Includes snails, clams, octopi, etc. Interesting Facts: - Phylum mollusca is composed of 3 main classes: Gastropods, bivalves, and cephalopods. - All cephalopods have some sort of interior shell, with the exception of the chambered nautilus, which does have an external shell. - Mollusks have been around for over 500 million years. Mollusk on my field guide: Mediterranean Scallop (Pecten jacobacus)