12 University of California Publications in Zoology [Vol.16 



The attached forms (pi. 4, figs. 93-99) are flagellates which upon 

 becoming attached to the wall of the rectum undergo rapid longi- 

 tudinal division until the whole rectal region is lined with a compact 

 layer of parasites. Longitudinal sections of the rectal region show 

 that while the upper part is lined wuth elongate flagellates (pi. 2, figs. 

 54^55) the lower, more posterior part is covered with small oval forms 

 (pi. 4, figs. 93-99). The small oval forms show certain changes. The 

 length of the body decreases, while the width increases, and the slightly 

 flattened body becomes round. Both ends of the body become blunt 

 (pi. 4, fig. 99) and the undulating membrane tends to disappear 

 (pi. 4, fig. 97). The change in size at this time i.s evidently due to a 

 process of binary fission rather than to any shrinking on the part of 

 the organism. Along with the changes taking place in the external 

 form, there are changes going on in the nucleus and extranuclear or- 

 ganelles. The nucleus is relatively much larger (pi. 4, fig. 97) in 

 respect to the size of the body than in the elongate flagellate stage. 

 The vesicular character becomes more pronounced (pi. 4, figs. 94, 95) 

 and the chromatin in the form of karyosomes which vary in size and 

 staining capacity. The kinetonucleus comes to lie close to the nucleus 

 (pi. 4, fig. 95) but shows no tendency to migrate posterior to the 

 nucleus. There is no evidence of an "axostyle" present, but the rhizo- 

 plast can be readily observed in these forms as a faint line connecting 

 the nucleus directly with the kinetonucleus (pi. 4, fig. 96). 



The free forms (pi. 4, figs. 74-86) in the rectum are of great in- 

 terest because of their varied size and shape. The width of the body 

 increases at the expense of the length, as seen in plate 4, figures 74-80. 

 Some forms become almost spherical (pi. 4, figs. 85-92). The move- 

 ment of such forms is very slow. The undulating membrane is very 

 much in evidence (pi. 4, figs. 77, 78-81) in connection with the spiral 

 movements of the flagellate and in those to the right or to the left. 

 Such a flagellate as that seen in plate 4, figure 81, is similar to 

 Trypanosoma rotatorium in movement. The broad, flat forms as seen 

 on plate 4, figures 77 and 78, are not due to drying in the process of 

 fixing and staining. They are foimd in the living material. It is 

 possible that these forms may be degenerate ones or that they may 

 be rounding up to form spheres in preparation for multiple fission. 

 In support of the last hypothesis merozoites have been found in the 

 rectal region. 



The nucleus of the free forms of the rectal phase is also relatively 

 large (pi. 4, figs. 74 and 78). The chromatin frequently indicates 



