Slugs and snails are members of the phylum Mollusca, along with their crusty friends, the clams and oysters as well as squid, cuttlefish and octopus. Slugs have evolved from snails and many species still retain a vestigial shell hidden under their mantle. The remnant shell is used primarily for calcium storage, rather than protection.
The slug’s cousin, the snail, has an outer shell that he can pull his soft little body into when a raccoon grabs him. The snail needs to have large enough calcium deposits in its environment to form this more substantial shell. The shell serves as a safe refuge from smaller predators as well as protection during dry weather conditions.
The slug is a gastropod, which translates to “stomach-foot.” A slug or snail moves by rhythmic waves of muscular contraction on the underside of its foot. At the same time a layer of mucus is secreted, which helps smooth the slug’s path across the ground.
Slugs and snails have two pairs of retractable tentacles on their head. The upper, optic tentacles, have light-sensitive eyespots on the ends. Each eye-stalk can move independently and can be re-grown if lost. The eye-stalks are also used for smell. Lower on the slug’s face are two smaller tentacles for feeling and tasting. The rasping mouth parts that will chew up your lettuce seedlings are under these tentacles. Slugs eat using a radula, a rasping, tongue-like organ, covered with thousands of tiny tooth-like protrusions called denticles.
Body openings include the anus and genitals, which are hidden under the mantel. The odd position of these openings so far forward on the body is a result of the slugs evolutionary descent from snails. In a snail these openings must be outside the shell (nobody wants to poop in their own shell after all). Slugs are hermaphrodites with both male and female sex organs. A large respiratory pore, usually on the right side, called a pneumostome, leads to a single lung.
The keel or carina is a ridge that runs the length of the back in some slug species.
As you can see, the slug is a very specialized creature, carefully constructed for living in moist places, feeding on vegetable matter, (although some species are omnivorous and enjoy the occasional earthworm or fellow gastropod,) and living life at a thoughtful, Buddha-like pace.