EFFECT OF OVERCROWDING ON GROWTH OF WATER SNAILS. 53
Snails planted out thus occasionally die, otherwise they all
grow up to maturity and about 80 % to 90 % lay eggs. Snails
left in the nursery never grow up unless very few in number, say
three or four. And if nurseries are examined it is found that
the sizes of the snails are inversely proportional to the numbers.
All sorts of factors can be varied without varying the result,
such as temperature, exposure to light, etc. For instance,
my first year's breeding was carried on indoors, the jars being
on windowsills, some facing north, others east. Of course the
latter got much more direct sunlight, and must have been
warmer. All the remaining four years the jars have stood in
the garden, and got still better light exposure.
Also the growth of the snail does not depend on the growth
of the weed. Pond weed is not essential. Colton states that a
young L. columella (an American species closely resembling
L. pereger) will grow up if put into a jar with pond water alone,
the young algae whose spores are present in the water developing
as fast as the young snail needs them. This experiment I have
tried two or three times, but only succeeded once.
Table I.
Mean length of singles, 15.5 mm.
Mean length of pairs, 11.7 mm.
Table I. gives the measurements of a number of shells of
pedigree snails taken during two years. Some died, some were
killed, nearly all had laid eggs. It will be seen that on the
average the length of singles is greater than that of snails reared
as pairs, in the proportion of I : 1.32, or the volume in the
ratio of 1 : 2.3. However, a few of the snails in pairs are larger