PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
141 
retires into some sheltered nook, such as the crevice of an old 
wall, the hollow of a decayed tree, or the cleft of a rock. 
Failing any such natural hiding-place, it will bury itself a little 
way under the surface of the soft soil,—its head generally being 
uppermost. Frequently a number of snails group themselves 
together during hybernation, adhering to each other’s shells. 
Having selected its retreat, its next care is to close up 
the mouth of its shell for protection from cold and from the 
dryness of the air. This it does by constructing across the 
opening a horny barrier, or diaphragm, as it is called, formed of 
hardened mucus. It then retires to the furthest extent into 
its shell, and there constructs a second and similar barrier. In 
each of these diaphragms a small aperture is left to admit of 
respiration. The Homan snail of Kent and Surrey (Helix 
pomatia) forms a solid diaphragm of shelly material, which 
may be found in spring after the snail emerges from its winter 
quarters. During hybernation no growth takes place, and pul¬ 
sation of the heart is said also to cease. The only vital process 
which appears to be carried on is a slow respiration. It 
frequently happens that the snail does not survive its winter 
privations, as in spring groups of empty shells may sometimes 
be found, glued together, as they were by their inmates, who 
have not had strength to liberate themselves from their volun¬ 
tary prisons. But if it does survive, then, with the first 
return of warmth, it breaks open its double doors by dissolving 
their edges in a fresh secretion of mucus. Frequently, hollow 
impressions may be observed on limestone rocks where snails 
are in the habit of hybernating, produced either by the wearing 
action of the foot, or rasped by the tongue to obtain lime for 
the formation of the shell. 
During the ensuing spring and summer the snail will grow 
rapidly, and by the following autumn will have attained its full 
size. Should the summer be dry and hot. it will hide itself for 
some days or weeks in the same way that it does during the 
severity of winter, but with this difference, that the head will be 
downmost. During such an aestivation, the vital functions are 
not so completely suspended as they are in hybernation, and 
growth does not cease. By the end of summer the snail will be 
busy depositing its eggs in a secure hiding-place, and this brings 
us to the close of a cycle of snail existence. Very little is known 
as to the usual duration of a snail’s life under natural conditions, 
but it is thought probable that the majority do not survive their 
second hybernation. In captivity, however, they have fre¬ 
quently been known to live six or eight years; and there are 
instances on record of even greater longevity. Their powers of 
enduring privation are very great. Thus, a specimen of an 
African snail, Helix desertorum, was fixed to a tablet in the 
British Museum in mistake for an empty shell in March, 1846. 
In March, 1850, it softened the glue with which it was fixed, and 
crawled off its card after a fast of four years. This fact is 
vouched for by Dr Woodward, the Assistant-Curator of the 
Museum, in his Manual of the Mollusca. 
Such is a very brief resume of some of the events which mark 
a snail’s career. Let us now take up a mature specimen, such as 
we may find on some parts of Kinnoull Hill, and make a closer 
nspection of the creature itself. Perhaps we shall find in doing 
so that it has a more highly-organised body than we gave it 
credit for. If we place our specimen, say, on a large leaf, and 
wait until it has recovered from its alarm on being disturbed 
we shall presently see a shapeless grey mass emerging from the 
shell, and from the centre of this mass the head is first cau¬ 
tiously protruded; and then from the latter the horns gradually 
unfold themselves. While it is crawling slowly away, we may 
note that the part of the body which is protruded from the shell 
consists, broadly speaking, of three principal parts—first, the 
thick fleshy disc on which it crawls, called the “ foot;” second, 
the pliable membrane which lines the shell, and forms a kind of 
hood over the head, called the “mantle;” and third, the head 
itself. Besides these, there is the part of the animal remaining 
within the shell, which comprises chiefly the digestive system 
and other vital organs. 
The muscular system is of a highly - developed type, 
and presents points of special interest; but in studying these it 
is necessary to bear in mind that the muscles, with the exception 
of those connected specially with the shell, are attached to the 
inner surface of a pliable integument or skin. In this they pre¬ 
sent a marked contrast, on the one hand, to those of such 
animals as insects, crustaceans, Ac., whose bodies are enveloped 
in a horny or shelly covering; and, on the other, to those of 
vertebrate animals, whose muscles have their attachment in a 
hard internal framework. The sheU of the snail, therefore, as 
we shall see more fully afterwards, partakes more of the charac¬ 
ter of a protection than of a true exo-skeleton. 
The structure of the foot, and the way in which locomotion is 
effected, form a curious physiological study. The action of the 
muscles can best be seen by allowing the snail to crawl along a 
pane of glass, so that we can watch the surface of the foot from 
beneath. A continual series of curved waves will appear to pass 
from front to back with a perfectly rythmical motion, their 
greatest intensity being in the central line. This appearance is 
produced by the rapid contraction and expansion of each set of 
muscles in succession; so that if the actual rate of progression is 
slow, it is the result of a large amount of muscular action on a 
minute scale. On a dry summer evening it has often been 
observed that a distinct musical sound is produced by a snail 
crawling on a window-pane, this proving how perfectly rythmical 
the wave-motion is. 
Coming now to the head, the chief points of interest are the 
structure of the tentacles or “horns,” andthe arrangement of the 
teeth. The former are remarkable for the mechanism by which 
the snail is able to put them out or withdraw them at will. This 
is done, not by drawing the entire tentacle intoaieceptacle, but 
by folding it in upon itself in precisely the same way that the 
finger of a glove is drawn in upon itself when it is turned inside 
out. Each tentacle consists of a hollow tube with muscular 
walls arranged in a series of rings, and with muscles leading 
from its extremity into the head. It is by the contraction of 
these latter muscles that the tentacle is withdrawn, while its 
protrusion is effected by the alternate expansion of the muscular 
rings. It may occur to some of you to wonder what happens, 
during this process, to the delicate nerves contained within the 
tentacle, which cannot be stretched beyond their normal limit. 
