Animals similar to these ancient molluscs, which are called monoplacophorans, are still found today in deep ocean habitats, and their body structure reveals that molluscs had segmented ancestry.
Animation shows how other mollusc groups evolved from the ancestral body plan.
In most molluscs, the mantle edge secretes a hard, calcified shell that serves to protect the soft body. The diversity of mollusc shells provides a fascinating case history in the adaptation of form and function-from the shells of limpets, clams, and tooth shells to the huge shells of ancient ammonites to the thin shell, or pen, of the squid. Some molluscs, such as the octopus, nudibranch, and land slug, have evolved strategies for defense that eliminate the need for a shell. The shell of chitons, class Polyplacophora, is a series of eight bony plates embedded in the dorsal surface, again reflecting a segmented ancestry. Chitons are largely herbivores, as can be seen by watching the action of the toothed radula as the chiton scrapes diatoms from a rock.