258 Liver Rot of Sheep 
or no sucli spots could possibly have been re-stocked from some pool or 
spring above, etc. 
The exact date of hatching in such cases is often difficult to fix 
accurately, as the young when discovered only measure -4 mm. to -5 mm. 
in length and being often few in number and the ditch heavily grass 
grown, it is often almost impossible to detect them. These very 
young forms may crawl upon stones, etc. The whole matter is complex 
and would require very prolonged experiment to disentangle. The 
outstanding and important facts brought out by these observations are : 
that it is very difficult to get rid of L. truncatula, which will re-appear 
in ditches, etc., that have been completely dry for weeks and even 
months, and that apparently only thorough cleaning out, or a drought 
killing off all adults prior to the deposition of any ova, or ova about to 
hatch, will effectually clear anj^ given spot. Conditions vary so much 
from place to place in the same locality that L. truncatula is not likely 
to be eliminated completely by natural means. 
Obviously, should the ditches dry up before the deposition of ova 
the range of the species is seriously threatened, but once ova are deposited 
it would seem that the species stands a very excellent chance of survival 
no matter how hot or dry a time may follow, even if all the snails them¬ 
selves are killed thereby. It may be urged that a wet period followed 
by a second drought would kill the hatching young and thus eliminate 
the snail from the locality and this is evidently a real danger, for in no 
case did snails appear from ova experimentally dried, when they were 
almost hatched. Newly deposited ova recovered after desiccation for 
certain periods. The difficulty however would be largely met by my 
hypothesis of delayed hatching consequent on length of desiccation. 
Again it may be pointed out that L. truncatula lives and deposits its 
ova in those portions of streams, etc., that are most liable to be affected 
by drought: namely the shallows and margins. Such situations are 
less liable to constant submergence, and even then, since some weeks 
must elapse before hatching can take place, it is obvious that only in 
a very wet year (or in permanently wet places) will hatching take place 
before autumn and this fits in with what I have observed. 
How far these observations may tally with other areas, I do not 
know: different conditions may produce variations. It is however 
certain from my experience that ova masses just about to hatch will die 
if completely dried; while freshly deposited ova dried 34 to 36 hours 
hatch in about six weeks (the normal time). Ova dried for longer 
periods take longer to hatch and the numbers resulting are much 
