The four major groups of mollusks (phyla Mollusca) are: 1) chitons; 2) gastropods, including snails, slugs (mostly marine, but some freshwater), and nudibranches. This is the largest and most diverse group of mollusks (around 40,000 different species); 3) bivalves, including clams, oysters, and mussels; and 4) cephalopods, including squids and octopuses. Although mollusks vary widely in external appearance, some share the following body plan: a muscular foot, usually used for movement; a mass containing most of their internal organs; and a mantle, or a fold, of tissue that drapes over the mass and secretes a shell (that is, in organisms that have a shell).