Class Bivalvia includes clams, oysters and mussels. Bivalves are surrounded by two shells joined by a hinge ligament. Class Gastropoda includes snails. Gastropods usually have a one piece shell. Class Cephalodopa includes cuttelfish, squids, and octopi.
Cephalopods are the most active and intelligent of the mollusks.
Class Polyplacophora includes Chitons. Those have a close resemblance to the hypothetical primitive form of mollusks. They have an elongated body with a mantle embedded with 8 dorsal plates.