EFFECT OF OVERCROWDING ON GROWTH OF WATER SNAILS. 55
the facts of experimental dwarfism, as the rest of my work has
brought me to believe. Or it may be that excess of food was
not always present in the above experiments. The jars were
usually looked at once a week, and in some instances all the
lettuce had disappeared. I have carried through another set
with L. stagnalis this year, looking through the jars twice
each week, but the result has been the same. Possibly the food
has not been suitable. I think that the snails will not eat
perfectly fresh lettuce. I have seen snails refuse lettuce for over
a week. On the other hand, when a leaf is half decayed they will
usually be found on the rotten portions. The same applies to
water-cress. In the earlier experiments I purchased the lettuce
and water-cress at the shops and divided it up among my jars.
Latterly I have regarded all leaves of either plant fit for human
food as unfit for my experiments, and have used solely those
leaves rejected by the cook as unfit for making her salad.
In conclusion, I consider it probable that the food factor is the
real explanation, but must leave a firm proof for further experi-
ments. I should like to add a few words on two subjects cognate
to the above discussion.
First. What is the normal food of water snails ? It cannot
be lettuce, and it is easy to observe that they do not eat Elodea.
They may constantly be seen browsing on the glass even when it
appears clear. However, with a pocket lens minute algae can
usually be seen, and one can watch these being tucked into the
snail's mouth. No doubt when they browse on water weed they
also eat only the microscopic algae and leave the weed unharmed.
Sometimes, though rarely, my jars have got coated with a thick
scum, partly fungi, partly algae, so thick as to be quite opaque.
This scum both pereger and stagnalis eat greedily, and leave
tracks as seen in plate 3, fig. 2.
Second. What is the smallest jar in which a snail can be
reared to maturity provided food be supplied? If the eggs
are placed in test tubes they hatch out as well as in larger vessels.
I have hatched out several thousands this way. In all cases
where I have placed the eggs in water only, none have survived
more than three or four weeks, but in those in which I put also a
small shoot of Elodea a few snails, usually one or two, or at most
four, survived many months. Last spring I had several, from
six to ten months old, and in May I commenced to feed some of