All snails belong to the phylum Gastropoda (literally "stomach-footed"). Nearly all are covered with a single spiral shell. Given that there are around 75,000 species around the world, plus several thousand more extinct forms known from their fossils, there is obviously going to be quite a bit of variation in this group of univalves. Including some that are completely shell-less, such as land and sea slugs, and others whose shells are so rudimentary that they are barely noticeable. So I’ll be using some pictorial examples that I hope will illustrate some of the basic common features.
Most snails have a roughly conical or oval shell protecting their bodies. These shells are twisted into spiral levels known as whorls. Some snails have dozens of whorls while others, like the knobbed whelk above, have only six or seven. The whorls are largest at the base and each one gets progressively smaller as we approach the tip, known as the apex.
Notice that there is a large opening in the newest whorl of the snail. This is called an aperture. The inside edge is called the inner lip and the outside edge is the outer lip. These aperture lips are where new shell material is added. Also, the newest whorl containing the aperture is called the body whorl. All the older whorls above this one are collectively called the spire.
The whorls form as shell material is laid down by the mantle, which is a sheet-like organ that lies against the inside of the largest bottom whorl. The larval stage of this whelk, even before it hatches from its egg case, will only have one tiny whorl around its body. As it ages, additional levels are formed. This initial larval whorl is called the protoconch.
Two conditions would have to occur for this spiral shape to form. First, the whorls must have a central base for them rotate around. This base is a rod-shaped structure that runs right down the center of the shell and is called the columella. The second condition is that the shell material must not be laid down evenly. The outer edge builds up faster than the inner edge. The best example I can give to illustrate this is a spiral staircase.
Notice that the whole structure is supported by a central beam and that the part of each step closest to the center is narrower than the more distal part. This columella can also be seen in micrograph photography like the image below, which I found at this UK site.
One additional feature of some snail shells is that tubular structure at the very end. This is called the aperture canal and is used to house and protect the animal’s siphon, which is like a nose that is used to search for food. Not all snails have this canal, only carnivorous snails like the oyster drill below (also diaried here). Note the tiny siphon protruding out of the canal. This can be tucked back in at the slightest sign of danger to keep it from being bitten off by a predator.
So the presence or absence of the aperture canal can usually tell you what the animal inside feeds on. Herbivorous snails do not have one as a snail that eats algae does not need a sense of smell to locate their food. This isn’t foolproof, of course. There are always exceptions in the animal world. Like the moon snail. This is a fierce predator on clams, but since the food lives underground, and the snail itself travels underground, touch rather than smell is used to locate prey. I diaried moon snails here and touched briefly on this.
Obviously the primary use of the shell is to protect the soft body inside. But since the animal must be able to extend part of its body from the shell in order to move around and eat, the aperture represents a pretty large chink in the snail’s armor. To protect this opening there is a structure attached to the foot called an operculum. This trap-door structure seals the opening shut when the foot is retracted back into the shell. This not only prevents predators access, but for intertidal species it protects them from drying out during low tide. The operculum appears to be not actually part of the shell because it is not normally made of calcium but of proteins similar to our fingernails. However, the rest of the shell, while mostly calcium carbonate, also contains proteins used to form a matrix for the calcium to form around. Sort of like the iron lattice used in cement buildings. And it is created by the mantle as well. Here’s a good image of a completely withdrawn snail with the operculum tightly sealing up the aperture.
There's one very interesting difference in those last two images above. The moon snail (top) has a hole in the bottom. This is called an umbilicus and is where the mantle inserts calcium to form the central columella. In this species the umbilicus is always left open, but in most species, like the apple snail below it, the umbilicus is covered up.
There are two other features of the shell whorls I’d like to mention. You can identify the whorls pretty clearly on most species because they are separated by an indentation called a suture.
Also, many shells, such as the oyster drill pictured above, have little raised ridges called axial ribs. The ribs are formed, like tree rings, because the growth rates vary throughout the year. During warmer months the growth increases, resulting in noticeably thicker portions of each whorl. Sometimes the differences in growth are so great that the ridges form spines that may extend out quite a bit. Like in the axial ribs of this murex.
One last note: I described the apex of the shell as being the "top" and the body whorl as the "bottom". This is because most diagrams you’ll come across show the shells in this vertical position. But snails actually lie horizontally, so the apex is actually the back end of the snail and the body whorl is in the front. There. Now you know more about a snail shell than you ever could have wished for.
Other diaries in this series can be found here.