Chitons are primitive Ocean|marine molluscs in the class biology class Polyplacophora ormerly the class Amphineura There are 900 to 1,000 living species of chitons Their size ranges from 0.5 to 30&nbsp cm in length most are quite small ITIS ID 78807 rename Barnes Barnes Robert D1982. Invertebrate Zoology Philadelphia PA Holt Saunders 381389 ISBN 0-03-0567475 All the living species are in the subclass Neoloricata and so may be called loricates Popular names are sea cradles or coat of mail shells They have a single shell that sticks to rocks, like a limpet The shell is made of hard plates that fit tightly together, overlapping one another The shell plates are surrounded by a structure known as a girdle