Experiments on the Development of the Liver-fluke. 5 
live rather lon<jer, and in a feebly alkaline solution of peptone 
I have been able to keep them alive for three days. The cilia 
were not lost, but their motion after the first day became very 
sluggish ; the embryos increased a little in size, but no advance 
in organization took place, except perhaps that the cellular 
parenchyma became arranged into rather more definite spherical 
masses. 
2. Farther Development of the Embryo. — What is the ultimate 
fate of this embryo? and by what stages is it connected with the 
adult fluke as found in the bile-ducts of the sheep? Its instinct 
evidently prompts it to bore its way into some object, and its 
organization is so simple, and its resemblance to the embryos of 
other Distomida; whose life-history has been made out is so close, 
that there is little reason for supposing its further development 
to depart from the ordinary type among the Distomidae : that 
is, we should expect to find it make its way into some animal — 
most probably a mollusc — and then, losing its cilia, be meta- 
morphosed into a brood-sac or sporosac, from which, in the first 
or later generation, larval cercarian forms will be produced by 
internal gemmation ; and after a period of quiescence these 
cercariae will, if they gain access to a suitable vertebrate host, 
give rise to adult sexual flukes. The embryos of Distoma nodu- 
losum and Distoma trigonocephalum are very like that of Fasciola 
hepatica ; the former gives rise to a sporocyst in Bithynia ten- 
taculata, and the latter to a redia in certain species of Paludina. 
This intermediate host will most probably be a mollusc, for 
very few instances are recorded of the occurrence of the sporosac 
form in any other than a molluscan host. There may indeed be 
more than one bearer for the brood-sac of Fasciola, for although 
some larval forms of trematodes are restricted to a single host, 
such as the Leucochloridium paradoxum of Succinea amphibia, 
others are less particular, and may be found in several different 
molluscs ; thus the rediae of Cere, echinata occur in Paludina 
vivipara, Limnceus stagnalis and Planorbis corneas, and the sporo- 
cysts of Cere, armata in the two last. 
Assuming that there is an intermediate molluscan bearer, the 
question still remains. In what form does the fluke enter the 
sheep ? and in what way ? Does it enter with the food ? or the 
drink? or by some other method? In the case of other Disto- 
midae the migration into the ultimate host is, so far as is known, 
always passive,* and the parasite is taken up with the food. 
* Wagener (' EiiigeweiJewiirmer,' p. 41) was unable to find the cysts of 
Cercaria mncrocerca, wliich develops into Distoma cygnoides of the frog, and 
suggested originally that this cercaria may wander directly into the frog, but did 
not prove the point experimentally. Thiry, however, (' Zeitschrift fiir wissen- 
schaftliche Zoologie,' vol. x. p. 27G) has succeeded in finding the cysts of Gere, 
macrocerca in all piirts of small Lirnnxi. 
