What you call the shell that sticks hard to rocks?
Many animals have been filmed using rocks and sticks as tools. One of the most common ways an animal breaks apart hard bits of food or a hard shell is by using a rock as a hammer. Often times they find another rock to place the hard food on and hit with a smaller rock. They seem to have learned what size and kind of rocks to use.
Soft shell crabs are soft and are eaten with the entire crab. Soft shell crabs are actually hard shell crabs that have shed their shells to allow for growth. Shells do not grow with the crab. Crabs retain a soft shell when they shed their hard shells; then that soft shells begins to harden-up as the crab grows.
mollusks use their hard shells, if they have any, to protect themselves. also they use things in their environments, such as rocks etc... Mollusks use their Shells for protection. most gastropods have a hard shell they can retreat into bivalves have two hard shells they can lock themselves up into. Limpets have dorsal plates.