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Thus, even among warm-blooded animals like the bears and dormice,
hibernation actually occurs to a very considerable degree; but it is far
more common and more complete among cold-blooded creatures, whose bodies
do not need to be kept heated to the same degree, and with whom,
accordingly, hibernation becomes almost a complete torpor, the breathing
and the action of the heart being still further reduced to very nearly
zero. Mollusks in particular, like oysters and mussels, lead very
monotonous and uneventful lives, only varied as a rule by the welcome
change of being cut out of their shells and eaten alive; and their
powers of living without food under adverse circumstances are really
very remarkable. Freshwater snails and mussels, in cold weather, bury
themselves in the mud of ponds or rivers; and land-snails hide
themselves in the ground or under moss and leaves. The heart then
ceases perceptibly to beat, but respiration continues in a very faint
degree. The common garden snail closes the mouth of his shell when he
wants to hibernate, with a slimy covering; but he leaves a very small
hole in it somewhere, so as to allow a little air to get in, and keep up
his breathing to a slight amount. My experience has been, however, that
a great many snails go to sleep in this way, and never wake up again.
Either they get frozen to death, or else the respiration falls so low
that it never picks itself up properly when spring returns. In warm
climates, it is during the summer that mollusks and other mud-haunting
creatures go to sleep; and when they get well plastered round with clay,
they almost approach in tenacity of life the mildest recorded specimens
of the toad-in-a-hole.

For example, take the following cases, which I extract, with needful
simplifications, from Dr. Woodward.

‘In June 1850, a living pond mussel, which had been more than a year out
of water, was sent to Mr. Gray, from Australia. The big pond snails of
the tropics have been found alive in logs of mahogany imported from
Honduras; and M. Caillaud carried some from Egypt to Paris, packed in
sawdust. Indeed, it isn’t easy to ascertain the limit of their
endurance; for Mr. Laidlay, having placed a number in a drawer for this
very purpose, found them alive after five years’ torpidity, although
in the warm climate of Calcutta. The pretty snails called cyclostomas,
which have a lid to their shells, are well known to survive
imprisonments of many months; but in the ordinary open-mouthed
land-snails such cases are even more remarkable. Several of the enormous
tropical snails often used to decorate cottage mantelpieces, brought by
Lieutenant Greaves from Valparaiso, revived after being packed, some for
thirteen, others for twenty months. In 1849, Mr. Pickering received
from Mr. Wollaston a basketful of Madeira snails (of twenty or thirty
different kinds), three-fourths of which proved to be alive, after
several months’ confinement, including a sea voyage. Mr. Wollaston has
himself recorded the fact that specimens of two Madeira snails survived
a fast and imprisonment in pill-boxes of two years and a half duration,
and that large numbers of a small species, brought to England at the
same time, were all living after being inclosed in a dry bag for a
year and a half.’

Whether the snails themselves liked their long deprivation of food and
moisture we are not informed; their personal tastes and inclinations
were very little consulted in the matter; but as they and their
ancestors for many generations must have been accustomed to similar long
fasts during tropical droughts, in all likelihood they did not much mind
it.