52 THE ESSEX NATURALIST.
to grow irrespective of the size of the jar if they were adequately
fed. In some cases he fed the snails on lettuce leaves; in others
he used jars which were already coated with a thick green scum.
Since July, 1924, I have tried feeding L. pereger on lettuce
or water-cress, and have confirmed his results. Whereas up to
that date I had never got snails to grow to maturity and lay in
jars smaller than 80 oz., say 2 litres, I have since then repeatedly
grown singles and pairs to maturity in 20 oz. or 11 oz. jars. The
snails also grow larger than by the older method. The largest
attained 22 mm. in a 11 oz. jar. The largest obtained previously
in an 80 oz. jar were 19 mm. long. This seems to prove that lack
of food is the main if not the only factor. Also it definitely
disproves that excess of excrement or of carbonic acid or lack
of oxygen is the cause. For in many of my successful jars the
excrement formed a thick layer at the bottom, at least half an
inch deep. And in proportion to the size of the jar the conce -
tration of excrement must have been much greater than in any
jars where artificial feeding was not employed.
Most of my observations were made on snails I bred for
Prof. Boycott, who was investigating the inheritance of the
sinistral variety of L. pereger. The usual method was to take
snails hatched in a jar, which I call the nursery, and plant them out
either singly or in pairs in jars of about 2 litres capacity. The
water was generally taken from a pond in my garden. In each
jar was placed some Elodea, and in most Lemna minor and L.
trisulca were also present. As these animals are hermaphrodite
the singles lay as freely as the pairs, though not as a rule so early
in the year. For the first three years planting out was all done
in the spring; examining and counting the young took place in
the summer and autumn. Many of the jars were then discarded,
but the remainder were saved as nurseries for the next year's
planting. But when I found that two months sufficed for a
generation in warm weather, I tried to rear two generations a
year and have done this to a great extent in 1925 and 1926. In
one or two instances I have raised three generations in a year.
For observing the rate of growth I have made use of some of these
pedigree snails, but I have also bred a good number from wild
snails, both L. pereger and L. stagnalis, so that on the whole I
have bred over 1,000 snails to maturity, the majority of which
laid eggs.