Well let's not forget that most seashells begin life as swimmers. The larval mollusk that hatches from the egg (in many families) is a free-swimming organism, and only settles down to a more sedentary existence after it become weighed down by that newly developed calcium carbonate anchor we call a shell. (Not all mollusks follow this pattern though - in some the larval form passes within the egg, and a shelled minature of the adult hatches out.) As for mollusks which continue swimming when mature, not many shelled species do so, for the obvious reason that the weight of a shell is counter-productive if your purpose is to remain suspended in the water. Swimmers that have a substantial shell either restrict their swimming to short bursts in emergency situations (Pecten, Lima) or they have a method of compensating for the weight of the shell (the gas-filled buoyancy chambers in Nautilus). The best molluscan swimmers though, have eliminated the shell altogether, or have moved it to an internal location. Of course, in avoiding the weight of a shell, they have also forfeited the protection which a hard shell provides, and therefore rely on speed or other special adaptations for protection. The best swimmers among the Mollusca are the Cephalopods. They can all handle themselves pretty well in open water, whether shell-less (Octopus), externally shelled (Nautilus), Internally shelled (Spirula), externally pseudo-shelled (Argonauta) or internally pseudo-shelled (Loligo and other squids). Among the bivalves, the Pectinidae (scallops or pectens) and Limidae (file shells) are the best swimmers, but they must maintain a high level of activity to stay up in the water. As soon as they rest, they sink to the bottom. Some species of Solenidae (razor clams and jackknife clams), such as Ensis, sometimes swim "accidentally". The rapid motions of the large foot are actually an effort to dig into the sand, but if the foot isn't in contact with the sand, it pulls them through the water in a crazy zigzag manner, until they find the substrate, and rapidly dig into it. Among the gastropods, I believe the best swimmers are the Aplysiidae or "sea hares", a family with an odd fan-shaped internal shell. Some of them can easily outdistance a diver. Some of the nudibrachs (sea slugs) are free swimming, and so are the pelagic Pteropods (sometimes called butterfly snails), in which the edges of the foot are expanded into two wide flaps resembling wings. These do have a shell, but it is extremely thin and lightweight. Paul M. Rhode Island