56
THE ESSEX NATURALIST.
these on lettuce. By autumn several had reached 11 mm.
None of those fed on Elodea only exceeded 4 mm., although one
is now (Dec, 1926) over 17 months old. I also placed in May
and June some snails from two nurseries into test tubes and fed
them with lettuce. At the start they were between three and five
mm. long, and many have now reached 11 mm. The tubes
contained from 8 to 14 cc. of water only.
The conditions in these tubes were certainly very abnormal
compared with the jars. The water went very foul and swarmed
with infusoria, also it was often acid, whereas in the jars it was
always neutral or alkaline. The snails often got coated with a
thick crust of excrement or fungoid growth mixed with bacteria,
but in spite of these handicaps few died, and growth was not
prevented. None of them have laid eggs, so I cannot prove
that they are mature, but snails often lay at 10 mm., so they
are large enough to do so. Next year I may be successful and
get eggs in test tubes or in vessels not much bigger*.
CONCLUSION.
Snails can be dwarfed by overcrowding to a much greater
extent than land animals. If overcrowding had the same effect
on human beings slum children would be only 1ft. 2in. high at
18 years of age. But the effect in snails is temporary ; they
grow up normally as soon as given room to grow.
The cause of this phenomenon is still doubtful, but probably
is lack of food. If this is so, it is surprising how tolerant snails
are to semi-starvation without any permanent damage.
Similar dwarfing has been noted with frog tadpoles, young
fish, etc., and is probably fairly general among aquatic animals.
I do not know, however, whether it reaches the same extent
as in Limnaea.
EXPLANATION OF PLATES.
Plate 3, Fig. i. Towards the top of the photograph is seen one
adult snail out of several which were raised to maturity from a single
egg capsule. Above it are eight of its young ones about a week old.
Below it are 44 of its brothers and sisters which had remained in the
nursery, and remained small.
Date of laying of the original capsule not recorded.
Date of hatching two or three days round about July 4th, 1926,
Date of photograph Sept. 6th, 1926.
* Since this paper was read two of the snails reared in test tubes were transferred to a
small bottle holding 23 c.cm. and have laid eggs.