They are related to other sea-snails, and also garden snails, but have lost their large external shells. Instead protection is provided by poisonous stinging cells which are actually taken from their prey. When sea slugs feed on anemones and sea mats they consume the stinging cells, but do not eat them. Somehow the sea slugs can then internally transport these stinging cells and store them in special bumps on their backs. Here they provide a nasty concentrated stinging defence against any would be predators. Sea slugs have external gills which look like feathers gathered in a ring towards the back end. They come in a bewildering variety of bright colours warning predators that they are poisonous to eat!