What is a mollusc?
A mollusc is an animal with a soft body and no bones. The word comes from a Latin word meaning "soft". Molluscs live in the sea, in rivers and ponds, and on land in damp places.
Look at the body in three parts:
- Head: has the mouth and often eyes and feelers.
- Foot: a strong muscle used to crawl, dig or grab.
- Mantle: a thin cloak of skin over the body. It makes the shell.
Many molluscs also have a rough "tongue" with tiny teeth called the radula. A snail uses it to scrape leaves like a file. Molluscs are the second biggest animal group after arthropods.
Gastropods: snails and slugs
The name means "stomach-foot": the animal seems to walk on its belly. A snail has one shell, usually a spiral. A slug has lost its shell. Gastropods crawl on a wide foot with a slippery slime trail, so they can glide even over rough ground. Examples: garden snail, apple snail, slug, sea hare.
When danger comes, the snail pulls its soft body inside the shell like going into a house.
Bivalves: clams, mussels and oysters
"Bi" means two and "valve" means shell door. A bivalve has two shells joined by a hinge and held shut by strong muscles. They have no clear head. Most sit in sand or on rocks and filter tiny food from the water they pull in. A clam also pushes its foot out to dig. Examples: clam, mussel, oyster, scallop. A pearl forms when a grain of sand irritates an oyster and the mantle wraps it in shiny layers.
Cephalopods: squid, octopus and cuttlefish
The name means "head-foot": the foot has turned into arms around the head. A squid has 10 arms (two are long tentacles), an octopus has 8. The shell is small and hidden inside the body (cuttlefish and squid) or gone (octopus). They have big eyes and the biggest brains among invertebrates. To move fast, they squirt water out of a tube called the siphon. Many can change colour and some squirt dark ink to escape.
Why do molluscs matter?
Molluscs are food, give pearls and shell craft, and clean water (bivalves filter it). Some snails eat garden crops and some carry diseases, so they can also be pests. Many shells are made of calcium carbonate, the same material as chalk.
Try it
Try it at home: After rain, find a snail and watch it for two minutes (do not touch the face). Is its shell one piece or two? Does it glide on a wet trail? Draw it and name the foot, shell and feelers. Then use the 3D above: drag the slider and count how many pieces each shell has.
Key formulas and definitions
- Mollusc = soft body + foot + mantle (+ shell in most)
- Gastropod = 1 shell (snail); Bivalve = 2 shells (clam); Cephalopod = shell inside or none (squid)
- Gastropod = "stomach-foot"; Cephalopod = "head-foot"
- Radula = rasping tongue; Siphon = water jet tube of squid
- Squid: 10 arms; Octopus: 8 arms
Worked examples
1. An animal has a soft body, a spiral shell and a wide foot. Which group is it?
One spiral shell with a wide crawling foot means a gastropod, such as a snail.
2. A sea animal has two shells joined by a hinge and no clear head. Name its group and one use of it.
Two shells and no head: bivalve (clam, mussel, oyster). Uses: food, pearls, and cleaning water by filtering.
3. Why can a squid swim faster than a snail can crawl?
A squid squirts water from its siphon, which pushes it forward like a jet. A snail only glides slowly on its foot.
4. A shop has 3 clams, 4 snails and 2 squids. How many outside shells are there in all?
Clams: 3 × 2 = 6. Snails: 4 × 1 = 4. Squids: 2 × 0 = 0. Total = 10 shells.
Common mistakes
- Thinking a slug is not a mollusc. It is a gastropod that lost its shell.
- Saying all molluscs have an outside shell. A squid's shell is inside and an octopus has none.
- Calling an octopus a fish. It has no backbone and no fins, so it is an invertebrate.
- Mixing up bivalve and gastropod: bi = two shells, gastro = one shell and a belly foot.