Slug is a common non-scientific word which is most often applied to any gastropod mollusk whatsoever that has a very reduced shell, a small internal shell, or no shell at all. A slug-like body is an adaptation which has occurred many times in various groups of snails. The word "slug" is used in everyday parlance specifically to differentiate gastropods with a very reduced shell or no shell at all from those which have a large and noticeable shell, which are commonly known as snails. But in reality slugs and snails are not very different at all from one another, apart from the fact that snails have a functional coiled shell into which they can retract. Simply speaking, slugs are snails. The common name "slug" is perhaps most often applied to land species, but the word has also been applied to many marine species. The largest group of marine shell-less gastropods or sea slugs are the nudibranchs, but there are in addition many other