What’s that hole on the side of slugs? This is how a slug breathes! All land slugs have a pneumostome (say it with me: new-mah-stome) on the right side of their mantle! Oxygen enters through this opening and moves inside the hollow cavity (called a pallial cavity) where gas exchange takes place. This entire system is also called a pallial lung. Certain marine and freshwater species, lack a pneumostome and instead use gills for respiration. Land slugs evolved to live in terrestrial or semi-aquati