208 University of California Publications in Zoology [Vol. 20 



sides. We have been unable to detect them as separate fibers parallel 

 to the axostyles as in Hartmann's (1910) diagrammatic figure of 

 G. tnuris. The intracytoplasmic sections destain with diiBculty, and, 

 next to the peristome and axostyles, are the heaviest fibrillar parts of 

 the neuromotor system. There is no basal graniile at or near the point 

 of emergence of these fiagella. Their position indicates that they are 

 possibly the homologues of the flagellum of the undulating membrane 

 in trichomonad flagellates. 



The free ventral flagella (vent, f., fig. A) emerge close together at 

 the level of the posterior margin of the cystostome (pi. 23, figs. 2, 3) 

 and appear to merge with the axostyles at about their level. We are 

 not able to trace tliem forward as distinct fibers to their respective 

 blepharoplasts. In life, these fiagella are extraordinarily active, 

 moving in uni.son. and in fixed material usually maintain their 

 parallelism. 



The posterior flagcUa (post, f., fig. A) are free extensions of the 

 axostyles emerging at the tip of the tail and are without basal granules. 



The axostyles (ax., fig. B) are two tapering, closely parallel rods 

 of deeply staining or grayish fibrillar substance passing posteriorly 

 from the two blepharoplasts. They and the adjacent parabasals con- 

 stitute the most striking elements of the neuromotor system in the free 

 stages, and especially in the cysts in the process of multiple fission. 

 The blepharoplasts are lodged in the anterior ends of the axostyles and 

 are not distinguishable from the axostyle except by adequate destain- 

 ing. The axostyles are continued distallj' as the free posterior fiagella 

 and are thus morphologically the intracytoplasmic parts of such 

 fiagella. The lively movements of the tail in whose ventral surface 

 they lie demonstrate the motor function of these axial organs. 



The question as to the homology of this organ of the component 

 cells of Giardia to the axostyles of other polymastigote flagellates is 

 an interesting one. We regard it as homologous with the slender rod 

 in Chilomastix davainei which we (Kofoid and Swezy, 1920) called 

 the parastyle. In Chilomastix, this organ lies in a position with 

 regard to other organs, notably the cytostome and blepharoplast, 

 homologous to that of the axostyle in the right cell of Giardia, and in 

 the left cell to that which it would occupy in the mirror image of 

 Chilomastix. It does not, however, reach the posterior end of Chilo- 

 mastix nor form a free flagellum at its tip. It differs from the axostyle 

 of Trichomonas and related genera in the development of the free 



