24 MOLLUSCA. 



revive, especially if it be moistened with a little water, 

 hurst 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 quiescence 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 



