Sue,

I have cleaned shells from old collections that had the animal still
inside -- very dried and hard.  If you try to pull it out like it is you can
chip the aperture and in the case of nerites you can break the operc.  So, I
soak the shell in soapy water for a day or so.  Then heat the water with the
shells in the microwave for 20 seconds or just short of a boil.  This is
usually sufficient to loosen the animal and make it soft enough to remove.
However, with some tightly closed opercs you may need to soak it another day
or so after the heating has slightly opend the operc seal.  Of course this
soaking will reactivate the smell -- less than a popular thing to do in the
kitchen (where I always soak them).

Tom E. in New Mexico, USA