The Parts of a Gastropod Shell
by Peggy Williams
When you try to identify shells from the shell
books, it’s important to understand the terminology that describes the shell. Here are some terms:
If you hold the shell with the aperture towards
you and down, the animal will be standing on its
head! At the top of the shell in the position is the
apex, consisting (if it’s not broken off) of the
protoconch. The protoconch is the larval shell,
consisting of 2-4 whorls (turns of the shell) and it
can tell you a lot about the animal’s lifestyle.
If the protoconch is especially large and bulbous (fat) for the size of the shell, that animal probably lays eggs that crawl directly out of the egg
case when they hatch. Other mollusks have a swimming larval stage which can last from a few hours
to up to a year, depending on the species. Needless
to say, these animals need lightweight shells to
maintain their position in the water until they are
ready to settle to the bottom and start growing, so
the protoconch will be small and quite often broken off. Direct-developing eggs (that crawl out of
the egg case) need to be larger and stronger to survive on the bottom with all its predators, so the
protoconch is heavier.
Below the protoconch are the adult whorls.
They generally increase rapidly in size (according
to a measureable formula) and are usually 5-9 in
number. Of course, each whorl covers up the
“lower” part of the preceding whorl, so the final
or body whorl is the only one you can see in its
entirety.
Usually you can see the division between the
whorls, called the suture. Often the shell has a little
shelf at the “top” of the whorl called the shoulder.
If there are spines on the shell they are usually at
the shoulder, and there may be additional ones on
the body whorl.
The opening through which the animal protrudes is the aperture, or mouth, with an outer lip
and an inner columella. Often the lip is thickened,
which helps to protect the shell from being broken
by crab claws. If the thickness is pronounced, it is
called a varix, and some animals dissolve the varix
when they grow more shell and some don’t. The
latter will have thickened vertical places on the
early whorls called previous varices (plural of
varix). One group, the Distorsios, have not only a
thick varix but many teeth on both the outer lip
and the columella which makes the opening especially small. This family does not dissolve the varix
or teeth when growing, so it has to grow over that
old shell material, distorting the shell.
Some molluscan families have a final, adult
whorl which looks different from the previous
whorls. A good example is the Strombus family,
the “conchs”. All of the immature whorls have a
thin lip, without a varix or any other thickening.
However, when they approach adulthood, they add
a wide, flaring lip to the final whorl. This adds
weight to the shell and protection for the animal as
it moves below the shell.
At the “lower” end of the aperture is the opening through which the animal protrudes its siphon,
which is the breathing, eating, and sensing organ.
It crawls along with the siphon extended ahead of
the shell to sniff out its environment and prey. To
protect the siphon as much as possible it builds the
siphonal canal, a partial tube which just fits the
siphon. Generally, rock-dwelling mollusks have a
long siphonal canal to better protect the siphon
while sand-burrowing mollusks can extend the siphon far beyond the siphonal canal, above the surface of the sand when they are buried.
Raised or incised lines, spines, and knobs help
strengthen the shell without making it too heavy.
Each species has a unique series of sculpture features - that’s how we tell the different species apart
without DNA! Raised or scratched lines can be spirals (around the whorl) or ribs (across the whorl).
If there are both spirals and ribs they may make
little squares in the surface of the shell - this is
called cancellate sculpture - and sometimes knobs
are formed at the intersections.
Copyright Peggy Williams; may be used by shell club publications with attribution to Peggy Williams
and www.Shelltrips.com