The shell
The most eyecatching part of the snail is of course the shell.It consists of several layers:
A thin outer layer called periostracum(A) made of organic substance called conchiolin,the color pigments of the shell are usually located here.
Often,when the snail gets older,the periostracum starts flaking and wears off.
A thicker inner layer called ostracum(B,C) made of inorganic calcium carbonate,in some species color pigments are located here as well.
A inner layer called hypostracum(D).This also the layer where nacre or mother of pearl can be found.
The shell grows larger by secretions of the mantle's epithelium near the shell edge.
The shellwall is made thicker by calcium carbonate crystals which are secreted by glands which are located on the epithelium of the mantle.
The layers of the shell
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Most shells are coiled in a clockwise direction.In the family of the Clausiliidae and some species of the genera Vertigo they are coiled to the left.
You can tell which way a shell is coiled by holding it with the aperture towards you.If the aperture is on the right side it is dextral(clockwise),
if it is on the left side it is sinistral(anti-clockwise).
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[1]Some shell features [2]Sinistral and dextral A.fulica
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The body is attached to the shell by a muscle which is attached to the central column,the columella.The oldest part of the shell
is the apex.From the apex the shell grows downwards,every 360 degrees turn is a whorl.The area where two whorls meet is called
the suture.This can be shallow,or deep.The last whorl of the shell is called the bodywhorl,this leads to the shellmouth or aperture.
Some shells have a hollow part on the underside called the navel or umbilicus.This is very narrow to wide and shallow or deep,
or might not be visible at all.
When a snail grows older the shell grows slowly wider.This might happen evenly so that the shell grows regularly(like in the
genus Discus),or unevenly.
The mouth of the shell is called aperture,it can be round,semicircular,trilobate or auriculate.
The edge of the mouth,the margin,can be shaped in a number of ways. Simple(for example family Zonitidae),or folded outward
(genus Helicigona).Sometimes it is folded outward and then tucked in again,this is an infolded margin(genus Granaria).
Often the margin is made stronger by a thickening,this is known as the lip.The thickening of the lip is a sign that the snail
reach adulthood.The lip sometimes has the same color as the rest of the shell,but quite often it has a different coloring.
 
[1]teeth in the aperture [2]ribbing on the shell of Cerion uva
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In some families the aperture has teeth or lamellae.In some snailfamilies this is very complicated(Clausillidae,Pupillidae,Chondrinidae).Often
they run till deep inside the shell,sometimes they are
located around the margin.In the family of the Clausiliidae another feature is to be found:the clausilium.
This is an elastic plate attached to the columella which covers the aperture when the snail is retracted in its shell.
The surface of the shell can be smooth(Cochlicopa),but is mostly grooved.This grooving can be axially or spirally,very faint
or pronounced.Raised areas between the grooves produce ribbing(for example genus Granaria).In spiral grooving the grooves are mostly fine
so that they are more like
spiral lines.Where there are both axial and spiral grooves a reticulated pattern is formed.Sometimes this is so fine it is only visible with a microscope.
Some shells possess spines (Acanthinula aculeata),these start already in the layer under the periostracum.
Other shells have hairs,these can be permanent(genus Trichia)or only present on juvenile shells.
The thickness of the shell is very variable.Usually snails in wet conditions have thinner shells than the snails which live
in warmer climates.Shells come in all sorts of colors,brown is often seen in European species.The color of the shell can be very variable within
a species(Cepaea nemoralis).
A reason for this seems to be that it helps the species to survive in a certain habitat.For example Song-thrushes might mainly eat the snail
with yellow banded shells in shadowy woods,the ones with reddish shells will be relatively safe.
Shells have many different shapes and sizes,they are usually described using the following terms:
flat(wider than high),high(taller,than wide)and spherical.Within these three groups you can distinguish a number of shapes.In tall shells this is:
tapering,conical,cylindrical,
fusiform and ovoid.Quite often there is a combination of two types:ovoid-cylindrical for example.In flat shells the term discoidal is often used.
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[1]discoidal:Discus patulus[2]spherical:Helix aspersa[3]tapering:Cochlicella acuta[4]fusiform:Cochlicopa lubrica
Photo's:Femorale
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Shells with an abnormal shape are to be found in every species,for example scalariform where the whorls are loosely
coiled.More well known are the shells which are coiled in the opposite direction of what they normally are.Among collectors these
sinistral shells are highly sought after.
Within certain snail- and slugfamilies a reduction of the shell has taken place.This is most obvious in slugs of the
genus Arion where under the skin chalky pieces form the remainder of the shell.
In another familie of slugs,the Limacidae,the shell is formed by an oval plate covered by the mantle.In between stages can be found in the family of the Vitrinidae.Vitrina pellucida has a shell big enough for the animal to retract into,while Semilimax has a
very small shell only protecting part of the body.
Pomacea bridgesi:shell and operculum
picture:www.applesnail.net
One of the characteristics of the Prosobranchs is the operculum,which is attached to the back of the foot.
This is used as a sort of door and protects the snail against enemies and loss of moisture.
It can be corneous or calcified(partly).
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