A class within the phylum Mollusca whose members are popularly called chitons, coat-of-mail shells, or sea cradles. Polyplacophora, or Loricata, was formerly combined with Aplacophora in the class Amphineura. All chitons are marine, and they typically live in the intertidal zone, although some live in deeper waters. They are found from subarctic to tropical latitudes but are most abundant in warmer waters. There are roughly 750 chiton species living today.