Here we have an amazing picture of a Banana slug (Ariolimax sp.), which I captured a photo of it as it roamed across the forest floor in Muir Woods. There are three species of Banana slug known, one of which, Ariolimax columbianus, is the second largest terrestrial slug in the world. It can reach up to 25 cm (10 in) long. All live in the foggy, forest belt on the pacific coast from Central California to Southern Alaska.
These guys posses a pneumostome, a hole which is visible in this picture as a dimple a bit down from the head. The pneumostome opens into a highly vascularized cavity which acts as a simple lung for these animals.
They are very prone to dehydration, so tend to inhabit areas that are moist, and produce lots of mucus. At the tail end of the slug, you can see some debris caught in the slug's mucus plug. This plug covers the caudal pit, where the mucus is produced. Their mucus not only keeps them moist, and helps them move, but also protects them as it has an anesthetic-like chemical making them unappetizing to other creatures.
Finally, Banana slugs also use their mucus as a mode of communication. During mating season, they will incorporate pheromones into their mucus, making it easier for another slug to find them.