8 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



complete, it is abandoned by the animal. The eggs vary in size accord- 

 ing to the magnitude of the species producing them. They are nearly 

 globular, one axis being somewhat longer than the other, white and 

 opaque. They consist, in general, of an external, semi-calcareous, elas- 

 tic membrane investing the whole, the interior surface of which is usually 

 studded with numerous rhombic, microscopic crystals of carbonate of 

 lime, some species, however, having a hard enveloping calcareous shell, 

 of the consistence of that of a bird's egg ; of an inner thin, transparent, 

 shining membrane which immediately encloses a transparent and some- 

 what viscid fluid, analogous to the albumen of birds' eggs ; of the albu- 

 men itself, and of the vitellus, which, possessing the same degree of 

 transparency as the albumen, cannot be distinguished from it at this 

 time. The elastic eggs, when first laid, are often flaccid, and seemingly 

 only half full of fluid, but they soon absorb moisture and become dis- 

 tended. The embryo animal, with its shell, is observable in the albu- 

 minous fluid in a few days after the egg is laid. Its exclusion takes 

 place, under ordinary circumstances, in from twenty to thirty days, 

 according to the state of the atmosphere. Warmth and humidity hasten 

 the process, while cold and dryness retard it to an almost indefinite 

 extent. The hatching of eggs laid late in the autumn is often inter- 

 rupted by the approach of cold weather and of snow, and delayed until 

 the next spring. In some few species the young is hatched from the 

 egg before exclusion. 



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, 1 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 hibernation. 2 Withdrawing into the shell, it forms over the 

 aperture a membranous covering, consisting of a thin, semi-transparent 

 mixture of lime mucus or gelatine, in some species opaque and thick, 

 secreted from the collar of the animal. This membrane is called the 



1 In Florida some species continue active during the whole winter. 



2 The same process is adopted by the species of the Pacific Region to protect themselves 

 from the effects of the dry season. 



