Patrick, the gastropods include many herbivorous families (Cypraeidae,
Littorinidae, Haliotidae, Trochidae, Turbinidae, Neritidae, Strombidae), but
also many carnivorous ones (Conidae, Buccinidae, Cassidae, Fasciolariidae,
Olividae, Mitridae, Muricidae, Turridae, Volutidae), so it's pretty hard to
generalize.  All Cephalopods are carnivorous.  Almost all chitons are
herbivorous.  Bivalves are pretty hard to classify, since they just filter
out of the water whatever small particles come along.
Paul M.