrosomes outside of the nuclear membrane which remain intact 

 during mitosis. Daughter parabasals, axostyles, and peristomal fibers 

 are formed by contact outgrowth of the daughter structure or by 

 distad splitting of the parent structures. Anterolaterals are certainly 

 duplicated by new outgrowth. 



Mitosis in the cyst resembles that in the free stage except that free 

 flagella are lacking and duplication of the neuromotor elements lags 

 unevenly after nuclear division. Dedifferentiation of free flagella, 

 cytostome, and the entire peristomal fiber occurs in the cyst. 



Encystment provides for dispersal of the parasite to new hosts and 

 affords opportunity for multiple mitosis to the 8-zooid, 16-nucleate 

 somatella. Plasmotomy was not seen in the cysts. Binary fission in 

 the c.ysts was not observed. Interpretations of the cysts as provision 

 for conjugation and for autogamy are erroneovis. There is no evidence 

 that there is an Octomitus stage (Hartmann, 1910) in the life-cycle of 

 Giardia. 



Giardia is bilaterally symmetrical. Fission is in the longitudinal, 

 frontal plane. It is asexual reproduction. There is no evidence of a 

 transition through or a return to a one-cell stage. The symmetry of the 

 right side of Giardia is that of the one-celled Chilomastix and the 

 organs of that side are homologous with those of a Chilomastix. The 

 organs of the left side of Giardia are those of a Chilomastix, but they 

 are those of a morphologically reversed Chilomastix, i.e., they form a 

 mirror image of those of the right side. Giardia may be derived from 

 Chilomastix by a morphological reversal of one daughter nucleus and 

 its attendant neuromotor s.vstem at mitosis and the persistence of a 



