There are many different oceanic mollusks, such as clams, octopus and oysters, but there are also mollusks that live on land, such as snails and slugs. Mollusks are the second largest group of invertebrates in the world, with somewhere between 50,000 and 200,000 living species.
Mollusks can be separated into a number of smaller categories, such as univalve, bivalve and cephalopods. Cephalopods are a group that contains cuttlefish, squid and octopus. Univalve mollusks are those that only have one shell, such as snails, while bivalve mollusks are those with two connecting shells. Virtually all univalves have a protective shell, which grows in a spiral pattern. The only exception is slugs, which are also considered univalve despite the fact that they have lost the ability to grow a shell through evolution.
Examples of bivalve mollusks include scallops and clams. Although snails are technically univalve, they actually belong to the group known as gastropods, which also includes both sea and land slugs, limpets, sea cucumbers and sea hares. Both univalve and bivalve mollusks are often prized for their extraordinary shells. Basically all shells that are found on beaches across the world come from some species of mollusk. Some of the most prized mollusk shells include conch, cowrie and whelk, which are all univalve gastropods.