24 MOLLUSCA, 
revive, especially if it be moistened with a little water, 
burst open the lid and begin to crawl. If the animal be 
again exposed to a low temperature, it again secretes ma- 
terials for a new lid, and speedily returns to its slumbers or 
inactivity. The first formed opercula, in these animals, al- 
ways contain a considerable proportion of carbonate of lime, 
a material which is found in smaller quantity in those of 
after formation. If the animal has revived frequently dur- 
ing the winter, the last formed opercula will be observed to 
consist chiefly of animal matter, and to be very thin. The 
first formed winter lid of the Helix Pomatia is of the con- 
sistence of card-paper. 
All the land shelly mollusca appear to have the power of 
passing into a state of qguzescence resembling torpidity, at 
pleasure, and independent of low temperature. Thus, even 
in midsummer, if we place in a box specimens of the Helix 
hortensis, nemoralis aut arbustorum, without food, in a day 
or two they form for themselves a thin operculum, become 
attached to the side of the box, and assume a dormant con- 
dition. In this state of apparent torpidity they may be kept 
for several years. No ordinary change of temperature pro- 
duces any effect upon them, but they speedily revive if 
moistened or plunged in water. Even in their natural 
haunts, they are often found in this dormant state, during 
the summer season, especially when there is a continued 
drought. Thus the Helix nemoralis may frequently be 
observed several feet from the ground, and attached to the 
stem or leaves of plants, with the mouth of the shell closed. 
But it is not drought which influences these terrestrial 
shelly mollusca to assume this quiescent state. The Sue- 
cinea putris, a species in ordinary cases delighting in mois- 
ture, may readily be observed during summer in a dormant 
