The pulmonate ("lunged") gastropods include land and freshwater snails and slugs. All pulmonates have lost their ancestral ctenidia, and the vascularized mantle has become a lung, which fills with air by contractions of the mantle floor.  The lung opens to the outside by a small opening called the pneumostome (seen on the right side of the animal).  Slugs often replace shelled snails where calcium is lacking. Although they lack shells, slugs may derive some protection by secreting a sticky and noxious mucus that may be toxic to some predators.