316 
Distoma liiteum n. sp. etc. 
in the jar, but it was noted on several occasions that, when they came 
in contact with the body of a snail, they became more qrdescent, and 
moved about over its surface by means of the sucker for a time, after 
which they apparently resumed their swimming. 
The cercariae escaped from the snail, one at a time, and, in a case 
specially observed, at intervals of about ten minutes. They then 
moved towards the surface of the water or towards the light. After 
swimming about actively for about 24 hours, they were observed to be 
moving away from the light, and downwards. They then began to 
crawl, often with a looping movement, along the bottom of the dish, 
never on the vegetation in the water. Finally they appeared to be 
endeavouring to attach themselves to the dish, the head meanwhile 
being bent downwards and the tail moving rapidly. Though closely 
watched in this attitude they were never observed to become encysted, 
but, after a time lost their tails, became swollen and died. No en¬ 
cysted forms were found on the walls of the vessel in which the water 
was contained nor on vegetable matter or mud from the lake from which 
the snails were procured. It was however noted as a peculiar fact 
that, of the several hundreds in the dish, not even the dead remains 
were found, except in a few cases. To check this observation more 
carefully a dozen cercariae were then removed from the dish containing 
the snails and put in a test tube, and all of them after a time sank 
to the bottom, became swollen and died. The solution of this diffi¬ 
culty, and, at the same time, the key to the whole life history, became 
evident on the finding of an encysted form in the body of the snail 
itself; this suggested that the cercaria might, after a free life, again 
return to the snail, and become encysted in it. Further search showed 
that, in nearly all cases at a certain time, cysts could be found in the 
snail in a patch of 20 to 200 in the pericardium, clustered round the 
auricle. 
It was difficult to discover exactly how the cercariae entered the body 
of the snail, as this was effected apparently within the mantle cavity. 
It was observed several times that cercariae on touching the surface of 
the body of the snail immediately ceased the wriggling movement of 
the tail and began to crawl over the surface, in and out of the mantle 
cavity. One observation however was made which indicated that the 
cercariae gain access to the pericardium, not by boring through the 
skin, but by the nephridial opening. In this particular case the snail 
under observation had the mantle cavity widely open, and the gill 
protruding; about twenty cercariae were crawling apparently indis- 
