EFFECT OF OVERCROWDING ON GROWTH OF WATER SNAILS. 51
One may also infer from Semper's work that snails ultimately
reach a limiting size which also corresponds to the size of the jar.
In none of his experiments did he grow the snails to maturity,
so that this fact perhaps is not established from his work. In
my repetition of his work I have always gone on till some of the
snails at least had laid eggs, and I always find that it is the
snails in the large jars which grow to full size and lay eggs ; as a
rule those in the smaller jars do not.
Semper's second conclusion was that if varying numbers of
snails are placed in a series of jars of the same size, their growth
is inversely proportional to their number, in other words, each
snail behaves as if the jar were partitioned off and each had only
its own share of the total volume to grow in. This fact also
has been confirmed by the majority of other observers.
Plate 4 fig. 1 shows a set of snails, all bred in 20 oz. jars
with varying numbers in different jars, and you will see at once
the striking difference of size. Jars 1-4 contained one snail each,
jars 5-8 three snails each, and jars 9-12 ten snails each. One shell
from No. 5 was accidentally lost, and the same applies to Nos. 10
and 12 also. To explain the phenomenon Semper assumes that
there is some unknown chemical in the water which is exhausted
during the snails' growth. He excludes the lack of food because
there was always a surplus of water weed, Elodea, in all the jars.
He also excludes the effect of excrement, because if ten snails
be reared in one jar and a single one in another jar of equal
size, at first the ten snails make ten times the amount of excre-
ment. But the single snail grows so rapidly that it soon
makes as much or more excrement than its brothers. And the
same argument applies to the theory that lack of oxygen or
excess of carbonic acid can produce the phenomenon.
De Varigny suggested a different explanation. He found
that if one jar be immersed in another, but in free communication
with it, the snail or snails in the small jar were dwarfed just
the same. His small jar in some experiments was made by tying
a gauze partition over the bottom of a wide glass tube and
immersing the whole in the larger jar. This was raised and
replaced several times a day to ensure mixing of the, water. He
also used cages with muslin sides erected on a framework of glass
rods.
Popovici-Basnosanu, however, found that he could get snails