Re: does a snail grow his own shell?

At last some-one has asked me a question which has a simple answer! And the answer to Andrew's question is: "Yes, snails grow their own shells".

The shell, and the tissue which makes it (the mantle), are distintive features of animals in the Phylum Mollusca. The most diverse group of molluscs are the gastropods (Class Gastropoda), the animals which are commonly referred to as snails. These two key features of the molluscs probably evolved very early in the evolutionary history of the group. Barnes, Calow & Olive have this to say:

What they mean by this is that the earliest molluscs (which probably looked a bit like the animal in the drawing on the right, only simpler) probably had a tough protective layer—the cuticle—over their back made by their skin—the epidermis. This cuticle probably consisted of proteins, and scales or plates made of calcium carbonate. (Calcium carbonate, CaCO₃, is the material which shells are now made of now.)

You can also see that these early molluscs probably had a simple, flat shell, like a dish.

Above: Drawing of a simple mollusc. The shell and mantle are labelled. Other organ systems are: gut (brown); heart and blood vessels (red); reproductive organs (blue); gills (purple). [Courtesy of the BIODIDAC image library.]

Over time, the scales or plates joined to become the shell (or shells in bivalves, and plates in chitons), and the epidermis evolved to become a special tissue—the mantle—which makes and repairs the shell.

The shell of a modern mollusc is a complex structure consisting of many different layers, some with proteins and some without. The animal maintains the shell throughout its life and adds to it as it grows.

Above: Photograph of a mussel with one shell removed. The mantle is labelled. It lies right next to the shell. The other large, thin tissues inside the shell are the gills (also labelled) which the animal uses to breathe and to feed. [Courtesy of the BIODIDAC image library.]

Finally, it is worth noting that, while the shell is a distinctive feature of the molluscs, not all molluscs have a shell and neither do all gastropods. Two strange groups of molluscs (the chaetodermomorphs and the neomeniomorphs) never have shells. Octopuses (one kind of cephalopod mollusc) don't have shells. And the gastropods called "nudibranchs", "slugs" and "sea slugs" also usually lack shells (although some slugs have a small plate).

Reference material

Books

Classification

These are the groups of molluscs recognised by Barnes, Calow & Olive. Other books have slightly different groupings.

PHLUM MOLLUSCA
Class	Kind of animal
Chaetodermomorpha	A group of strange, shell-less and worm-like animals, from 2 mm to 14 cm in length, which live in burrows in sand and mud.
Neomeniomorpha	These animals are similar in shape to the chaetodermomorphs, although they can be up to 30 cm in length, but are carnivores feeding on cnidarians (sea anemones, hydroids, corals).
Monoplacophora	These animals were thought to be extinct until, in 1952, some were dredged up from deep ocean trenches in the Pacific Ocean. They are all small (3 mm to 3 cm), have a simple, dish-like and feed on material in sand or mud.
Polyplacophora	These are the "chitons": animals with eight plates and common herbivore on rocky shores around the world.
Gastropoda	The gastropods are the animals known as "snails". Some are shell-less but in those with shells, the shell can take on many elaborate shapes with curls, spines, knobs and projections. They are most common in the sea but also live in freshwater and on land.
Bivalvia	These are the clams, mussels and oysters: molluscs with two shells.
Scaphopoda	The scaphopods are the "tusk shells", a name that very accurately describes the shape of their shells (like an elephant's tusk). They are not often seen alive but their shells are common on beaches worldwide.
Cephalopoda	This group includes the squid, octopus and cuttlefish. These are undoubtedly the most active of the molluscs and amongst the most active of all invertebrates. They are predators, often feeding on crustaceans (crabs, lobsters and shrimp). They are also the largest of the invertebrates—giant squid can be more than 20 m in length—and the most intelligent.

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