48
ON THE EFFECT OF OVERCROWDING ON
THE GROWTH OF THE WATER SNAILS
LIMNAEA PEREGER AND L. STAGNALIS.
By F. M. TURNER, M.D., B.A., B.Sc.
(With Two Plates.)
[Read 29th January, 1927.]
NEARLY every keeper of an aquarium must be aware of
the fact from his own observation, or have heard, that
overcrowding retards the growth of young animals. Over-
crowding to an excessive degree results in the death of the
animals, as many a novice has found to his cost. But the over-
crowding of which I am going to speak has no such effect, the
animals live in apparent health for weeks and months, even
years ; and if the overcrowding is not too great, may even
breed.
But I think that most are not aware of the full extent to
which this influence can work, although it is more than 70 years
ago since Hogg, working with L. stagnalis, reported as follows :
"If the young animal be kept in fresh water alone without
vegetable matter of any kind, it still retains its cilia, and attains
only a small size." In a later note he says : "My attempt to arrest
the development of some young animals is still continued with
perfect success. They have remained in the same narrow glass
cell, at the stage of growth before referred to, viz., about the
size the animal usually attains during the first two or three weeks
of its existence. They are now six months old, alive and well,
the cilia are retained around the tentacles in constant activity,
whilst other animals of the same brood and age, placed in a
situation favourable to growth, have attained their full size,
and have now produced young, which are of the size of their elder
relations."
I have not succeeded in repeating this experiment exactly,
although some of those to be described later are rather close to it,
but it is not necessary to withhold all vegetable matter, nor to
use a very small narrow cell to get results almost as striking.
Plate 3, fig. 1 shows 53 snails turned out of their jars and
crawling about in a porcelain dish. The eight in the top row
are only a few days old, all the remainder were hatched from
one egg capsule on or about July 4th, 1926. They were hatched