OCTOMITUS SALMONIS, A PARASITIC FLAGELLATE OF TROUT 
15 
The writer has found no reason to believe that a transverse division, as de- 
scribed by Moore (1924), ever occurs. Individuals showing a transverse constric- 
tion in some part of the body are quite common in some of the writer’s preparations, 
but there is every reason to believe that such a constriction has nothing to do with 
division. It has been noticed frequently that when an individual is passing 
through a small opening the body may become strongly constricted as the animal 
squeezes through the narrow space. When mounted in the thick mucus from tho 
posterior end of the intestine, numerous individuals that are worming their way 
through small openings of this kind can often be seen in the same field. 
It is a notable fact that fission is evidently more or less periodic, since in the 
majority of the writer’s preparations dividing forms are rare, while in a few instances 
they are very abundant. Just what factors are concerned in stimulating division is 
still somewhat doubtful, but there is evidence that there may be a sudden increase 
in the rate of fission when the host fishes are subjected to unfavorable conditions, 
such as a rise in temperature or a deficiency of oxygen or food. 
DEVELOPMENT OF CYSTS 
The cysts of Octomitus salmonis are ordinarily rare in the intestines of infected 
fish, even though the flagellates may be abundant, but a few can usually be found 
if a diligent search is made. In several instances they were present in large numbers, 
and it is worthy of note that these cases were all found in early spring. However, 
the evidence at hand is too meager to justify the conclusion that the cysts are more 
abundant at this season, although this may very well be the case. 
When first formed, the cysts are ovoidal, rarely spherical, and are surrounded 
by a very thin transparent membrane. No definite structure can be made out in 
the living cysts, but in some cases rapid vibratile movements can be plainly seen. 
These are evidently due to the flagella that are retained for a short time after 
encystment. The axostyles are indistinguishable, but the nuclei usually can be 
recognized, and small vacuoles can be faintly discerned in some cases. The cysts 
usually stain a deep brown when treated with iodine, due to the presence of gly- 
cogen, which occurs in the form of fine granules or, in some cases, in irregular masses. 
This affords an easy method of demonstrating the presence of cysts, which stand 
out conspicuously against the more lightly stained background. It is of interest 
to note that ordinarily no trace of glycogen can be seen in the flagellated forms, 
but in one instance, when the cysts were exceptionally abundant, numerous 
flagellates were observed, which contained varying amounts of glycogen in the 
posterior half of the body, where it was evidently being stored in preparation for 
encystment. The cysts are usually about 7 microns wide by 10 microns long, but 
may reach a length of 12 microns with a corresponding decrease in the width. 
Figure 15 shows a newly formed cyst in which the paired nuclei, blepharo- 
plasts, flagella, and axostyles can be distinguished, but the texture of the cytoplasm 
is much looser than before encystment. This is characteristic of the cysts and is 
probably due to extensive formation of glycogen, which, of course, disappears as a 
result of the treatment to which the cysts are subjected. The flagella disappear 
