142 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
This is provided for by allowing them to coil upon themselves in 
the form of a spiral spring. 
The structure of the mouth next claims our attention, and 
when we have examined it we shall find that it is precisely such 
as to meet the requirements of a creature feeding on soft vege¬ 
table tissues. The upper part is provided with a hard horny 
jaw, presenting a curved and indented cutting edge. On the 
floor of the mouth is a long ribbon-like tongue, whose surface is 
entirely studded with minute flinty teeth arranged in the most 
beautifully symmetrical manner. This rasp-like tongue works 
backwards and forwards against the edge of the jaw, and draws 
in young leaves and shoots, which are cut off by the latter. 
These ribbon-tongues, or odontophores, are most interesting 
objects to study under the microscope, and the manner in which 
the teeth are arranged is characteristic in each species. They 
are easily obtained by carefully dissecting the head after the 
snail has been killed. In the garden-snail the number of these 
teeth is 14,175, arranged in 135 rows of 105 each. It is evident 
that with continual wear and tear these teeth must get broken 
and blunted, but provision is made for this by the tongue being 
constantly renewed from behind; so that while the front portion 
is being worn away, a freshly-armed portion is coming forward 
to take its place. 
The chief interest attaching to the mantle is that it is 
the part of the animal which is concerned in the work of 
building up and ornamenting the shell. While it envelopes the 
animal, it lines the shell, and corresponds with it in shape. Its 
margin, where it encircles the head with a kind of collar, is con¬ 
siderably thickened, and it is this portion which is chiefly em¬ 
ployed in adding to the edge of the shell; while the whole sur¬ 
face is engaged in forming the inner lining. The colour and 
markings of the shell are the result of pigment-glands with 
which the mantle is provided. There are many interesting 
questions connected with the structure of shells, their mode of 
formation, analogies, &c., which it would carry me much be¬ 
yond the limits of the present paper to enter into. For the 
present, therefore, I will only ask you to bear in mind that the 
shell is not to be looked upon as a structure distinct from the 
rest of Ihe animal, but simply as a hardened or calcified portion 
of the mantle, and that its primary use is to serve as a protection 
to the heart and lungs. 
The nervous system is characteristic of the sub - king¬ 
dom, and differs from that of both higher and lower groups 
in being entirely unsymmetrical. It may be broadly described as 
consisting of three centres, or ganglia, connected by nerve-fibres, 
and giving off nerves to all parts of the body. The functions 
with which these centres are severally connected will be seen 
by the positions which they occupy, the first and most impor¬ 
tant being in the head; the second in the foot; and the third in 
close proximity to the heart and lungs. Of the organs of sense, 
the eyes are situated at the extremities of the two longer ten¬ 
tacles in the form of small bulbs. It is probable that they are 
capable of receiving impressions of light, but not of form. The 
organs of hearing—we can hardly call them "ears”—are situated 
at the base of the tentacles, and consist of little vesicles or sacs 
filled with fluid, in which float particles of lime known as 
“otoliths.” Supposed organs of smell have been detected, but 
the presence of the senses of smell and taste can with greater 
probability be inferred from the selection evinced by the snail in 
the choice of its food, and from the considerable distances it has 
been known to travel in quest of a savoury morsel. The entire 
skin constitutes the seat of the sense of touch, but the four 
tentacles serve especially as “ feelers,” and are constantly waved 
to and fro to guard against the approach of danger. 
The remaining details of structure are more of anatomical than 
of general interest, and may be dismissed in a few sentences. 
The digestive system occupies the greater portion of that part of 
the animal which is contained within the inner whorls of the 
shell, and is lemarkable for the large development of the liver. 
The heart consists of a single auricle and ventricle : the former 
to receive the purified blood from the lungs, and. the latter to 
propel it throughout the system. The blood of the snail is a 
clear celourless fluid, and is conveyed directly from the system 
back to the lungs without the intervention of the heart. The 
lungs are constructed upon the simplest type, consisting of a 
single pulmonary chamber, or cavity, formed by a fold of the 
mantle. The inner surface of this chamber is lined with a net¬ 
work of minute blood-vessels, in which the blood is purified and 
oxygenated by the air introduced into the chamber in the pro¬ 
cess of breathing. If we watch a snail as it crawls along, we 
may observe a small round opening at the right side of the neck, 
which slowly contracts and expands. This is the breathing 
aperture, and leads directly into the breathing chamber. The 
alternate expansion and contraction of this chamber is effected 
by the rise and fall of its muscular floor. 
Having concluded our hasty dissection and examination of the 
snail, we will return to the woods, and try what we can learn 
further of its habits. To do this to advantage, we must set out 
in the early morning, or after sundown in the evening; and, if 
the weather is damp, so much the better for our purpose. From 
this circumstance in itself, we may learn an important fact in 
snail economy, namely, that these creatures invariably shrink 
from the sun’s rays. Their bodies must constantly be bathed in 
the slime or mucus which is poured out from glands with 
which the skin is copiously provided, and exposure to the sun 
would soon diy up and harden this secretion. During the heat 
of the day, therefore, in dry weather, the snail hides under 
stones, moss, or leaves, or anything that will afford it shelter, 
and only comes out in search of food after the dew has moistened 
every leaf and stone, and prepared a path over which it can 
crawl without injury to its slimy consitution. For the same 
reason, autumn, when the fields and woods are moist and the 
soil rich with decaying vegetable matter, and before the frost 
has yet hardened the ground, is the season in which the snail- 
world revels. 
The slow rate of progression of the snail is proverbial, and is 
to be accounted for, as already pointed out, by the laborious 
mode in which locomotion is effected. It first occurred to an 
Am erican n aturalist, Air Thompson, to calculate what this rate 
actually is, and the conclusion he arrived at was that 16 days 
and 14 hours is the time that a moderately expeditious snail 
would take to accomplish a mile! I cannot vouch for the accu- 
