Mollusca (or mollusks) are a very diverse groups of animal with at about 85,000 living species. Mollusks are the largest marine phylum, comprising about 23% of all known marine organisms. Mollusks include clams, scallops, oysters, mussels, limpets, chitons, and snails (snails are gastropods—the account for about 80% of invertebrate species). Cephalopods are mollusks and include octopuses, squid, cuttlefish, and nautilus.
• Mollusks all have unsegmented soft bodies with a "head" and a "foot" region (they may not look like a head or foot!).
• Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons.
• Many have shells, either calcareous, or made of proteins and chitin.
• Most mollusks have eyes.
• Mollusks have a mantle with a body cavity (used for breathing and excretion), and the presence of a radula (something tongue-like).
• All mollusks larvae nervous system, blood circulation system, and often complex digestive system.
• All produce eggs that emerge as larvae or miniature adults.
Mollusks appeared in the Cambrian Period and have diversified into their multiple forms. A large group called ammonites dominated the oceans during the Mesozoic era, but vanished with many other species at the K/T Boundary extinction event. Their distant relatives, squids, that do not have calcareous shells, survived the K/T extinction event. Another distant relative, the nautilus, also survived the K/T event.