MOLLUSCA. 23 
been observed to thrive on the food which it spontaneously 
yields, to execute their accustomed motions, and above all, 
to propagate their kind, we shall be disposed to conclude, 
that patient suffering has been mistaken for health and 
vivaciousness for the power of accommodation. 
The influence of the seasons, in regulating the motions 
and habits of molluscous animals, has been but little attend- 
ed to. Those which live in the water, avoid the effects of 
low temperature, on the approach of winter, by retiring to 
the deeper parts of the lakes or rivers in which they reside. 
This migration, however, does not, in many cases, furnish 
the requisite security, so that they betake themselves to 
burrowing in the mud in which they repose until increasing 
warmth invites them to return to the open water. 
Among the naked terrestrial mollusca, it may be observ- 
ed, that they burrow in holes of the earth, under the roots 
of trees or among moss, and there screen themselves from 
sudden changes of temperature, and appear to spend the 
winter in a state of torpidity. 
The different kinds of shelly mollusca which inhabit the 
land, such as those belonging to the genera Helix, Buli- 
mus, and Pupa, not only retire to crevices of rocks and other 
places, for safety in the winter season, but they form an 
operculum or lid for the mouth of the shell, calculated to 
exclude the access of the air, and by the intervention of 
which they likewise adhere to the wall of their dwelling. A 
rise of temperature, however, especially if accompanied by 
moisture, excites their revival and motion, and the lid be- 
comes detached. If we bring, for example, the Helix ne- 
moralis, from its cold abode, and in an apparently torpid 
state, with the mouth of its shell closed by the lid and ad- 
hering to a stone, into a warm apartment, it will speedily 
