12 
BULLETIN OF THE BUREAU OF FISHERIES 
This should be mounted on a slide in a drop of water, in which the parasites will 
remain alive and active for 10 to 15 minutes. Since no other parasites are likely 
to be encountered, which could be confused with Octomitus, examination with the 
low power of a compound microscope usually is sufficient. At this magnification 
the parasites can be distinguished readily as colorless, minute, pear-shaped organisms 
darting rapidly about in all directions. 
When the parasites are very abundant, the intestinal wall just behind the 
stomach usually is thin and transparent and distinctly yellowish rather than brown- 
ish, which is the normal color. In acute cases it may exhibit a distinctly reddish 
color due to congestion. However, since the appearance of the intestinal wall is 
to a considerable extent dependent on the character of the food, too much reliance 
should not be placed on the yellow transparent appearance as a diagnostic character. 
MORPHOLOGY OF THE FLAGELLATE 
The flagellate usually is pyriform or ovoidal in shape, tapering gradually 
toward the posterior end, which is more or less distinctly truncate (figs. 1 to 4). 
Rounded individuals are quite common, however, and other variations in form are 
by no means infrequent. They vary considerably in size, also, the majority being 
about 6 to 8 microns in width by 10 to 12 microns long. 
But little of the internal structure can be distinguished in the living flagellates, 
which are usually very active and swim swiftly about by means of rapidly moving 
flagella. Furthermore, the animals are so minute that even when fixed and properly 
stained it is very difficult to make out the finer details of their structure. 
A thorough study of more carefully fixed and stained material has convinced 
the writer that his previous description (Davis, 1923) of the structural details was 
inaccurate in several respects. This is especially true of the cytostome, which was 
described as a slightly concave structure on one side of the anterior end. It is 
now believed that there is no constant flattened or concave region, nor is there a 
definite ventral side. On the contrary, the anterior end is usually uniformly 
rounded on all sides. Although there is no flattened cytostome, the protoplasm in 
this region has a quite different structure from the rest of the body and can be 
plainly distinguished in both living and stained individuals. In the former it 
appears as a clear hyaline region occupying the anterior third of the animal, which is 
sharply marked off from the more opaque and granular protoplasm composing the 
remainder of the body. When stained with hematoxylin, this region usually appears 
darker than the rest of the body, but sometimes may be lighter in color (figs. 
1 to 4) . In the living animal this region is very mobile and changes shape rapidly, 
sometimes being thrust out for some distance in front and again retracted so that 
the anterior end becomes enlarged and rounded. It seems probable that this 
mobile region has been specially modified to enable the animal to cling to the 
intestinal wall and possibly even to bore into the tissues. In other words, it is 
apparently a rudimentary cytostome and homologous with the more complex 
structure characteristic of Giardia. A similar mobile region has been described by 
Schmidt (1920) in Octomitus intestinalis truttse. 
When in active movement, the organism rotates rapidly on its long axis and, 
as noted above, there is no definite ventral side as described in a previous paper. 
