The molluscs demonstrate remarkable morphological diversity, a characteristic that has confused molluscan taxonomy from the group’s inception.  The Latin root molluscus means soft, and many soft-bodied invertebrates have been added and removed this group until Cuvier’s modern approximation in 1795 (Brusca and Brusca 2003).  Mollusca is the second largest invertebrate phylum after the arthropods.  Some 93,000 extant species have been described, but the thinking is this number represents only about half of the living species.  70,000 fossil species are also known.  Most classifications recognize ten molluscan classes (two extinct).  One class, the gastropods (snails and slugs), contains about 80% of mollusc species.

A very rich molluscan fossil record dates back 500 million year to the Precambrian.  The evolutionary origins of molluscs are still disputed, but recent well-respected molecular phylogenetic analyses place the molluscs in the Lophotrochozoa, along with annelids, brachiopods, bryozoans and several other phyla (Halanych et al. 1995).  Relationships within the Mollusca are also unclear and disputed; some recent analyses challenge whether this enormous phylum is a natural, monophyletic group (Sigwart and Sutton 2007 and references therein).