Another attribute of mollusks is that they have a mantle, which is responsible for secreting the shell. For the most part, this shell is used for protection.
Mollusks have a definite body plan, including a foot for locomotion, a head, and a visceral mass.
Not all mollusks have a head however. Some of the more common mollusks are the snails, clams, and octopus. In the class cephalopoda, the organisms have internal shells. The phylum annelida, contains the organisms that have many septae, or body segments. These organisms include the earthworms and leeches.