J. D. F. Gilchrist 
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criminately on the surface of the foot, the body and in and out of the 
mantle cavity. When the animal was placed under the microscope, 
the nephridial opening was observed near the base of the gill at its 
anterior right side. The nephridial aperture in some molluscs can best 
be found by observing the living animal in this way, when it is seen to 
open and close at intervals—in this case at intervals of 18 seconds. The 
cercariae, in their active looping movements, passed rather quickly over 
all the exposed surface, and several of them passed over the nephridial 
aperture, others even turned away when it opened in front of them. So 
persistent was this active movement that it seemed as if it had a definite 
purpose, perhaps procuring food. One was observed however to crawl 
rapidly into the opening, and soon disappeared into it. The aperture 
had been watched for about half an hour, and, after several hours, on 
re-examination, the snail, which had been kept in a watch glass was 
found to have only two cercariae crawling over it. There can be little 
doubt that this observation, together with the fact that the cysts are 
lodged in the pericardium, indicates that the cercariae enter the snail 
by the nephridial aperture. 
A cercaria, crawling about on the surface of the snail, showed indi¬ 
cation of the beginning of a change. The projecting shoulders were 
occasionally well marked, and behind them, on the neck a number of 
small hooks or bristles were seen. 
On another occasion, in March, some snails were procured, in which 
were found a large number of cysts (over 500), and near them were 
some very active cercariae not encysted. Some of them possessed 
tails and some were without. Some possessed two sets of spines, and 
were apparently about to encyst. 
There seems therefore to be no doubt but that the cercariae in 
question are represented by the cysts. Other means were however- 
taken to confirm this. A number of free cercariae were put with a 
snail in a test-tube, and a similar number in another test-tube without 
a snail. In about an hour the first had disappeared, while the dead 
remains of the second lot were found at the bottom of the tube. 
This does not altogether exclude the possibility that the cyst may 
have arisen from cercariae which had never been outside the snail; 
this possibility however was disposed of by the finding of a snail with a 
perfectly healthy liver, but badly infected with cysts. 
Such cysts were found in aestivating snails in the mud. 
The cysts were 0-156 mm. in diameter and transparent. Each 
could be observed to contain a small Distoma either at rest or revolving 
