Crustacean or mollusk? Can you tell the difference?

You can probably tell a mussel from a barnacle perfectly, but do you know which one is a crustacean and which one is a mollusk?

Crustaceans (from the Latin crusta, “crust, bark”) have a hard or flexible shell and can be of two types: elongated body (such as prawns, shrimp, shrimp, lobster, crayfish or crayfish) and short body (such as spider crab, crab, velvet crab or barnacle).

Mollusks (from the Latin molluscus , meaning "soft") have a soft body that can be covered by one shell (univalves or gastropods), two shells (bivalves), or be naked and have tentacles (cephalopods). Among the univalve mollusks (with a single shell) are the limpet, the periwinkle, and the whelk. Bivalve mollusks (with two shells) include mussels, razor clams, scallops, cockles, queen scallops, wedge clams, and oysters. Finally, among the cephalopods are the squid, cuttlefish, octopus , and giant squid.