2 2 Experiments on the Development of the Liver-fluke. 
In addition to these, numerous other cases have been examined. 
At Woodeaton certain fields have been known to give the rot 
regularly for more than five years. Avion ater was the com- 
monest mollusc here, and many specimens were examined. The 
road leading to Oxford over Woodeaton Common has a very 
bad reputation for giving rot, but more than a score of the same 
slugs (and these were the only molluscs discoverable) afforded 
no trematodes. 
A consignment of slugs was received from a field in Lincoln- 
shire notoriously liable to flukes, which is surrounded by ditches 
containing brackish water, which occasionally fills them for 
months together. The slugs comprised many species of Avion 
atev and Limax agrcstis, with a single Avion hovtensis, but none 
of them contained any cercariae. From sheep-rotting fields at 
Swindon A. atcr, L. agrcstis, Limnceus tvuncatulus and Succinea 
amphibia were obtained, and slugs have been obtained from 
similar localities at Abingdon and Hincksey, but none of these 
contained any cercarian form. 
It will be seen that large numbers of our two common slugs 
have been collected at a critical time from infected ground, and 
all have proved to be free from any form of trematode, whether 
in the brood-sac or encysted state. The only cercaria met with 
in any slug during the investigation is Cercavia limacis, and this 
is, indeed, the only one which has been described in A. ater 
or L. agvestis, except some free but immature distomes found by 
Dujardin, and which have probably nothing to do with Fasci- 
ola. In aquatic molluscs cercarian forms are in certain localities 
very abundant, and I have not found any sheep-rotting ground 
where careful search has not revealed some water-snails, though 
in some instances they have been scarce, as at Wytham. In 
these one or two suspicious forms have been discovered, but 
they have not been abundant enough to enable me to perform 
the necessary feeding experiments upon rabbits. 
The failure to find the cercaria of Fasciola even in the very 
large numbers of molluscs which have been worked upon furnishes 
us with negative evidence only ; and in this connection it must 
be remembered that where the larval forms of trematodes do 
occur, they are frequently found in vast numbers in a single 
host. As many as 10,000 cercaria; may be given off in a single 
day by a slug infested with Cercavia limacis. 
One of the conditions most necessary for the prevalence of rot 
is moisture ; and whilst low-lying ground is thus naturally more 
exposed to it, rot may occur on upland (as at Wytham) if there 
be sufficient moisture to allow the egg of the fluke to develop ; 
that is, any heavy or ill-drained land may give the rot. In the 
neighbourhood of Hampton Poyle and Bletchington, north of 
