460 
DORIS L. MACKINNON. 
looking grubs were generally found to have the richest intes- 
tinal fauna. Probably such an association should rather be 
looked on as one of symbiosis. Certain of these protozoa have 
no doubt become closely adapted to their peculiar environment, 
and could flourish there only ; but others of them, such as the 
flagellates Polymastix and Monocer como n as, are found 
in other larval insect hosts besides Tipula; while some are 
even less particular in their choice of host. The occurrence 
of these last is always sporadic and quite unpredictable. 
These are the ‘‘ parasites facultatifs,^^ which are of special 
value in helping us to All in the gap between strict parasites 
and free-living forms. 
To such a group of facultative parasites belongs the 
beautiful little monoflagellate, named by Alexeieff (1912) 
Rhizomastix gracilis. Alexeieff based his description on 
a single infection of an axolotl. I have recently found in 
Tipula a cercomonad which seems to me morphologically 
indistinguishable from Rhizomastix gracilis, and I see no 
good reason for creating a new species, even though the 
host be in the one case a vertebrate, and in the other an inver- 
tebrate. The infected leather- jackets were found in swampy 
ground near a stream — in much the sort of environment, that 
is to say, which would also be suitable for amphibians. 1 
share with a few other protistologists the belief that animals 
totally unrelated, but frequenting the same feeding grounds, 
are liable to infection by the same “ facultative protozoan 
parasites. It would be well if the enthusiastic creators of 
new species would keep this possibility in mind. 
The material at my disposal for the study of Rhizomastix 
was not much richer than that from Alexeiefl^s axolotls. 
Only two larvae out of hundreds were found infected, and of 
these one so sparingly that only a dozen flagellates were found 
in the stained preparation. But in the other the organism 
was relatively abundant, and I found a large number of its cysts. 
I am thus able to add something to the little-known life-cycle 
of this cercomonad, and though the account is necessarily 
incomplete, I do not hesitate to publish it, believing as I do 
