Development of the Liver-Fluke. 
449 
whilst still in the snail, or encysted on grass, &c. For this 
purpose it will be necessary to feed sheep with the suspected 
larval form. 
I have endeavoured to confirm these results of Professor 
Leuckart in the case of young L. pereger, and have met with 
partial success, although my experiments have been restricted 
by the scarcity of the needful material. Fluked livers have 
been very rare during the past spring, and I have had great 
difficulty in securing the comparatively small number of fluke- 
eggs with which I have worked. I exposed about eighty examples 
of L. pereger, varying in length from 1'4 mm. to 4 mm., to 
infection. I have measured young L. pereger which have just 
escaped from the egg, and find that they are as nearly as 
possible 1 mm. long, so that many of my snails must have been 
sufficiently small. Although they were exposed to infection 
from a larger number of eggs than my examples of L. triin- 
catulus, only a few of the smallest were found to contain 
embryos, and none of these proceeded far in their development. 
Some of the smallest of the specimens of L. pereger died very 
soon, possibly from injury done them by the parasite ; but more 
than three-fourths, including all the larger ones, entirely escaped 
infection. Those snails which have reached the size of 3 mm. 
seem to enjoy an immunity which may be due to the greater 
firmness of their tissues. It will be seen, however, that 
L. pereger does not form so suitable a host as L. truncatulus, for 
in the case of the latter species all the examples, both large and 
small, that I have experimented with have so far proved to have 
been infected. As L. truncatulus is said to be a very widely 
distributed species, and is amphibious in its habits, it will 
evidently be by far the more dangerous bearer. 
Leuckart says of the head-papilla of the embryo of Fasciola 
hepatica, that it " seems to function as a tactile organ." But 
I have no doubt that it really has the function I assigned to it 
in my former paper, viz. that it is a boring-organ* The papilla 
is ordinarily short (its length is about "006 mm.), the end is 
quite blunt, or may have a slight depression in the middle. 
But when the embryo begins to bore into any object, the papilla 
is protruded, and takes the shape of a cone with a sharp point 
and a bulbous base. I have repeatedly observed an embryo 
attempting to bore into the foot of a small snail placed upon 
the same slide with it under the microscope. The point of the 
head-papilla is directed towards the tissue of the snail, and 
the embryo turns round on its longitudinal axis, the cilia 
working actively and pressing the anterior end against the 
* ' Eoyal Agricultural Socifif y's Journal,' vol. xvii., p. 7. 
VOL. XVIII. — S. S. ' G 
