These molluscs are unusual in that instead of having one big shell (like limpets or whelks) their armour is made up of eight overlapping plates. Under this exciting exterior is the creature's greenstuff; mouth at the front, algae-scraping radula to drag food into the oesophagus, a stomach and a coiled bowel that makes and excretes pellets of digested algal biofilm residue. There are gills down each side, under the shells. One group is carnivorous, trapping prey under its shell before consuming it.
Someone, a scientist of heroic patience, has suggested that like limpets, chitons have a homing instinct. Their sensory organs protrude between the plates of the shell to tell the chiton (of which there are an estimated 750 living species) what is happening and are wonderfully named: aesthetes.
Chitons are mollucs, class polyplacophora. For a brief and intelligent introduction to these creatures, the UCMP page cannot be beaten.