6 Experiments on the Development of the Liver-Jluke. 
The cercaria, when fully formed, makes its way out of the sporo- 
sac which has produced it, and encysts itself either in the same 
mollusc, or more frequently it passes out into the water, and 
after a short free life encysts itself in another snail of the same 
species, or it may be of a different species. But with all 
aquatic cercaria; an encysted stage seems to be necessary, and no 
instance is known of direct development of the cercaria into the 
mature form without the intervention of a period of quiescence. 
\\ hen free cercariae have been given to the animals which form 
their natural abode in the adult state, these cercariap have always 
failed to develop further.* 
The sporosacs of Distomida? have only been found in molluscs 
and fish, but the encysted larvae are more widely distributed, and 
have been found not only in molluscs, but also in the aquatic 
larva» of insects (^Perla, Ephemera, «Scc.) ; in Crustacea, especially 
Gammarus ; in Hydraclina ; rarely in worms ; in small fish, 
tadpoles, frogs and newts ; that is, in animals which serve as food 
for other creatures. Some cercariae encyst themselves on plants, 
or on the sides of the vessel which contains them ; indeed, the 
instinct to encyst is so imperative at a certain moment of their 
lives, that it takes place wherever the cercariae happen to be. 
Judging from the analogy with other flukes, it is most probable 
that sheep pick up the young liver-flukes whilst in the encysted 
state with their food. It is not likely to be taken up with the 
water drunk by the sheep, for free cercariae would perish. More- 
over, rabbits are exceedingly liable to flukes — 1 have found as 
many as fifty in a single rabbit's liver— and sportsmen deny that 
they ever drink. It has been conjectured that the sheep takes 
up the fluke by eating the mollusc which harbours the larvae. 
Sheep and many other animals are passionately fond of salt, and 
might possibly eat snails intentionally on account of their saline 
taste. Mr. C. Spence Bate, F.R.S., says that "as a rule, farmers 
encourage the snails, as they say that the sheep are fond of them." 
But other observers have come to the opposite opinion, and it 
would seem more probable that the molluscs, if eaten at all, are 
swallowed accidentally, being mixed with the food. In this case 
of course any large mollusc could not be taken up. 
Or, infection may be produced in another way. The cer- 
caria of Easciola may encyst itself at the moist roots of plants, 
and then be taken up when the sheep is grazing. In examining 
* In tlie case of Distuma nodulosum,ii Acerina cernua 'the pope), which is one 
of its ultimate hosts, be fed with cysts, the included distonies develop to the 
adult condition in the intestine, and produce eggs; but if fed with free cercariae 
these encyst themselves on the outer surface of tlie intestine, and do not come to 
perfection until the host itself falls a prey to some larger species of lish, as the 
perch. In this instance the same animal may serve either as intermediate or 
ultimate host. — 0. von Linstow, Archiv fiir Natunjeschichte, 1873, j). 1. 
