|A chiton showing the eight-part shell that signifies their classification into Class Polyplacophora ("many plate bearing").|
I find these unusual molluscs to be extremely fascinating. Phylogenetic classification places them as basal molluscs, what would also be called a primitive type. As such, they may be the most like the soft-bodies animals from which all molluscs came. They are most commonly found in tidal areas, stuck fast to rocks. Their mantle girdles act like suction cups to attach them to the surface and conserve moisture while the tide is out. Their attachment can be so firm that they may only be removed with great effort. There are over six hundred species, all of which are marine and most of which live in what is known as the "rocky intertidal zone," where they will regularly be exposed to open air. However, some species live in quite deep water or in association with other living things.
The most obvious characteristic of chitons is the eight-part shell. Like all molluscs with an external shell, it is used for protection. In addition, the mantle may be armed to further protect the animal. Interestingly enough, chitons may have shell plates that are internal and/or reduced in size. These examples are more like worms in appearance than their fully plated cousins.
|A chiton's underside, mouth to the left|
Overall, chitons are not very significant to humans. Though eaten at one time by those who were native to the Pacific coast of North America, they are not any longer.
Sources are Animal Diversity Web and the University of California Museum of Paleontology. Images are from Wikimedia Commons under Creative Commons licenses: one, two.