The word `` slug '' is also often used as part of the common name of any gastropod mollusc that has no shell , has a very reduced shell , or has only a small internal shell ( this is in contrast to the common name snail , which applies to gastropods that have a coiled shell large enough that the animal can retract its soft parts fully into it ) .
Snails are gastropods with a coiled shell large enough for the animal can retract inside it .