With over 50,000 currently living species, it is second only to the Phylum Arthropoda. These are bivalves, which are mollusks with two shells that are hinged at the mid-dorsal line and drawn tightly together by powerful adductor muscles. They include clams, scallops and oysters. These are univalves, which is usually coiled creating a place of retreat for the animal when in danger. They have a sharp radula used for grazing on plants or tearing at animal flesh.
Most species are marine, but there are also many freshwater and terrestrial species.
There is one type possessing a shell called the chambered nautilus. They use tentacles for movement like creeping on the ocean floor and also to capture prey. In the middle of these tentacles is a powerful beaklike jaw to bite prey. They are also known for their ability to shoot ink in self-defense, and escape with their jet propulsion movement. As a mussel or clam grows, the mantle secretes the shell in layers, each beyond the last.