 Chitons from Australia. Photo: Bill and Mark Bell (Source). |
Apart from the well-known Mollusc groups (Gastropods, Cephalopods, Bivalves and Scaphopods) several other smaller groups are distinguished. Apart from the Monoplacophorans with the living fossil Neopilina there are molluscs groups which have not yet developed a shell in the common sense of the word, such as, for example, the Polyplacophorans (Chitons) and the so-called shell-less molluscs or worm molluscs with the Solenogasters and Caudofoveatans.
The difference between the various groups of molluscs can well be seen looking at the Evolution of the Mollusc Shell.