4 LAND AND FRESH-WATER SHELLS OP N, A. [PART I. 



days, according to the state of the atmosphere. Warmth and 

 humidity hasten the process, wliile cold and dryness retard it to 

 an almost indefinite extent. The hatching of eggs laid late in 

 the autumn is often interrupted by the approach of cold weather 

 and of snow, and delayed until the next spring. 



The young animal gnaws its way out of the egg, and makes 

 its first repast of the shell which it has just left. It consists at 

 first of about one and a half whirls, the umbilicus being minute, 

 but open. Its growth is rapid, and it has usually increased in 

 magnitude three or four times, before the close of the first year. 



In the month of October, or at the epoch of the first frost, the 

 snail ceases to feed, becomes inactive, and fixes itself to the 

 under surface of the substance by which it is sheltered, or 

 partially burrows in the soil, and with the aperture of the shell 

 upward, disposes itself for its annual sleep or hybernation. 

 Withdrawing into the shell, it forms over the aperture a mem- 

 branous covering, consisting of a thin, semi-transparent mixture 

 of lime mucus or gelatine, secreted from the collar of the animal. 

 This membrane is called the ejnphragm. It is formed in this 

 manner : The animal being withdrawn into the shell, the collar 

 is brought to a level with the aperture, and a quantity of mucus 

 is poured out from it and covers it. A small quantity of air is 

 then emitted from the respiratory foramen, which detaches the 

 mucus from the surface of the collar, and projects it in a convex 

 form, like a bubble. At the same moment, the animal retreats 

 farther into the shell, leaving a vacuum between itself and the 

 membrane, which is consequently pressed back by the external 

 air to a level with the aperture, or even farther, so as to form a 

 concave surface, where, having become desiccated and hard, it 

 remains fixed. These operations are nearly simultaneous, and 

 occupy but an instant. As the weather becomes colder the 

 animal retires farther into the shell, and makes another septum, 

 and so on, until there are sometimes as many as six of these 

 partitions. The circulation becomes slow, the pulsations of the 

 heart, which in the season of activity vary from forty to sixty in 

 a minute, according to the temperature of the air, decrease in 

 frequency and strength, until they at length become imperceptible. 

 The other functions of the body cease, and a state of torpidity 

 succeeds, which is interrupted only by the reviving heat of the 



