The principle physiological problem in moving from a marine to a fresh water habitat is maintaining the proper water-salt balance in the cells and tissues of the animal. I don't know much about how molluscs have accomplished this, but I know a little about the parallel problem in vertebrates. Presumably something analygous has occurred in molluscs. The salt concentration in the body cells of marine fishes is lower than that of the surrounding ocean. As a result, water tends to pass out of their bodies by osmosis. They need to be able to retain needed water while excreting the excess salt that is inevitably taken in with their food. As a result, their kidneys have very small and relatively sparse renal corpuscles (or in some cases tubules only, with no corpuscles at all) which filter water slowly; water reabsorbing renal tubules (also found in mammals); and special salt-excreting cells in their gills. Fresh water fishes have the opposite problem. The necessary concentration of salts in their tissues is greater than that of the surrounding water. Therefore water tends to flow into their bodies by osmosis. These animals must be able to excrete large volumes of water while retaining the needed physiological salts. Their kidneys have large and numerous renal corpuscles which carry on selective filtration and excretion of water at a high rate; and they lack water-reabsorbing kidney tubules. Most amphibians have inherited this fresh water fish type of kidney structure, which is why they become dehydrated so quickly if they are not able to soak up water through their skin. So presumably molluscs, in their gradual invasion of fresh water, have likewise developed methods of excreting water and retaining salts to counter the effects of osmosis. And then there are the terrestrial molluscs - but that's another story. Paul M.