I think that you would need to accept that every individual detail has been potentially lost through the long evolutionary history of the group, and that a mollusc is anything that can be traced back to a proto-mollusc, which can be defined as having a mantle, radula and foot. Allen Aigen [log in to unmask] ---------- "Monfils, Paul" <[log in to unmask]> writes: From: "Monfils, Paul" <[log in to unmask]> To: [log in to unmask] Subject: Re: Conchology 101 Question Date: Mon, 9 Sep 2002 11:06:52 -0400 Bivalves don't have a radula, but I believe all mollusks have a mantle (even though they don't all produce a shell) and a foot (even though it is modified beyond recognition in cephalopods). Paul M. Allen Aigen [log in to unmask]