Oyster or slug, eg (7)
I believe the answer is:
mollusc
(Other definitions for mollusc that I've seen before include "Creature such as a snail or mussel" , "Invertebrate such as a snail" , "Invertebrate with a soft unsegmented body, usually inside a shell" , "one likely to be shelled" , "Eg, snail or slug" .)