All except bivalves have a radula, which is an organ used for scraping. There are at least 47,000 species of mollusks. 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.