C. L. Walton 
■2(33 
I have obtained strong evidence that the thorough cleaning of ditches 
completely destroys L. truncatula (at any rate for that season). 
7. It is observable that these roadside snails are frequently infected 
with Distomum and this in successive generations, evidently the result 
of the passing and re-passing of the sheep, some of which are infected, 
thus supplying these ditches, etc., with a constant succession of ova. 
Cercariae. 
Hundreds of samples of snails were examined and cercariae obtained 
during every month of the year except August. In many cases they 
appeared in the water in the collecting tubes soon after the snails were 
obtained, often in large numbers. Samples were examined in order to 
ascertain the distribution of infection; and (as already mentioned) 
some ditches showed re-infection after the re-appearance of young snails. 
Extensive experiments were conducted in order to gain some idea 
as to the relative number of cercariae given off by individual snails of 
various ages and sizes. In one series of experiments, specimens of 
L. truncatula were placed singly in glass tubes filled with clean water, 
and as soon as possible after collection. The water was renewed daily 
after counting the number of cercariae present. The results of a 
typical experiment is given in Table III. These experiments brought 
out several points: 
(a) Cercariae were not given off by any snail of less than 4-5 mm. 
in length and are not abundant until a length of over 5 mm. is attained, 
and above that size they may be regarded as dangerous. 
( b ) As many as 70 cercariae have been naturally obtained from a 
specimen 7 mm. in length within 48 hours; while 40 to 50 were often 
obtained. 
Table III. Table showing the number of Cercariae obtained from ten 
specimens of L. truncatula, collected near Pontrhydygroes, Oct. 22, 1914. 
Length of specimens in millimetres 
3*5 4 5 5 5 6 
6 
6 
8 
' 
8 
Total 
Aver¬ 
age 
Oct. 23 
i — 
40 
8 
16 
52 
117 
11-7 
» 24 
— .— — — — — 
40 
5 
1 
7 
53 
5-3 
25 
— — __ — — — 
20 
2 
7 
15 
44 
4-4 
» 26 
_____ X — 
X 
3 x 
4 x 
25 x 
32 
3-2 
Totals 
— — — — X — 
100 
18 
28 
99 
246 
24-6 
x = snail dead: remainder moribund. 
