Mollusks. 8933 
Winding in the intetior, and invariably terminating in a cup-shaped cavity. These 
holes are the winter retreat of a peculiar species of snail—Helix saxicava—which, 
during the summer months, inhabit the surrounding wood, retiring to their stony habi- 
tations during the winter. The snails in question not only live in the holes, but exca- 
vate them themselves at the rate of about half an inch during each hybernation. In 
their selection of winter quarters they never choose the upper surface of the rock, but 
always a more or less vertical face of it, and that which is least exposed to the pre- 
vailing wind and rain, keeping the actual entrance to their dwelling narrow, although 
the mouth of the hole is expanded, thus using every precaution to render their habi- 
tations secure against the weather during the most inclement season of the year. The 
mode in which they effect this excavation has hitherto been a source of considerable 
controversy amongst the French savans, some affirming it! to be mechanical, others 
chemical. From the investigations, however, of my friend M. Bouchard-Chatereaux, 
the well-known savant of Boulogne-sur-Mer, who bas studied their peculiarities for 
years, and who, living so near to them, has had opportunities of observation which 
render him the most able authority on the subject, it would appear that the chemical 
theory is the correct one, and that the excavation is made by the dissolution of the 
rock by means of an acid given off from the fore part of the foot of the snail. 
M. Bouchard has pointed out that the Helix saxicava, when in its winter retreat, un- 
like the rest of its tribe, which form one or several epiphragms over the mouth of their 
shells, protrudes half its body from the shell, and fixes itself firmly against that por- 
tion of the rock which it selects for its winter habitation. Here it remains motionless 
until the return of spring again brings it forth from its hole. During this period it, 
as it were, sucks the rock away in the form of a cup, leaving the stone around the exca- 
vation, for about one-sixteenth of an inch in depth, impregnated with a sort of greasy 
matter, which disappears after exposure to the atmosphere. If the snail be forced 
from its hold, and litmus paper be applied to its foot and the place it has just left, it 
at once shows the presence of an acid by turning violet, more or less tinged with red 
according to the degree of moisture on the spot to which it is applied. Moreover, the 
surface of the rock is furrowed all over with tracks leading from one hole to another, 
doubtless caused by the corrosive action of the feet of the snails in the passage to and 
fro during many centuries. The Helix saxicava also differs from other tribes, in the 
fact that, whilst they generally congregate in heaps during the winter, it is almost 
always solitary. When more than one are found in the same hole, they are invariably 
at some distance from each other, as though fearing the action of the acid on their 
shells. Again, whilst in summer its tracks are marked by the usual slimy, shiny mucus, 
in its progress over the rocks, on the approach of winter none of this is perceptible. 
Hence it would seem that the acid secretion is quite distinct from the mucus, and is 
probably derived from a separate system of organs. The facts here stated seem to me 
to prove the chemical nature of the proceeding. The effect of the gastric juice of 
animals in dissolving calcareous substances is well known, and some very interesting 
experiments made by M. Bouchard prove that that of fishes possesses an equivalent 
power. May we not regard these facts as additional proofs of the wisdom and bene- 
volence of the Almighty in furnishing the means not only of comfort, but also of the 
preservation of life? It is well known that when, from any cause, the stomach is 
allowed to remain empty for a lengthened period, the gastric juice acts prejudicially 
upon it, destroys its coats, and at the same time the life of the creature. The Helix 
saxicava has no employment for its stomach during winter: may we not believe that 
