A mollusk is a type of invertebrate animal that belongs to the phylum Mollusca. This diverse group includes creatures such as snails, clams, and octopuses. Mollusks typically have soft bodies, and many are protected by a hard shell made of calcium carbonate. They can be found in various environments, from oceans to freshwater and even on land. Mollusks are characterized by their unique body structure, which usually includes a muscular foot for movement, a visceral mass containing internal organs, and a mantle that secretes the shell. They play important roles in ecosystems and are also significant in human diets and industries.