A marine invertebrate animal that has some resemblance to a terrestrial slug, although many are brightly coloured with external gills and tentacles on their backs. Most are snails (class Gastropoda, order Nudibranchia) that have lost or internalized their shells. They swim well, many have skin glands that secrete sulphuric acid, and some have and deploy nematocysts (stinging threads) they have retained from prey they have eaten, replacing them every few days. Sea slugs vary in size from 4 mm to 600 mm long. There are about 2300 species of nudibranchs, found in all oceans.