any one of a large group of animals having no backbone, soft bodies not composed of segments, and usually covered with a hard shell of one or more parts. The shell of mollusks is secreted by a covering mantle and is formed on snails, clams, oysters, whelks (large, often edible, marine snails) and mussels. Slugs, octopuses and squids have no shell. —Handbook for Preclears Glossary (app 9.7.90)