=Underground Snails.=--Not only do many of the land Mollusca aestivate, or hibernate, as the case may be, beneath the surface of the soil, but a certain number of species live permanently underground, like the mole, and scarcely ever appear in the light of day. Our own little _Caecilianella acicula_ lives habitually from 1 to 3 feet below ground, appearing to prefer the vicinity of graveyards. _Testacella_, the carnivorous slug, scarcely ever appears on the surface during the day, except when driven by excessive rain, and even then it lurks awhile under some protecting cover of leafage. There is a curious little _Helix_ (_tristis_ Pfr.), peculiar to Corsica, which is of distinctly subterranean habits. It lives in drifted sand above high-water mark, always at the roots of _Genista Saltzmanni_, at a depth which varies with the temperature and dryness of the air. In hot and very dry weather it buries itself nearly 2 feet below the surface, only coming up during rain, and burying itself again immediately the rain is over. Like a _Solen_, it often has a hole above its burrow, by which it communicates with the air above, so as to avoid being stifled in the sand. The animal, in spite of its dry habitat, is singularly soft and succulent, and exudes a very glutinous mucus. It probably descends in its burrow until it arrives at the humid stratum, the persistence of which is due to the capillarity of the sand.[112] I am assured by Mr. E. L. Layard that precisely similar underground habits are characteristic of _Coeliaxis Layardi_, which lives exclusively in sand at the roots of scrub and coarse grass at East London. =Rock-boring Snails.=--Cases have sometimes been recorded, from which it would appear that certain species of snails possess the power of excavating holes in rocks to serve as hiding-places. At Les Bois des Roches, ten miles from Boulogne, occur a number of solid calcareous rocks scattered about in the wood. The sides of the rocks which face N.E. and E. are covered with multitudes of funnel-shaped holes, 1½ inch in diameter at the opening and contracting suddenly within to ½ inch. Sometimes the holes are 6 inches deep, and terminate, after considerable windings, in a cup-shaped cavity. _Helix hortensis_ inhabits these holes, and has been observed to excavate them at the rate of ½ inch each hibernation, choosing always the side of the rock which is sheltered from the prevailing rains. It does not form an epiphragm, but protrudes part of its body against the rock. That the snails secrete an acid which acts as a solvent seems probable from the fact that red litmus paper, on being applied to the place where the foot has been, becomes stained with violet.[113] _Helix aspersa_ is said to excavate holes 10 to 12 cm. deep at Constantine,[114] and _H. Mazzullii_ is recorded as perforating limestone at Palermo.[115]