Development of the Liver-Fluke. 
447 
these water-snails, the large numbers picked up from the bottom 
of the aquarium showed that the snails do not swallow the eggs 
accidentally, but intentionally, no doubt mistaking them for food. 
The eggs and the contained embryos are uninjured by their 
passage through the digestive tract, and may be swallowed over 
and over again. Such being the case, eggs will undoubtedly 
be often hatched during their passage along the digestive tract ; 
and if the embryos are not killed by the action of the digestive 
juices, they have only to pass through the walls of the intestine 
or up the bile-ducts into the liver (an organ very frequently 
infested with sporocysts or rediae) to find themselves, if in a 
suitable mollusc, in a favourable place for further development. 
That the embr^'os hatched out in the intestine may escape 
injury from the digestive juices of the snail, I have been able 
to observe directly in the case of the embryo of the liver-fluke. 
An embryo of this species was seen escaping from an egg in 
the intestine of a small LimncBUS jiereger, and during the three 
hours it was under observation it remained alive and active, 
although bathed in the fluid contents of the intestine. In this 
case, the embryo was prevented, partly by the pressure of the 
cover-glass, and partly by the presence of other eggs, portions 
of undigested food, &c., from escaping from the intestine. But 
in another example of the same species of snail, an active 
embryo was found free in the body-cavity, and as the digestive 
canal had not been ruptured, it must have bored its way 
through the walls of the intestine, which contained very 
numerous eggs. 
It is clear, therefore, that those snails which have this 
habit of swallowing the eggs may be exposed to infection 
with trematode-larvae. The species in which I have more 
especially noticed the habit are sufficiently numerous (viz. 
Limnccus stagnalis, L. pereger, L. auricularis, L. palnstris, Pla~ 
norbis marginatus, P. carinatus, Bithynia tentacidata, as well as 
three slugs, Arion ater, Limax agrestis, and L. cinereiis) to allow 
us to assume that it is very generally prevalent amongst our 
land and fresh-water molluscs. 
Von Willemoes-Suhm published in 1873 a list of the trema- 
tode embryos then known.* Since then, that of Distoma trigo- 
* ' Zeitschrift fiir wissenschaftliche Zoologie,' vol. xxiii., p. 341. As no list of 
these embryos exists in English, it may be useful to give it here. The nineteen 
ciliated embryos are : Fasciola hqmtica, Distoma hians, D. laitreatum, D. vivi- 
jmriim, D. triqonocephaluvi, D. nodulosum, D. cygnoides, D. longtcolle, D. glohi- 
2)onim, D. folium, D. lanceolatum, Billiarzia lixmatohia (see Dr. Cobbold's work 
on ' Parasites '), D. innnatum, D. signatum, Monostoma mutahile, M. flavum, 
M. caintellatmn, Amphistoma suhclavatum, an embryo obtained from a retort- 
shaped egg out of Anas. The embryo of D. lanceolaftim has the cilia on the 
anterior half of the body only. The eleven unciliated embryos are, Monostoma 
