Mollusks mostly live in the water, but not always: land-based snails and slugs are Mollusks, too.
Mollusks are invertebrates, meaning they have no backbones. Their soft bodies are “single segments” — all one piece. If you wonder whether this is true of an octopus, because surely that has “legs” that are a distinct part of their body, they are actually all one part of the same body, sort of like gloop dripping through your fingers — there are no joints.
Most Mollusks secrete calcium carbonate to make shells that will outlive them for quite a while.
There are three types of Mollusks: bivalves, cephalopods and gastropods.
- Bivalves includes things with two shells, such as oysters, cockles, mussels and clams;
- Cephalopods include octopus, squid and cuttlefish. They have no skeleton nor any shells;
- Gastropods include snails, both land and sea, limpets and whelks.