Slugs and snails both belong to the molluscan group called the gastropods. They are strikingly similar in all but one blatantly obvious respect: slugs are snails that have lost their shells.
Shell-loss is something that has happened many times among the gastropods, including the air-breathing pulmonates (the group that contains terrestrial slugs and snails, as well as pond snails and even a few marine species).
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Some species are still in the process of losing them. The ear-shelled slug of Europe, for example, sports a small, limpet-like shell on its rear end.
But why would so many soft-bodied gastropods have ditched their shells? Well, as caravan owners know only too well, taking a bulky protective shelter everywhere you go hampers manoeuvrability and your ability to park in tight spaces.
Streamlined slugs can go places their shelled relatives cannot. And to compensate for their lack of physical protection, many slugs – both marine and terrestrial – defend themselves with toxic or unpalatable chemicals.