Land snails generally have
the same basic shape, that being a spirally (usually dextrally) coiled shell
into which the snail’s body can withdraw in times of environmental stress,
hibernation, or in effort to provide protection from
predators.
The shell itself is
comprised of two layers. The
ostracum is the inner layer, made of calcium carbonate (CaCO 3). It is the thickness of the ostracum that
determines the opacity of the shell.
Covering the ostracum is the periostracum, made of a scleroprotein called
conchiolin (which is the substance that, along with CaCO 3, makes up pearl and
mother of pearl). Either of these
layers may contain pigmentation that gives each species its characteristic
markings.
Each whorl (or coil) of the
shell is separated by a join, called a suture and may exhibit transverse growth
ridges along with bands or stripes of colour. In some species, an umbilicus is
present, which is an opening running along the central axis of the
shell.
Some prosobranch snails have
a horny plate that the snail uses to seal off the entrance of the shell to
protect it. This structure is
referred to as the operculum.
Pulmonate snails, which includes Giant African Land Snails, do not have
an operculum, as the inside of their lower shell mantle has a cavity
(pneumostone) that acts as a lung (hence pulmonate – lung snails). Pulmonate snails do, however, seal off
their shell entrance, but with a film of mucus known as an epiphragm, which may
be thin and transparent, or thick and opaque, depending on the species and state
of hibernation. The
Clausiliidae family of pulmonates also has a plate to close off the entrance to
the shell, but this is referred to as a clausilium, and is not related to the
operculum of prosobranch species, but has evolved separately to perform a
similar function, in an example of convergent evolution.
Number of whorls should be counted as shown
here – 3 whorls.
The snail’s body is divided
into two parts: the extendable
foot, on which the snail moves via a series of muscular contractions, and the
internal visceral mass, containing the animal’s organs. The foot carries the head, which usually
bears two pairs on tentacles, the upper of which house the eyes at the
tips. The lower tentacles are a
sensory organ. These can be
inverted, or invaginated (withdrawn inside the body). The head also contains the odontophore,
which is a rudimentary tongue, bearing an abrasive surface called the radula.
The snail uses the radula to scrape food into its gullet, which runs across the
top of the head. (Food can actually
be seen, when observed closely, travelling down the gullet.) The anus and urinary tract open
near to the front of the foot, under the shell lip near the
pneumostome.
The lower surface of the
foot is appropriately called the sole.
The snail moves, as aforementioned, with a series of contractions of the
muscle of the foot, aided by the adhesive mucus famously secreted by the
snail. The mucus is excreted from
an organ called the pedal gland at the front of the foot, below the mouth
opening.
Variations according to
species and groups include the presence of only one pair of tentacles in the
case of most prosobranch snails. In
this case, the eyes are located not at the tip of the tentacles, but at the
base.