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.