Asked by: Sally Thompson, Burnley
Slugs belong to a group of animals called molluscs. While slugs have no apparent shell, some species may have a reduced shell, or one that is internal, and therefore not visible. It is believed that the first mollusc-like animals were shell-less, with reduced or internal shells emerging later, and external shells evolving after that. This may seem strange, as molluscs have soft bodies, making them easy, vulnerable prey. But while shells do give some protection, they can also slow you down, so molluscs have extremely diverse survival strategies, which include camouflage, colouration, detaching their back appendages (as some lizards do) and chemical defences.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.