Polyplacophorans are most commonly known as chitons. They are also sometimes known as sea cradles or "coat-of-mail shells", or more formally as loricates, and occasionally as polyplacophores.
Chitons have a dorsal shell, which is composed of eight separate shell plates or valves. These plates overlap somewhat at the front and back edges, and yet articulate well with one another. Because of this, although the plates provide good protection for impacts from above, they nonetheless permit the chiton to flex upward when needed for locomotion over uneven surfaces, and also allow the animal to slowly curl up into a ball when it is dislodged from the underlying surface. The shell plates are surrounded by a structure known as a girdle.
Chitons live worldwide, in cold water, warm water, and in the tropics. Most chiton species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone. They live on hard surfaces, such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and ar
e exposed to the air and light for long periods. Others live subtidally. A few species live in deep water, as deep as 6,000 m (20,000 ft). Chitons are exclusively and fully marine. This is in contrast to the bivalves, which were able to adapt to brackish water and fresh water, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.