Empty Shells

• Aquarists often see empty snail shells in their aquariums and gardeners frequently spot empty snail shells in the soil. The shell is left after the snail dies simply because it decomposes more slowly. The snail's soft body breaks down quickly leaving behind the hard shell, which is made of calcium. In aquariums, other creatures such as hermit crabs may reuse snail shells after the snail dies.
  
  

Shell Growth

• Snails are soft-bodied creatures called mollusks, as are clams, oysters, octopus and squid. Some species of snails live on land while other species live in water. All snails are born with a shell, which grows in a spiraling pattern as the snail grows. The shell grows at the rim by adding ever-larger rings. A snail cannot survive without its shell. If it leaves the shell, the snail is dead or about to die.
  
  

Withdrawing Inside

• Sometimes a snail withdraws into its shell, making it appear as if it has left the shell. The garden snail, or Helix aspersa, retreats into its shell if it is disturbed, just as a turtle would. It also pulls into its shell to protect is slimy body from drying out in arid weather. The snail plugs its shell holes with mucus and secretes a cocoon of mucus around its body. It waits in a state of suspended animation until the dry weather passes.
  
  

Shell Repairs

• Snails have different types of shells. They can be flat, round, spiral or pointed. If a shell is cracked or chipped around the rim, a snail can repair its shell by healing over the breaks during the normal course of its growth. If holes or breaks are away from the rim, however, they can be fatal to the snail.
  
  

Slugs

• Snails carry their shells on their backs as they secrete mucus and slither along with their flat-bottomed feet. Slugs are sometimes mistakenly called snails. Slugs and snails are very similar; however slugs do not have shells. Both belong to the group of soft-bodied creatures known as gastropods.