The pneumostome or breathing pore is a respiratory opening of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods. It is an opening in the right side of the mantle of a stylommatophoran snail or slug. Air enters through the pneumostome into the animal's single lung, the air-filled mantle cavity. Inside the mantle cavity the animal has a highly vascularized area of tissue that functions as a lung.