These animals include snails , slugs , clams , mussels , octopus , squid , and many other kinds of animals without backbones that are often found in the sea , but sometimes in lakes and rivers , and on land . Most of them have shells , but not all of them . Mollusks include snails and slugs , clams , octopus and squid , and numerous other kinds , many ( but by no means all ) of which have shells .